[deliverable/linux.git] / mm / huge_memory.c

/*
 *  Copyright (C) 2009  Red Hat, Inc.
 *
 *  This work is licensed under the terms of the GNU GPL, version 2. See
 *  the COPYING file in the top-level directory.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/highmem.h>
#include <linux/hugetlb.h>
#include <linux/mmu_notifier.h>
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/shrinker.h>
#include <linux/mm_inline.h>
#include <linux/swapops.h>
#include <linux/dax.h>
#include <linux/kthread.h>
#include <linux/khugepaged.h>
#include <linux/freezer.h>
#include <linux/pfn_t.h>
#include <linux/mman.h>
#include <linux/memremap.h>
#include <linux/pagemap.h>
#include <linux/debugfs.h>
#include <linux/migrate.h>
#include <linux/hashtable.h>
#include <linux/userfaultfd_k.h>
#include <linux/page_idle.h>

#include <asm/tlb.h>
#include <asm/pgalloc.h>
#include "internal.h"

enum scan_result {
        SCAN_FAIL,
        SCAN_SUCCEED,
        SCAN_PMD_NULL,
        SCAN_EXCEED_NONE_PTE,
        SCAN_PTE_NON_PRESENT,
        SCAN_PAGE_RO,
        SCAN_NO_REFERENCED_PAGE,
        SCAN_PAGE_NULL,
        SCAN_SCAN_ABORT,
        SCAN_PAGE_COUNT,
        SCAN_PAGE_LRU,
        SCAN_PAGE_LOCK,
        SCAN_PAGE_ANON,
        SCAN_PAGE_COMPOUND,
        SCAN_ANY_PROCESS,
        SCAN_VMA_NULL,
        SCAN_VMA_CHECK,
        SCAN_ADDRESS_RANGE,
        SCAN_SWAP_CACHE_PAGE,
        SCAN_DEL_PAGE_LRU,
        SCAN_ALLOC_HUGE_PAGE_FAIL,
        SCAN_CGROUP_CHARGE_FAIL
};

#define CREATE_TRACE_POINTS
#include <trace/events/huge_memory.h>

/*
 * By default transparent hugepage support is disabled in order that avoid
 * to risk increase the memory footprint of applications without a guaranteed
 * benefit. When transparent hugepage support is enabled, is for all mappings,
 * and khugepaged scans all mappings.
 * Defrag is invoked by khugepaged hugepage allocations and by page faults
 * for all hugepage allocations.
 */
unsigned long transparent_hugepage_flags __read_mostly =
#ifdef CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS
        (1<<TRANSPARENT_HUGEPAGE_FLAG)|
#endif
#ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
        (1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG)|
#endif
        (1<<TRANSPARENT_HUGEPAGE_DEFRAG_FLAG)|
        (1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG)|
        (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);

/* default scan 8*512 pte (or vmas) every 30 second */
static unsigned int khugepaged_pages_to_scan __read_mostly = HPAGE_PMD_NR*8;
static unsigned int khugepaged_pages_collapsed;
static unsigned int khugepaged_full_scans;
static unsigned int khugepaged_scan_sleep_millisecs __read_mostly = 10000;
/* during fragmentation poll the hugepage allocator once every minute */
static unsigned int khugepaged_alloc_sleep_millisecs __read_mostly = 60000;
static struct task_struct *khugepaged_thread __read_mostly;
static DEFINE_MUTEX(khugepaged_mutex);
static DEFINE_SPINLOCK(khugepaged_mm_lock);
static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
/*
 * default collapse hugepages if there is at least one pte mapped like
 * it would have happened if the vma was large enough during page
 * fault.
 */
static unsigned int khugepaged_max_ptes_none __read_mostly = HPAGE_PMD_NR-1;

static int khugepaged(void *none);
static int khugepaged_slab_init(void);
static void khugepaged_slab_exit(void);

#define MM_SLOTS_HASH_BITS 10
static __read_mostly DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);

static struct kmem_cache *mm_slot_cache __read_mostly;

/**
 * struct mm_slot - hash lookup from mm to mm_slot
 * @hash: hash collision list
 * @mm_node: khugepaged scan list headed in khugepaged_scan.mm_head
 * @mm: the mm that this information is valid for
 */
struct mm_slot {
        struct hlist_node hash;
        struct list_head mm_node;
        struct mm_struct *mm;
};

/**
 * struct khugepaged_scan - cursor for scanning
 * @mm_head: the head of the mm list to scan
 * @mm_slot: the current mm_slot we are scanning
 * @address: the next address inside that to be scanned
 *
 * There is only the one khugepaged_scan instance of this cursor structure.
 */
struct khugepaged_scan {
        struct list_head mm_head;
        struct mm_slot *mm_slot;
        unsigned long address;
};
static struct khugepaged_scan khugepaged_scan = {
        .mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
};

static DEFINE_SPINLOCK(split_queue_lock);
static LIST_HEAD(split_queue);
static unsigned long split_queue_len;
static struct shrinker deferred_split_shrinker;

static void set_recommended_min_free_kbytes(void)
{
        struct zone *zone;
        int nr_zones = 0;
        unsigned long recommended_min;

        for_each_populated_zone(zone)
                nr_zones++;

        /* Ensure 2 pageblocks are free to assist fragmentation avoidance */
        recommended_min = pageblock_nr_pages * nr_zones * 2;

        /*
         * Make sure that on average at least two pageblocks are almost free
         * of another type, one for a migratetype to fall back to and a
         * second to avoid subsequent fallbacks of other types There are 3
         * MIGRATE_TYPES we care about.
         */
        recommended_min += pageblock_nr_pages * nr_zones *
                           MIGRATE_PCPTYPES * MIGRATE_PCPTYPES;

        /* don't ever allow to reserve more than 5% of the lowmem */
        recommended_min = min(recommended_min,
                              (unsigned long) nr_free_buffer_pages() / 20);
        recommended_min <<= (PAGE_SHIFT-10);

        if (recommended_min > min_free_kbytes) {
                if (user_min_free_kbytes >= 0)
                        pr_info("raising min_free_kbytes from %d to %lu "
                                "to help transparent hugepage allocations\n",
                                min_free_kbytes, recommended_min);

                min_free_kbytes = recommended_min;
        }
        setup_per_zone_wmarks();
}

static int start_stop_khugepaged(void)
{
        int err = 0;
        if (khugepaged_enabled()) {
                if (!khugepaged_thread)
                        khugepaged_thread = kthread_run(khugepaged, NULL,
                                                        "khugepaged");
                if (IS_ERR(khugepaged_thread)) {
                        pr_err("khugepaged: kthread_run(khugepaged) failed\n");
                        err = PTR_ERR(khugepaged_thread);
                        khugepaged_thread = NULL;
                        goto fail;
                }

                if (!list_empty(&khugepaged_scan.mm_head))
                        wake_up_interruptible(&khugepaged_wait);

                set_recommended_min_free_kbytes();
        } else if (khugepaged_thread) {
                kthread_stop(khugepaged_thread);
                khugepaged_thread = NULL;
        }
fail:
        return err;
}

static atomic_t huge_zero_refcount;
struct page *huge_zero_page __read_mostly;

struct page *get_huge_zero_page(void)
{
        struct page *zero_page;
retry:
        if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
                return READ_ONCE(huge_zero_page);

        zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
                        HPAGE_PMD_ORDER);
        if (!zero_page) {
                count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED);
                return NULL;
        }
        count_vm_event(THP_ZERO_PAGE_ALLOC);
        preempt_disable();
        if (cmpxchg(&huge_zero_page, NULL, zero_page)) {
                preempt_enable();
                __free_pages(zero_page, compound_order(zero_page));
                goto retry;
        }

        /* We take additional reference here. It will be put back by shrinker */
        atomic_set(&huge_zero_refcount, 2);
        preempt_enable();
        return READ_ONCE(huge_zero_page);
}

static void put_huge_zero_page(void)
{
        /*
         * Counter should never go to zero here. Only shrinker can put
         * last reference.
         */
        BUG_ON(atomic_dec_and_test(&huge_zero_refcount));
}

static unsigned long shrink_huge_zero_page_count(struct shrinker *shrink,
                                        struct shrink_control *sc)
{
        /* we can free zero page only if last reference remains */
        return atomic_read(&huge_zero_refcount) == 1 ? HPAGE_PMD_NR : 0;
}

static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink,
                                       struct shrink_control *sc)
{
        if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
                struct page *zero_page = xchg(&huge_zero_page, NULL);
                BUG_ON(zero_page == NULL);
                __free_pages(zero_page, compound_order(zero_page));
                return HPAGE_PMD_NR;
        }

        return 0;
}

static struct shrinker huge_zero_page_shrinker = {
        .count_objects = shrink_huge_zero_page_count,
        .scan_objects = shrink_huge_zero_page_scan,
        .seeks = DEFAULT_SEEKS,
};

#ifdef CONFIG_SYSFS

static ssize_t double_flag_show(struct kobject *kobj,
                                struct kobj_attribute *attr, char *buf,
                                enum transparent_hugepage_flag enabled,
                                enum transparent_hugepage_flag req_madv)
{
        if (test_bit(enabled, &transparent_hugepage_flags)) {
                VM_BUG_ON(test_bit(req_madv, &transparent_hugepage_flags));
                return sprintf(buf, "[always] madvise never\n");
        } else if (test_bit(req_madv, &transparent_hugepage_flags))
                return sprintf(buf, "always [madvise] never\n");
        else
                return sprintf(buf, "always madvise [never]\n");
}
static ssize_t double_flag_store(struct kobject *kobj,
                                 struct kobj_attribute *attr,
                                 const char *buf, size_t count,
                                 enum transparent_hugepage_flag enabled,
                                 enum transparent_hugepage_flag req_madv)
{
        if (!memcmp("always", buf,
                    min(sizeof("always")-1, count))) {
                set_bit(enabled, &transparent_hugepage_flags);
                clear_bit(req_madv, &transparent_hugepage_flags);
        } else if (!memcmp("madvise", buf,
                           min(sizeof("madvise")-1, count))) {
                clear_bit(enabled, &transparent_hugepage_flags);
                set_bit(req_madv, &transparent_hugepage_flags);
        } else if (!memcmp("never", buf,
                           min(sizeof("never")-1, count))) {
                clear_bit(enabled, &transparent_hugepage_flags);
                clear_bit(req_madv, &transparent_hugepage_flags);
        } else
                return -EINVAL;

        return count;
}

static ssize_t enabled_show(struct kobject *kobj,
                            struct kobj_attribute *attr, char *buf)
{
        return double_flag_show(kobj, attr, buf,
                                TRANSPARENT_HUGEPAGE_FLAG,
                                TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG);
}
static ssize_t enabled_store(struct kobject *kobj,
                             struct kobj_attribute *attr,
                             const char *buf, size_t count)
{
        ssize_t ret;

        ret = double_flag_store(kobj, attr, buf, count,
                                TRANSPARENT_HUGEPAGE_FLAG,
                                TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG);

        if (ret > 0) {
                int err;

                mutex_lock(&khugepaged_mutex);
                err = start_stop_khugepaged();
                mutex_unlock(&khugepaged_mutex);

                if (err)
                        ret = err;
        }

        return ret;
}
static struct kobj_attribute enabled_attr =
        __ATTR(enabled, 0644, enabled_show, enabled_store);

static ssize_t single_flag_show(struct kobject *kobj,
                                struct kobj_attribute *attr, char *buf,
                                enum transparent_hugepage_flag flag)
{
        return sprintf(buf, "%d\n",
                       !!test_bit(flag, &transparent_hugepage_flags));
}

static ssize_t single_flag_store(struct kobject *kobj,
                                 struct kobj_attribute *attr,
                                 const char *buf, size_t count,
                                 enum transparent_hugepage_flag flag)
{
        unsigned long value;
        int ret;

        ret = kstrtoul(buf, 10, &value);
        if (ret < 0)
                return ret;
        if (value > 1)
                return -EINVAL;

        if (value)
                set_bit(flag, &transparent_hugepage_flags);
        else
                clear_bit(flag, &transparent_hugepage_flags);

        return count;
}

/*
 * Currently defrag only disables __GFP_NOWAIT for allocation. A blind
 * __GFP_REPEAT is too aggressive, it's never worth swapping tons of
 * memory just to allocate one more hugepage.
 */
static ssize_t defrag_show(struct kobject *kobj,
                           struct kobj_attribute *attr, char *buf)
{
        return double_flag_show(kobj, attr, buf,
                                TRANSPARENT_HUGEPAGE_DEFRAG_FLAG,
                                TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG);
}
static ssize_t defrag_store(struct kobject *kobj,
                            struct kobj_attribute *attr,
                            const char *buf, size_t count)
{
        return double_flag_store(kobj, attr, buf, count,
                                 TRANSPARENT_HUGEPAGE_DEFRAG_FLAG,
                                 TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG);
}
static struct kobj_attribute defrag_attr =
        __ATTR(defrag, 0644, defrag_show, defrag_store);

static ssize_t use_zero_page_show(struct kobject *kobj,
                struct kobj_attribute *attr, char *buf)
{
        return single_flag_show(kobj, attr, buf,
                                TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);
}
static ssize_t use_zero_page_store(struct kobject *kobj,
                struct kobj_attribute *attr, const char *buf, size_t count)
{
        return single_flag_store(kobj, attr, buf, count,
                                 TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);
}
static struct kobj_attribute use_zero_page_attr =
        __ATTR(use_zero_page, 0644, use_zero_page_show, use_zero_page_store);
#ifdef CONFIG_DEBUG_VM
static ssize_t debug_cow_show(struct kobject *kobj,
                                struct kobj_attribute *attr, char *buf)
{
        return single_flag_show(kobj, attr, buf,
                                TRANSPARENT_HUGEPAGE_DEBUG_COW_FLAG);
}
static ssize_t debug_cow_store(struct kobject *kobj,
                               struct kobj_attribute *attr,
                               const char *buf, size_t count)
{
        return single_flag_store(kobj, attr, buf, count,
                                 TRANSPARENT_HUGEPAGE_DEBUG_COW_FLAG);
}
static struct kobj_attribute debug_cow_attr =
        __ATTR(debug_cow, 0644, debug_cow_show, debug_cow_store);
#endif /* CONFIG_DEBUG_VM */

static struct attribute *hugepage_attr[] = {
        &enabled_attr.attr,
        &defrag_attr.attr,
        &use_zero_page_attr.attr,
#ifdef CONFIG_DEBUG_VM
        &debug_cow_attr.attr,
#endif
        NULL,
};

static struct attribute_group hugepage_attr_group = {
        .attrs = hugepage_attr,
};

static ssize_t scan_sleep_millisecs_show(struct kobject *kobj,
                                         struct kobj_attribute *attr,
                                         char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_scan_sleep_millisecs);
}

static ssize_t scan_sleep_millisecs_store(struct kobject *kobj,
                                          struct kobj_attribute *attr,
                                          const char *buf, size_t count)
{
        unsigned long msecs;
        int err;

        err = kstrtoul(buf, 10, &msecs);
        if (err || msecs > UINT_MAX)
                return -EINVAL;

        khugepaged_scan_sleep_millisecs = msecs;
        wake_up_interruptible(&khugepaged_wait);

        return count;
}
static struct kobj_attribute scan_sleep_millisecs_attr =
        __ATTR(scan_sleep_millisecs, 0644, scan_sleep_millisecs_show,
               scan_sleep_millisecs_store);

static ssize_t alloc_sleep_millisecs_show(struct kobject *kobj,
                                          struct kobj_attribute *attr,
                                          char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_alloc_sleep_millisecs);
}

static ssize_t alloc_sleep_millisecs_store(struct kobject *kobj,
                                           struct kobj_attribute *attr,
                                           const char *buf, size_t count)
{
        unsigned long msecs;
        int err;

        err = kstrtoul(buf, 10, &msecs);
        if (err || msecs > UINT_MAX)
                return -EINVAL;

        khugepaged_alloc_sleep_millisecs = msecs;
        wake_up_interruptible(&khugepaged_wait);

        return count;
}
static struct kobj_attribute alloc_sleep_millisecs_attr =
        __ATTR(alloc_sleep_millisecs, 0644, alloc_sleep_millisecs_show,
               alloc_sleep_millisecs_store);

static ssize_t pages_to_scan_show(struct kobject *kobj,
                                  struct kobj_attribute *attr,
                                  char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_pages_to_scan);
}
static ssize_t pages_to_scan_store(struct kobject *kobj,
                                   struct kobj_attribute *attr,
                                   const char *buf, size_t count)
{
        int err;
        unsigned long pages;

        err = kstrtoul(buf, 10, &pages);
        if (err || !pages || pages > UINT_MAX)
                return -EINVAL;

        khugepaged_pages_to_scan = pages;

        return count;
}
static struct kobj_attribute pages_to_scan_attr =
        __ATTR(pages_to_scan, 0644, pages_to_scan_show,
               pages_to_scan_store);

static ssize_t pages_collapsed_show(struct kobject *kobj,
                                    struct kobj_attribute *attr,
                                    char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_pages_collapsed);
}
static struct kobj_attribute pages_collapsed_attr =
        __ATTR_RO(pages_collapsed);

static ssize_t full_scans_show(struct kobject *kobj,
                               struct kobj_attribute *attr,
                               char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_full_scans);
}
static struct kobj_attribute full_scans_attr =
        __ATTR_RO(full_scans);

static ssize_t khugepaged_defrag_show(struct kobject *kobj,
                                      struct kobj_attribute *attr, char *buf)
{
        return single_flag_show(kobj, attr, buf,
                                TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
}
static ssize_t khugepaged_defrag_store(struct kobject *kobj,
                                       struct kobj_attribute *attr,
                                       const char *buf, size_t count)
{
        return single_flag_store(kobj, attr, buf, count,
                                 TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
}
static struct kobj_attribute khugepaged_defrag_attr =
        __ATTR(defrag, 0644, khugepaged_defrag_show,
               khugepaged_defrag_store);

/*
 * max_ptes_none controls if khugepaged should collapse hugepages over
 * any unmapped ptes in turn potentially increasing the memory
 * footprint of the vmas. When max_ptes_none is 0 khugepaged will not
 * reduce the available free memory in the system as it
 * runs. Increasing max_ptes_none will instead potentially reduce the
 * free memory in the system during the khugepaged scan.
 */
static ssize_t khugepaged_max_ptes_none_show(struct kobject *kobj,
                                             struct kobj_attribute *attr,
                                             char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_max_ptes_none);
}
static ssize_t khugepaged_max_ptes_none_store(struct kobject *kobj,
                                              struct kobj_attribute *attr,
                                              const char *buf, size_t count)
{
        int err;
        unsigned long max_ptes_none;

        err = kstrtoul(buf, 10, &max_ptes_none);
        if (err || max_ptes_none > HPAGE_PMD_NR-1)
                return -EINVAL;

        khugepaged_max_ptes_none = max_ptes_none;

        return count;
}
static struct kobj_attribute khugepaged_max_ptes_none_attr =
        __ATTR(max_ptes_none, 0644, khugepaged_max_ptes_none_show,
               khugepaged_max_ptes_none_store);

static struct attribute *khugepaged_attr[] = {
        &khugepaged_defrag_attr.attr,
        &khugepaged_max_ptes_none_attr.attr,
        &pages_to_scan_attr.attr,
        &pages_collapsed_attr.attr,
        &full_scans_attr.attr,
        &scan_sleep_millisecs_attr.attr,
        &alloc_sleep_millisecs_attr.attr,
        NULL,
};

static struct attribute_group khugepaged_attr_group = {
        .attrs = khugepaged_attr,
        .name = "khugepaged",
};

static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
{
        int err;

        *hugepage_kobj = kobject_create_and_add("transparent_hugepage", mm_kobj);
        if (unlikely(!*hugepage_kobj)) {
                pr_err("failed to create transparent hugepage kobject\n");
                return -ENOMEM;
        }

        err = sysfs_create_group(*hugepage_kobj, &hugepage_attr_group);
        if (err) {
                pr_err("failed to register transparent hugepage group\n");
                goto delete_obj;
        }

        err = sysfs_create_group(*hugepage_kobj, &khugepaged_attr_group);
        if (err) {
                pr_err("failed to register transparent hugepage group\n");
                goto remove_hp_group;
        }

        return 0;

remove_hp_group:
        sysfs_remove_group(*hugepage_kobj, &hugepage_attr_group);
delete_obj:
        kobject_put(*hugepage_kobj);
        return err;
}

static void __init hugepage_exit_sysfs(struct kobject *hugepage_kobj)
{
        sysfs_remove_group(hugepage_kobj, &khugepaged_attr_group);
        sysfs_remove_group(hugepage_kobj, &hugepage_attr_group);
        kobject_put(hugepage_kobj);
}
#else
static inline int hugepage_init_sysfs(struct kobject **hugepage_kobj)
{
        return 0;
}

static inline void hugepage_exit_sysfs(struct kobject *hugepage_kobj)
{
}
#endif /* CONFIG_SYSFS */

static int __init hugepage_init(void)
{
        int err;
        struct kobject *hugepage_kobj;

        if (!has_transparent_hugepage()) {
                transparent_hugepage_flags = 0;
                return -EINVAL;
        }

        err = hugepage_init_sysfs(&hugepage_kobj);
        if (err)
                goto err_sysfs;

        err = khugepaged_slab_init();
        if (err)
                goto err_slab;

        err = register_shrinker(&huge_zero_page_shrinker);
        if (err)
                goto err_hzp_shrinker;
        err = register_shrinker(&deferred_split_shrinker);
        if (err)
                goto err_split_shrinker;

        /*
         * By default disable transparent hugepages on smaller systems,
         * where the extra memory used could hurt more than TLB overhead
         * is likely to save.  The admin can still enable it through /sys.
         */
        if (totalram_pages < (512 << (20 - PAGE_SHIFT))) {
                transparent_hugepage_flags = 0;
                return 0;
        }

        err = start_stop_khugepaged();
        if (err)
                goto err_khugepaged;

        return 0;
err_khugepaged:
        unregister_shrinker(&deferred_split_shrinker);
err_split_shrinker:
        unregister_shrinker(&huge_zero_page_shrinker);
err_hzp_shrinker:
        khugepaged_slab_exit();
err_slab:
        hugepage_exit_sysfs(hugepage_kobj);
err_sysfs:
        return err;
}
subsys_initcall(hugepage_init);

static int __init setup_transparent_hugepage(char *str)
{
        int ret = 0;
        if (!str)
                goto out;
        if (!strcmp(str, "always")) {
                set_bit(TRANSPARENT_HUGEPAGE_FLAG,
                        &transparent_hugepage_flags);
                clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
                          &transparent_hugepage_flags);
                ret = 1;
        } else if (!strcmp(str, "madvise")) {
                clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
                          &transparent_hugepage_flags);
                set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
                        &transparent_hugepage_flags);
                ret = 1;
        } else if (!strcmp(str, "never")) {
                clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
                          &transparent_hugepage_flags);
                clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
                          &transparent_hugepage_flags);
                ret = 1;
        }
out:
        if (!ret)
                pr_warn("transparent_hugepage= cannot parse, ignored\n");
        return ret;
}
__setup("transparent_hugepage=", setup_transparent_hugepage);

pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
{
        if (likely(vma->vm_flags & VM_WRITE))
                pmd = pmd_mkwrite(pmd);
        return pmd;
}

static inline pmd_t mk_huge_pmd(struct page *page, pgprot_t prot)
{
        pmd_t entry;
        entry = mk_pmd(page, prot);
        entry = pmd_mkhuge(entry);
        return entry;
}

static inline struct list_head *page_deferred_list(struct page *page)
{
        /*
         * ->lru in the tail pages is occupied by compound_head.
         * Let's use ->mapping + ->index in the second tail page as list_head.
         */
        return (struct list_head *)&page[2].mapping;
}

void prep_transhuge_page(struct page *page)
{
        /*
         * we use page->mapping and page->indexlru in second tail page
         * as list_head: assuming THP order >= 2
         */
        BUILD_BUG_ON(HPAGE_PMD_ORDER < 2);

        INIT_LIST_HEAD(page_deferred_list(page));
        set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
}

static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
                                        struct vm_area_struct *vma,
                                        unsigned long address, pmd_t *pmd,
                                        struct page *page, gfp_t gfp,
                                        unsigned int flags)
{
        struct mem_cgroup *memcg;
        pgtable_t pgtable;
        spinlock_t *ptl;
        unsigned long haddr = address & HPAGE_PMD_MASK;

        VM_BUG_ON_PAGE(!PageCompound(page), page);

        if (mem_cgroup_try_charge(page, mm, gfp, &memcg, true)) {
                put_page(page);
                count_vm_event(THP_FAULT_FALLBACK);
                return VM_FAULT_FALLBACK;
        }

        pgtable = pte_alloc_one(mm, haddr);
        if (unlikely(!pgtable)) {
                mem_cgroup_cancel_charge(page, memcg, true);
                put_page(page);
                return VM_FAULT_OOM;
        }

        clear_huge_page(page, haddr, HPAGE_PMD_NR);
        /*
         * The memory barrier inside __SetPageUptodate makes sure that
         * clear_huge_page writes become visible before the set_pmd_at()
         * write.
         */
        __SetPageUptodate(page);

        ptl = pmd_lock(mm, pmd);
        if (unlikely(!pmd_none(*pmd))) {
                spin_unlock(ptl);
                mem_cgroup_cancel_charge(page, memcg, true);
                put_page(page);
                pte_free(mm, pgtable);
        } else {
                pmd_t entry;

                /* Deliver the page fault to userland */
                if (userfaultfd_missing(vma)) {
                        int ret;

                        spin_unlock(ptl);
                        mem_cgroup_cancel_charge(page, memcg, true);
                        put_page(page);
                        pte_free(mm, pgtable);
                        ret = handle_userfault(vma, address, flags,
                                               VM_UFFD_MISSING);
                        VM_BUG_ON(ret & VM_FAULT_FALLBACK);
                        return ret;
                }

                entry = mk_huge_pmd(page, vma->vm_page_prot);
                entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
                page_add_new_anon_rmap(page, vma, haddr, true);
                mem_cgroup_commit_charge(page, memcg, false, true);
                lru_cache_add_active_or_unevictable(page, vma);
                pgtable_trans_huge_deposit(mm, pmd, pgtable);
                set_pmd_at(mm, haddr, pmd, entry);
                add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
                atomic_long_inc(&mm->nr_ptes);
                spin_unlock(ptl);
                count_vm_event(THP_FAULT_ALLOC);
        }

        return 0;
}

static inline gfp_t alloc_hugepage_gfpmask(int defrag, gfp_t extra_gfp)
{
        return (GFP_TRANSHUGE & ~(defrag ? 0 : __GFP_RECLAIM)) | extra_gfp;
}

/* Caller must hold page table lock. */
static bool set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
                struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
                struct page *zero_page)
{
        pmd_t entry;
        if (!pmd_none(*pmd))
                return false;
        entry = mk_pmd(zero_page, vma->vm_page_prot);
        entry = pmd_mkhuge(entry);
        pgtable_trans_huge_deposit(mm, pmd, pgtable);
        set_pmd_at(mm, haddr, pmd, entry);
        atomic_long_inc(&mm->nr_ptes);
        return true;
}

int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                               unsigned long address, pmd_t *pmd,
                               unsigned int flags)
{
        gfp_t gfp;
        struct page *page;
        unsigned long haddr = address & HPAGE_PMD_MASK;

        if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
                return VM_FAULT_FALLBACK;
        if (unlikely(anon_vma_prepare(vma)))
                return VM_FAULT_OOM;
        if (unlikely(khugepaged_enter(vma, vma->vm_flags)))
                return VM_FAULT_OOM;
        if (!(flags & FAULT_FLAG_WRITE) && !mm_forbids_zeropage(mm) &&
                        transparent_hugepage_use_zero_page()) {
                spinlock_t *ptl;
                pgtable_t pgtable;
                struct page *zero_page;
                bool set;
                int ret;
                pgtable = pte_alloc_one(mm, haddr);
                if (unlikely(!pgtable))
                        return VM_FAULT_OOM;
                zero_page = get_huge_zero_page();
                if (unlikely(!zero_page)) {
                        pte_free(mm, pgtable);
                        count_vm_event(THP_FAULT_FALLBACK);
                        return VM_FAULT_FALLBACK;
                }
                ptl = pmd_lock(mm, pmd);
                ret = 0;
                set = false;
                if (pmd_none(*pmd)) {
                        if (userfaultfd_missing(vma)) {
                                spin_unlock(ptl);
                                ret = handle_userfault(vma, address, flags,
                                                       VM_UFFD_MISSING);
                                VM_BUG_ON(ret & VM_FAULT_FALLBACK);
                        } else {
                                set_huge_zero_page(pgtable, mm, vma,
                                                   haddr, pmd,
                                                   zero_page);
                                spin_unlock(ptl);
                                set = true;
                        }
                } else
                        spin_unlock(ptl);
                if (!set) {
                        pte_free(mm, pgtable);
                        put_huge_zero_page();
                }
                return ret;
        }
        gfp = alloc_hugepage_gfpmask(transparent_hugepage_defrag(vma), 0);
        page = alloc_hugepage_vma(gfp, vma, haddr, HPAGE_PMD_ORDER);
        if (unlikely(!page)) {
                count_vm_event(THP_FAULT_FALLBACK);
                return VM_FAULT_FALLBACK;
        }
        prep_transhuge_page(page);
        return __do_huge_pmd_anonymous_page(mm, vma, address, pmd, page, gfp,
                                            flags);
}

static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
                pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write)
{
        struct mm_struct *mm = vma->vm_mm;
        pmd_t entry;
        spinlock_t *ptl;

        ptl = pmd_lock(mm, pmd);
        entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
        if (pfn_t_devmap(pfn))
                entry = pmd_mkdevmap(entry);
        if (write) {
                entry = pmd_mkyoung(pmd_mkdirty(entry));
                entry = maybe_pmd_mkwrite(entry, vma);
        }
        set_pmd_at(mm, addr, pmd, entry);
        update_mmu_cache_pmd(vma, addr, pmd);
        spin_unlock(ptl);
}

int vmf_insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
                        pmd_t *pmd, pfn_t pfn, bool write)
{
        pgprot_t pgprot = vma->vm_page_prot;
        /*
         * If we had pmd_special, we could avoid all these restrictions,
         * but we need to be consistent with PTEs and architectures that
         * can't support a 'special' bit.
         */
        BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)));
        BUG_ON((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) ==
                                                (VM_PFNMAP|VM_MIXEDMAP));
        BUG_ON((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags));
        BUG_ON(!pfn_t_devmap(pfn));

        if (addr < vma->vm_start || addr >= vma->vm_end)
                return VM_FAULT_SIGBUS;
        if (track_pfn_insert(vma, &pgprot, pfn))
                return VM_FAULT_SIGBUS;
        insert_pfn_pmd(vma, addr, pmd, pfn, pgprot, write);
        return VM_FAULT_NOPAGE;
}

static void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
                pmd_t *pmd)
{
        pmd_t _pmd;

        /*
         * We should set the dirty bit only for FOLL_WRITE but for now
         * the dirty bit in the pmd is meaningless.  And if the dirty
         * bit will become meaningful and we'll only set it with
         * FOLL_WRITE, an atomic set_bit will be required on the pmd to
         * set the young bit, instead of the current set_pmd_at.
         */
        _pmd = pmd_mkyoung(pmd_mkdirty(*pmd));
        if (pmdp_set_access_flags(vma, addr & HPAGE_PMD_MASK,
                                pmd, _pmd,  1))
                update_mmu_cache_pmd(vma, addr, pmd);
}

struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
                pmd_t *pmd, int flags)
{
        unsigned long pfn = pmd_pfn(*pmd);
        struct mm_struct *mm = vma->vm_mm;
        struct dev_pagemap *pgmap;
        struct page *page;

        assert_spin_locked(pmd_lockptr(mm, pmd));

        if (flags & FOLL_WRITE && !pmd_write(*pmd))
                return NULL;

        if (pmd_present(*pmd) && pmd_devmap(*pmd))
                /* pass */;
        else
                return NULL;

        if (flags & FOLL_TOUCH)
                touch_pmd(vma, addr, pmd);

        /*
         * device mapped pages can only be returned if the
         * caller will manage the page reference count.
         */
        if (!(flags & FOLL_GET))
                return ERR_PTR(-EEXIST);

        pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
        pgmap = get_dev_pagemap(pfn, NULL);
        if (!pgmap)
                return ERR_PTR(-EFAULT);
        page = pfn_to_page(pfn);
        get_page(page);
        put_dev_pagemap(pgmap);

        return page;
}

int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                  pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
                  struct vm_area_struct *vma)
{
        spinlock_t *dst_ptl, *src_ptl;
        struct page *src_page;
        pmd_t pmd;
        pgtable_t pgtable;
        int ret;

        ret = -ENOMEM;
        pgtable = pte_alloc_one(dst_mm, addr);
        if (unlikely(!pgtable))
                goto out;

        dst_ptl = pmd_lock(dst_mm, dst_pmd);
        src_ptl = pmd_lockptr(src_mm, src_pmd);
        spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);

        ret = -EAGAIN;
        pmd = *src_pmd;
        if (unlikely(!pmd_trans_huge(pmd) && !pmd_devmap(pmd))) {
                pte_free(dst_mm, pgtable);
                goto out_unlock;
        }
        /*
         * When page table lock is held, the huge zero pmd should not be
         * under splitting since we don't split the page itself, only pmd to
         * a page table.
         */
        if (is_huge_zero_pmd(pmd)) {
                struct page *zero_page;
                /*
                 * get_huge_zero_page() will never allocate a new page here,
                 * since we already have a zero page to copy. It just takes a
                 * reference.
                 */
                zero_page = get_huge_zero_page();
                set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
                                zero_page);
                ret = 0;
                goto out_unlock;
        }

        if (pmd_trans_huge(pmd)) {
                /* thp accounting separate from pmd_devmap accounting */
                src_page = pmd_page(pmd);
                VM_BUG_ON_PAGE(!PageHead(src_page), src_page);
                get_page(src_page);
                page_dup_rmap(src_page, true);
                add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
                atomic_long_inc(&dst_mm->nr_ptes);
                pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
        }

        pmdp_set_wrprotect(src_mm, addr, src_pmd);
        pmd = pmd_mkold(pmd_wrprotect(pmd));
        set_pmd_at(dst_mm, addr, dst_pmd, pmd);

        ret = 0;
out_unlock:
        spin_unlock(src_ptl);
        spin_unlock(dst_ptl);
out:
        return ret;
}

void huge_pmd_set_accessed(struct mm_struct *mm,
                           struct vm_area_struct *vma,
                           unsigned long address,
                           pmd_t *pmd, pmd_t orig_pmd,
                           int dirty)
{
        spinlock_t *ptl;
        pmd_t entry;
        unsigned long haddr;

        ptl = pmd_lock(mm, pmd);
        if (unlikely(!pmd_same(*pmd, orig_pmd)))
                goto unlock;

        entry = pmd_mkyoung(orig_pmd);
        haddr = address & HPAGE_PMD_MASK;
        if (pmdp_set_access_flags(vma, haddr, pmd, entry, dirty))
                update_mmu_cache_pmd(vma, address, pmd);

unlock:
        spin_unlock(ptl);
}

static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
                                        struct vm_area_struct *vma,
                                        unsigned long address,
                                        pmd_t *pmd, pmd_t orig_pmd,
                                        struct page *page,
                                        unsigned long haddr)
{
        struct mem_cgroup *memcg;
        spinlock_t *ptl;
        pgtable_t pgtable;
        pmd_t _pmd;
        int ret = 0, i;
        struct page **pages;
        unsigned long mmun_start;       /* For mmu_notifiers */
        unsigned long mmun_end;         /* For mmu_notifiers */

        pages = kmalloc(sizeof(struct page *) * HPAGE_PMD_NR,
                        GFP_KERNEL);
        if (unlikely(!pages)) {
                ret |= VM_FAULT_OOM;
                goto out;
        }

        for (i = 0; i < HPAGE_PMD_NR; i++) {
                pages[i] = alloc_page_vma_node(GFP_HIGHUSER_MOVABLE |
                                               __GFP_OTHER_NODE,
                                               vma, address, page_to_nid(page));
                if (unlikely(!pages[i] ||
                             mem_cgroup_try_charge(pages[i], mm, GFP_KERNEL,
                                                   &memcg, false))) {
                        if (pages[i])
                                put_page(pages[i]);
                        while (--i >= 0) {
                                memcg = (void *)page_private(pages[i]);
                                set_page_private(pages[i], 0);
                                mem_cgroup_cancel_charge(pages[i], memcg,
                                                false);
                                put_page(pages[i]);
                        }
                        kfree(pages);
                        ret |= VM_FAULT_OOM;
                        goto out;
                }
                set_page_private(pages[i], (unsigned long)memcg);
        }

        for (i = 0; i < HPAGE_PMD_NR; i++) {
                copy_user_highpage(pages[i], page + i,
                                   haddr + PAGE_SIZE * i, vma);
                __SetPageUptodate(pages[i]);
                cond_resched();
        }

        mmun_start = haddr;
        mmun_end   = haddr + HPAGE_PMD_SIZE;
        mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

        ptl = pmd_lock(mm, pmd);
        if (unlikely(!pmd_same(*pmd, orig_pmd)))
                goto out_free_pages;
        VM_BUG_ON_PAGE(!PageHead(page), page);

        pmdp_huge_clear_flush_notify(vma, haddr, pmd);
        /* leave pmd empty until pte is filled */

        pgtable = pgtable_trans_huge_withdraw(mm, pmd);
        pmd_populate(mm, &_pmd, pgtable);

        for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
                pte_t *pte, entry;
                entry = mk_pte(pages[i], vma->vm_page_prot);
                entry = maybe_mkwrite(pte_mkdirty(entry), vma);
                memcg = (void *)page_private(pages[i]);
                set_page_private(pages[i], 0);
                page_add_new_anon_rmap(pages[i], vma, haddr, false);
                mem_cgroup_commit_charge(pages[i], memcg, false, false);
                lru_cache_add_active_or_unevictable(pages[i], vma);
                pte = pte_offset_map(&_pmd, haddr);
                VM_BUG_ON(!pte_none(*pte));
                set_pte_at(mm, haddr, pte, entry);
                pte_unmap(pte);
        }
        kfree(pages);

        smp_wmb(); /* make pte visible before pmd */
        pmd_populate(mm, pmd, pgtable);
        page_remove_rmap(page, true);
        spin_unlock(ptl);

        mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);

        ret |= VM_FAULT_WRITE;
        put_page(page);

out:
        return ret;

out_free_pages:
        spin_unlock(ptl);
        mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
        for (i = 0; i < HPAGE_PMD_NR; i++) {
                memcg = (void *)page_private(pages[i]);
                set_page_private(pages[i], 0);
                mem_cgroup_cancel_charge(pages[i], memcg, false);
                put_page(pages[i]);
        }
        kfree(pages);
        goto out;
}

int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
                        unsigned long address, pmd_t *pmd, pmd_t orig_pmd)
{
        spinlock_t *ptl;
        int ret = 0;
        struct page *page = NULL, *new_page;
        struct mem_cgroup *memcg;
        unsigned long haddr;
        unsigned long mmun_start;       /* For mmu_notifiers */
        unsigned long mmun_end;         /* For mmu_notifiers */
        gfp_t huge_gfp;                 /* for allocation and charge */

        ptl = pmd_lockptr(mm, pmd);
        VM_BUG_ON_VMA(!vma->anon_vma, vma);
        haddr = address & HPAGE_PMD_MASK;
        if (is_huge_zero_pmd(orig_pmd))
                goto alloc;
        spin_lock(ptl);
        if (unlikely(!pmd_same(*pmd, orig_pmd)))
                goto out_unlock;

        page = pmd_page(orig_pmd);
        VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
        /*
         * We can only reuse the page if nobody else maps the huge page or it's
         * part. We can do it by checking page_mapcount() on each sub-page, but
         * it's expensive.
         * The cheaper way is to check page_count() to be equal 1: every
         * mapcount takes page reference reference, so this way we can
         * guarantee, that the PMD is the only mapping.
         * This can give false negative if somebody pinned the page, but that's
         * fine.
         */
        if (page_mapcount(page) == 1 && page_count(page) == 1) {
                pmd_t entry;
                entry = pmd_mkyoung(orig_pmd);
                entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
                if (pmdp_set_access_flags(vma, haddr, pmd, entry,  1))
                        update_mmu_cache_pmd(vma, address, pmd);
                ret |= VM_FAULT_WRITE;
                goto out_unlock;
        }
        get_page(page);
        spin_unlock(ptl);
alloc:
        if (transparent_hugepage_enabled(vma) &&
            !transparent_hugepage_debug_cow()) {
                huge_gfp = alloc_hugepage_gfpmask(transparent_hugepage_defrag(vma), 0);
                new_page = alloc_hugepage_vma(huge_gfp, vma, haddr, HPAGE_PMD_ORDER);
        } else
                new_page = NULL;

        if (likely(new_page)) {
                prep_transhuge_page(new_page);
        } else {
                if (!page) {
                        split_huge_pmd(vma, pmd, address);
                        ret |= VM_FAULT_FALLBACK;
                } else {
                        ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
                                        pmd, orig_pmd, page, haddr);
                        if (ret & VM_FAULT_OOM) {
                                split_huge_pmd(vma, pmd, address);
                                ret |= VM_FAULT_FALLBACK;
                        }
                        put_page(page);
                }
                count_vm_event(THP_FAULT_FALLBACK);
                goto out;
        }

        if (unlikely(mem_cgroup_try_charge(new_page, mm, huge_gfp, &memcg,
                                           true))) {
                put_page(new_page);
                if (page) {
                        split_huge_pmd(vma, pmd, address);
                        put_page(page);
                } else
                        split_huge_pmd(vma, pmd, address);
                ret |= VM_FAULT_FALLBACK;
                count_vm_event(THP_FAULT_FALLBACK);
                goto out;
        }

        count_vm_event(THP_FAULT_ALLOC);

        if (!page)
                clear_huge_page(new_page, haddr, HPAGE_PMD_NR);
        else
                copy_user_huge_page(new_page, page, haddr, vma, HPAGE_PMD_NR);
        __SetPageUptodate(new_page);

        mmun_start = haddr;
        mmun_end   = haddr + HPAGE_PMD_SIZE;
        mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

        spin_lock(ptl);
        if (page)
                put_page(page);
        if (unlikely(!pmd_same(*pmd, orig_pmd))) {
                spin_unlock(ptl);
                mem_cgroup_cancel_charge(new_page, memcg, true);
                put_page(new_page);
                goto out_mn;
        } else {
                pmd_t entry;
                entry = mk_huge_pmd(new_page, vma->vm_page_prot);
                entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
                pmdp_huge_clear_flush_notify(vma, haddr, pmd);
                page_add_new_anon_rmap(new_page, vma, haddr, true);
                mem_cgroup_commit_charge(new_page, memcg, false, true);
                lru_cache_add_active_or_unevictable(new_page, vma);
                set_pmd_at(mm, haddr, pmd, entry);
                update_mmu_cache_pmd(vma, address, pmd);
                if (!page) {
                        add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
                        put_huge_zero_page();
                } else {
                        VM_BUG_ON_PAGE(!PageHead(page), page);
                        page_remove_rmap(page, true);
                        put_page(page);
                }
                ret |= VM_FAULT_WRITE;
        }
        spin_unlock(ptl);
out_mn:
        mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
out:
        return ret;
out_unlock:
        spin_unlock(ptl);
        return ret;
}

struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
                                   unsigned long addr,
                                   pmd_t *pmd,
                                   unsigned int flags)
{
        struct mm_struct *mm = vma->vm_mm;
        struct page *page = NULL;

        assert_spin_locked(pmd_lockptr(mm, pmd));

        if (flags & FOLL_WRITE && !pmd_write(*pmd))
                goto out;

        /* Avoid dumping huge zero page */
        if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
                return ERR_PTR(-EFAULT);

        /* Full NUMA hinting faults to serialise migration in fault paths */
        if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
                goto out;

        page = pmd_page(*pmd);
        VM_BUG_ON_PAGE(!PageHead(page), page);
        if (flags & FOLL_TOUCH)
                touch_pmd(vma, addr, pmd);
        if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
                /*
                 * We don't mlock() pte-mapped THPs. This way we can avoid
                 * leaking mlocked pages into non-VM_LOCKED VMAs.
                 *
                 * In most cases the pmd is the only mapping of the page as we
                 * break COW for the mlock() -- see gup_flags |= FOLL_WRITE for
                 * writable private mappings in populate_vma_page_range().
                 *
                 * The only scenario when we have the page shared here is if we
                 * mlocking read-only mapping shared over fork(). We skip
                 * mlocking such pages.
                 */
                if (compound_mapcount(page) == 1 && !PageDoubleMap(page) &&
                                page->mapping && trylock_page(page)) {
                        lru_add_drain();
                        if (page->mapping)
                                mlock_vma_page(page);
                        unlock_page(page);
                }
        }
        page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
        VM_BUG_ON_PAGE(!PageCompound(page), page);
        if (flags & FOLL_GET)
                get_page(page);

out:
        return page;
}

/* NUMA hinting page fault entry point for trans huge pmds */
int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
                                unsigned long addr, pmd_t pmd, pmd_t *pmdp)
{
        spinlock_t *ptl;
        struct anon_vma *anon_vma = NULL;
        struct page *page;
        unsigned long haddr = addr & HPAGE_PMD_MASK;
        int page_nid = -1, this_nid = numa_node_id();
        int target_nid, last_cpupid = -1;
        bool page_locked;
        bool migrated = false;
        bool was_writable;
        int flags = 0;

        /* A PROT_NONE fault should not end up here */
        BUG_ON(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)));

        ptl = pmd_lock(mm, pmdp);
        if (unlikely(!pmd_same(pmd, *pmdp)))
                goto out_unlock;

        /*
         * If there are potential migrations, wait for completion and retry
         * without disrupting NUMA hinting information. Do not relock and
         * check_same as the page may no longer be mapped.
         */
        if (unlikely(pmd_trans_migrating(*pmdp))) {
                page = pmd_page(*pmdp);
                spin_unlock(ptl);
                wait_on_page_locked(page);
                goto out;
        }

        page = pmd_page(pmd);
        BUG_ON(is_huge_zero_page(page));
        page_nid = page_to_nid(page);
        last_cpupid = page_cpupid_last(page);
        count_vm_numa_event(NUMA_HINT_FAULTS);
        if (page_nid == this_nid) {
                count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);
                flags |= TNF_FAULT_LOCAL;
        }

        /* See similar comment in do_numa_page for explanation */
        if (!(vma->vm_flags & VM_WRITE))
                flags |= TNF_NO_GROUP;

        /*
         * Acquire the page lock to serialise THP migrations but avoid dropping
         * page_table_lock if at all possible
         */
        page_locked = trylock_page(page);
        target_nid = mpol_misplaced(page, vma, haddr);
        if (target_nid == -1) {
                /* If the page was locked, there are no parallel migrations */
                if (page_locked)
                        goto clear_pmdnuma;
        }

        /* Migration could have started since the pmd_trans_migrating check */
        if (!page_locked) {
                spin_unlock(ptl);
                wait_on_page_locked(page);
                page_nid = -1;
                goto out;
        }

        /*
         * Page is misplaced. Page lock serialises migrations. Acquire anon_vma
         * to serialises splits
         */
        get_page(page);
        spin_unlock(ptl);
        anon_vma = page_lock_anon_vma_read(page);

        /* Confirm the PMD did not change while page_table_lock was released */
        spin_lock(ptl);
        if (unlikely(!pmd_same(pmd, *pmdp))) {
                unlock_page(page);
                put_page(page);
                page_nid = -1;
                goto out_unlock;
        }

        /* Bail if we fail to protect against THP splits for any reason */
        if (unlikely(!anon_vma)) {
                put_page(page);
                page_nid = -1;
                goto clear_pmdnuma;
        }

        /*
         * Migrate the THP to the requested node, returns with page unlocked
         * and access rights restored.
         */
        spin_unlock(ptl);
        migrated = migrate_misplaced_transhuge_page(mm, vma,
                                pmdp, pmd, addr, page, target_nid);
        if (migrated) {
                flags |= TNF_MIGRATED;
                page_nid = target_nid;
        } else
                flags |= TNF_MIGRATE_FAIL;

        goto out;
clear_pmdnuma:
        BUG_ON(!PageLocked(page));
        was_writable = pmd_write(pmd);
        pmd = pmd_modify(pmd, vma->vm_page_prot);
        pmd = pmd_mkyoung(pmd);
        if (was_writable)
                pmd = pmd_mkwrite(pmd);
        set_pmd_at(mm, haddr, pmdp, pmd);
        update_mmu_cache_pmd(vma, addr, pmdp);
        unlock_page(page);
out_unlock:
        spin_unlock(ptl);

out:
        if (anon_vma)
                page_unlock_anon_vma_read(anon_vma);

        if (page_nid != -1)
                task_numa_fault(last_cpupid, page_nid, HPAGE_PMD_NR, flags);

        return 0;
}

int madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
                pmd_t *pmd, unsigned long addr, unsigned long next)

{
        spinlock_t *ptl;
        pmd_t orig_pmd;
        struct page *page;
        struct mm_struct *mm = tlb->mm;
        int ret = 0;

        if (!pmd_trans_huge_lock(pmd, vma, &ptl))
                goto out_unlocked;

        orig_pmd = *pmd;
        if (is_huge_zero_pmd(orig_pmd)) {
                ret = 1;
                goto out;
        }

        page = pmd_page(orig_pmd);
        /*
         * If other processes are mapping this page, we couldn't discard
         * the page unless they all do MADV_FREE so let's skip the page.
         */
        if (page_mapcount(page) != 1)
                goto out;

        if (!trylock_page(page))
                goto out;

        /*
         * If user want to discard part-pages of THP, split it so MADV_FREE
         * will deactivate only them.
         */
        if (next - addr != HPAGE_PMD_SIZE) {
                get_page(page);
                spin_unlock(ptl);
                if (split_huge_page(page)) {
                        put_page(page);
                        unlock_page(page);
                        goto out_unlocked;
                }
                put_page(page);
                unlock_page(page);
                ret = 1;
                goto out_unlocked;
        }

        if (PageDirty(page))
                ClearPageDirty(page);
        unlock_page(page);

        if (PageActive(page))
                deactivate_page(page);

        if (pmd_young(orig_pmd) || pmd_dirty(orig_pmd)) {
                orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
                        tlb->fullmm);
                orig_pmd = pmd_mkold(orig_pmd);
                orig_pmd = pmd_mkclean(orig_pmd);

                set_pmd_at(mm, addr, pmd, orig_pmd);
                tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
        }
        ret = 1;
out:
        spin_unlock(ptl);
out_unlocked:
        return ret;
}

int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
                 pmd_t *pmd, unsigned long addr)
{
        pmd_t orig_pmd;
        spinlock_t *ptl;

        if (!__pmd_trans_huge_lock(pmd, vma, &ptl))
                return 0;
        /*
         * For architectures like ppc64 we look at deposited pgtable
         * when calling pmdp_huge_get_and_clear. So do the
         * pgtable_trans_huge_withdraw after finishing pmdp related
         * operations.
         */
        orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
                        tlb->fullmm);
        tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
        if (vma_is_dax(vma)) {
                spin_unlock(ptl);
                if (is_huge_zero_pmd(orig_pmd))
                        put_huge_zero_page();
        } else if (is_huge_zero_pmd(orig_pmd)) {
                pte_free(tlb->mm, pgtable_trans_huge_withdraw(tlb->mm, pmd));
                atomic_long_dec(&tlb->mm->nr_ptes);
                spin_unlock(ptl);
                put_huge_zero_page();
        } else {
                struct page *page = pmd_page(orig_pmd);
                page_remove_rmap(page, true);
                VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
                add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
                VM_BUG_ON_PAGE(!PageHead(page), page);
                pte_free(tlb->mm, pgtable_trans_huge_withdraw(tlb->mm, pmd));
                atomic_long_dec(&tlb->mm->nr_ptes);
                spin_unlock(ptl);
                tlb_remove_page(tlb, page);
        }
        return 1;
}

bool move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
                  unsigned long old_addr,
                  unsigned long new_addr, unsigned long old_end,
                  pmd_t *old_pmd, pmd_t *new_pmd)
{
        spinlock_t *old_ptl, *new_ptl;
        pmd_t pmd;

        struct mm_struct *mm = vma->vm_mm;

        if ((old_addr & ~HPAGE_PMD_MASK) ||
            (new_addr & ~HPAGE_PMD_MASK) ||
            old_end - old_addr < HPAGE_PMD_SIZE ||
            (new_vma->vm_flags & VM_NOHUGEPAGE))
                return false;

        /*
         * The destination pmd shouldn't be established, free_pgtables()
         * should have release it.
         */
        if (WARN_ON(!pmd_none(*new_pmd))) {
                VM_BUG_ON(pmd_trans_huge(*new_pmd));
                return false;
        }

        /*
         * We don't have to worry about the ordering of src and dst
         * ptlocks because exclusive mmap_sem prevents deadlock.
         */
        if (__pmd_trans_huge_lock(old_pmd, vma, &old_ptl)) {
                new_ptl = pmd_lockptr(mm, new_pmd);
                if (new_ptl != old_ptl)
                        spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
                pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd);
                VM_BUG_ON(!pmd_none(*new_pmd));

                if (pmd_move_must_withdraw(new_ptl, old_ptl)) {
                        pgtable_t pgtable;
                        pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
                        pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
                }
                set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
                if (new_ptl != old_ptl)
                        spin_unlock(new_ptl);
                spin_unlock(old_ptl);
                return true;
        }
        return false;
}

/*
 * Returns
 *  - 0 if PMD could not be locked
 *  - 1 if PMD was locked but protections unchange and TLB flush unnecessary
 *  - HPAGE_PMD_NR is protections changed and TLB flush necessary
 */
int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
                unsigned long addr, pgprot_t newprot, int prot_numa)
{
        struct mm_struct *mm = vma->vm_mm;
        spinlock_t *ptl;
        int ret = 0;

        if (__pmd_trans_huge_lock(pmd, vma, &ptl)) {
                pmd_t entry;
                bool preserve_write = prot_numa && pmd_write(*pmd);
                ret = 1;

                /*
                 * Avoid trapping faults against the zero page. The read-only
                 * data is likely to be read-cached on the local CPU and
                 * local/remote hits to the zero page are not interesting.
                 */
                if (prot_numa && is_huge_zero_pmd(*pmd)) {
                        spin_unlock(ptl);
                        return ret;
                }

                if (!prot_numa || !pmd_protnone(*pmd)) {
                        entry = pmdp_huge_get_and_clear_notify(mm, addr, pmd);
                        entry = pmd_modify(entry, newprot);
                        if (preserve_write)
                                entry = pmd_mkwrite(entry);
                        ret = HPAGE_PMD_NR;
                        set_pmd_at(mm, addr, pmd, entry);
                        BUG_ON(!preserve_write && pmd_write(entry));
                }
                spin_unlock(ptl);
        }

        return ret;
}

/*
 * Returns true if a given pmd maps a thp, false otherwise.
 *
 * Note that if it returns true, this routine returns without unlocking page
 * table lock. So callers must unlock it.
 */
bool __pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma,
                spinlock_t **ptl)
{
        *ptl = pmd_lock(vma->vm_mm, pmd);
        if (likely(pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
                return true;
        spin_unlock(*ptl);
        return false;
}

#define VM_NO_THP (VM_SPECIAL | VM_HUGETLB | VM_SHARED | VM_MAYSHARE)

int hugepage_madvise(struct vm_area_struct *vma,
                     unsigned long *vm_flags, int advice)
{
        switch (advice) {
        case MADV_HUGEPAGE:
#ifdef CONFIG_S390
                /*
                 * qemu blindly sets MADV_HUGEPAGE on all allocations, but s390
                 * can't handle this properly after s390_enable_sie, so we simply
                 * ignore the madvise to prevent qemu from causing a SIGSEGV.
                 */
                if (mm_has_pgste(vma->vm_mm))
                        return 0;
#endif
                /*
                 * Be somewhat over-protective like KSM for now!
                 */
                if (*vm_flags & VM_NO_THP)
                        return -EINVAL;
                *vm_flags &= ~VM_NOHUGEPAGE;
                *vm_flags |= VM_HUGEPAGE;
                /*
                 * If the vma become good for khugepaged to scan,
                 * register it here without waiting a page fault that
                 * may not happen any time soon.
                 */
                if (unlikely(khugepaged_enter_vma_merge(vma, *vm_flags)))
                        return -ENOMEM;
                break;
        case MADV_NOHUGEPAGE:
                /*
                 * Be somewhat over-protective like KSM for now!
                 */
                if (*vm_flags & VM_NO_THP)
                        return -EINVAL;
                *vm_flags &= ~VM_HUGEPAGE;
                *vm_flags |= VM_NOHUGEPAGE;
                /*
                 * Setting VM_NOHUGEPAGE will prevent khugepaged from scanning
                 * this vma even if we leave the mm registered in khugepaged if
                 * it got registered before VM_NOHUGEPAGE was set.
                 */
                break;
        }

        return 0;
}

static int __init khugepaged_slab_init(void)
{
        mm_slot_cache = kmem_cache_create("khugepaged_mm_slot",
                                          sizeof(struct mm_slot),
                                          __alignof__(struct mm_slot), 0, NULL);
        if (!mm_slot_cache)
                return -ENOMEM;

        return 0;
}

static void __init khugepaged_slab_exit(void)
{
        kmem_cache_destroy(mm_slot_cache);
}

static inline struct mm_slot *alloc_mm_slot(void)
{
        if (!mm_slot_cache)     /* initialization failed */
                return NULL;
        return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
}

static inline void free_mm_slot(struct mm_slot *mm_slot)
{
        kmem_cache_free(mm_slot_cache, mm_slot);
}

static struct mm_slot *get_mm_slot(struct mm_struct *mm)
{
        struct mm_slot *mm_slot;

        hash_for_each_possible(mm_slots_hash, mm_slot, hash, (unsigned long)mm)
                if (mm == mm_slot->mm)
                        return mm_slot;

        return NULL;
}

static void insert_to_mm_slots_hash(struct mm_struct *mm,
                                    struct mm_slot *mm_slot)
{
        mm_slot->mm = mm;
        hash_add(mm_slots_hash, &mm_slot->hash, (long)mm);
}

static inline int khugepaged_test_exit(struct mm_struct *mm)
{
        return atomic_read(&mm->mm_users) == 0;
}

int __khugepaged_enter(struct mm_struct *mm)
{
        struct mm_slot *mm_slot;
        int wakeup;

        mm_slot = alloc_mm_slot();
        if (!mm_slot)
                return -ENOMEM;

        /* __khugepaged_exit() must not run from under us */
        VM_BUG_ON_MM(khugepaged_test_exit(mm), mm);
        if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
                free_mm_slot(mm_slot);
                return 0;
        }

        spin_lock(&khugepaged_mm_lock);
        insert_to_mm_slots_hash(mm, mm_slot);
        /*
         * Insert just behind the scanning cursor, to let the area settle
         * down a little.
         */
        wakeup = list_empty(&khugepaged_scan.mm_head);
        list_add_tail(&mm_slot->mm_node, &khugepaged_scan.mm_head);
        spin_unlock(&khugepaged_mm_lock);

        atomic_inc(&mm->mm_count);
        if (wakeup)
                wake_up_interruptible(&khugepaged_wait);

        return 0;
}

int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
                               unsigned long vm_flags)
{
        unsigned long hstart, hend;
        if (!vma->anon_vma)
                /*
                 * Not yet faulted in so we will register later in the
                 * page fault if needed.
                 */
                return 0;
        if (vma->vm_ops)
                /* khugepaged not yet working on file or special mappings */
                return 0;
        VM_BUG_ON_VMA(vm_flags & VM_NO_THP, vma);
        hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
        hend = vma->vm_end & HPAGE_PMD_MASK;
        if (hstart < hend)
                return khugepaged_enter(vma, vm_flags);
        return 0;
}

void __khugepaged_exit(struct mm_struct *mm)
{
        struct mm_slot *mm_slot;
        int free = 0;

        spin_lock(&khugepaged_mm_lock);
        mm_slot = get_mm_slot(mm);
        if (mm_slot && khugepaged_scan.mm_slot != mm_slot) {
                hash_del(&mm_slot->hash);
                list_del(&mm_slot->mm_node);
                free = 1;
        }
        spin_unlock(&khugepaged_mm_lock);

        if (free) {
                clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
                free_mm_slot(mm_slot);
                mmdrop(mm);
        } else if (mm_slot) {
                /*
                 * This is required to serialize against
                 * khugepaged_test_exit() (which is guaranteed to run
                 * under mmap sem read mode). Stop here (after we
                 * return all pagetables will be destroyed) until
                 * khugepaged has finished working on the pagetables
                 * under the mmap_sem.
                 */
                down_write(&mm->mmap_sem);
                up_write(&mm->mmap_sem);
        }
}

static void release_pte_page(struct page *page)
{
        /* 0 stands for page_is_file_cache(page) == false */
        dec_zone_page_state(page, NR_ISOLATED_ANON + 0);
        unlock_page(page);
        putback_lru_page(page);
}

static void release_pte_pages(pte_t *pte, pte_t *_pte)
{
        while (--_pte >= pte) {
                pte_t pteval = *_pte;
                if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)))
                        release_pte_page(pte_page(pteval));
        }
}

static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                                        unsigned long address,
                                        pte_t *pte)
{
        struct page *page = NULL;
        pte_t *_pte;
        int none_or_zero = 0, result = 0;
        bool referenced = false, writable = false;

        for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
             _pte++, address += PAGE_SIZE) {
                pte_t pteval = *_pte;
                if (pte_none(pteval) || (pte_present(pteval) &&
                                is_zero_pfn(pte_pfn(pteval)))) {
                        if (!userfaultfd_armed(vma) &&
                            ++none_or_zero <= khugepaged_max_ptes_none) {
                                continue;
                        } else {
                                result = SCAN_EXCEED_NONE_PTE;
                                goto out;
                        }
                }
                if (!pte_present(pteval)) {
                        result = SCAN_PTE_NON_PRESENT;
                        goto out;
                }
                page = vm_normal_page(vma, address, pteval);
                if (unlikely(!page)) {
                        result = SCAN_PAGE_NULL;
                        goto out;
                }

                VM_BUG_ON_PAGE(PageCompound(page), page);
                VM_BUG_ON_PAGE(!PageAnon(page), page);
                VM_BUG_ON_PAGE(!PageSwapBacked(page), page);

                /*
                 * We can do it before isolate_lru_page because the
                 * page can't be freed from under us. NOTE: PG_lock
                 * is needed to serialize against split_huge_page
                 * when invoked from the VM.
                 */
                if (!trylock_page(page)) {
                        result = SCAN_PAGE_LOCK;
                        goto out;
                }

                /*
                 * cannot use mapcount: can't collapse if there's a gup pin.
                 * The page must only be referenced by the scanned process
                 * and page swap cache.
                 */
                if (page_count(page) != 1 + !!PageSwapCache(page)) {
                        unlock_page(page);
                        result = SCAN_PAGE_COUNT;
                        goto out;
                }
                if (pte_write(pteval)) {
                        writable = true;
                } else {
                        if (PageSwapCache(page) && !reuse_swap_page(page)) {
                                unlock_page(page);
                                result = SCAN_SWAP_CACHE_PAGE;
                                goto out;
                        }
                        /*
                         * Page is not in the swap cache. It can be collapsed
                         * into a THP.
                         */
                }

                /*
                 * Isolate the page to avoid collapsing an hugepage
                 * currently in use by the VM.
                 */
                if (isolate_lru_page(page)) {
                        unlock_page(page);
                        result = SCAN_DEL_PAGE_LRU;
                        goto out;
                }
                /* 0 stands for page_is_file_cache(page) == false */
                inc_zone_page_state(page, NR_ISOLATED_ANON + 0);
                VM_BUG_ON_PAGE(!PageLocked(page), page);
                VM_BUG_ON_PAGE(PageLRU(page), page);

                /* If there is no mapped pte young don't collapse the page */
                if (pte_young(pteval) ||
                    page_is_young(page) || PageReferenced(page) ||
                    mmu_notifier_test_young(vma->vm_mm, address))
                        referenced = true;
        }
        if (likely(writable)) {
                if (likely(referenced)) {
                        result = SCAN_SUCCEED;
                        trace_mm_collapse_huge_page_isolate(page_to_pfn(page), none_or_zero,
                                                            referenced, writable, result);
                        return 1;
                }
        } else {
                result = SCAN_PAGE_RO;
        }

out:
        release_pte_pages(pte, _pte);
        trace_mm_collapse_huge_page_isolate(page_to_pfn(page), none_or_zero,
                                            referenced, writable, result);
        return 0;
}

static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
                                      struct vm_area_struct *vma,
                                      unsigned long address,
                                      spinlock_t *ptl)
{
        pte_t *_pte;
        for (_pte = pte; _pte < pte+HPAGE_PMD_NR; _pte++) {
                pte_t pteval = *_pte;
                struct page *src_page;

                if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
                        clear_user_highpage(page, address);
                        add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
                        if (is_zero_pfn(pte_pfn(pteval))) {
                                /*
                                 * ptl mostly unnecessary.
                                 */
                                spin_lock(ptl);
                                /*
                                 * paravirt calls inside pte_clear here are
                                 * superfluous.
                                 */
                                pte_clear(vma->vm_mm, address, _pte);
                                spin_unlock(ptl);
                        }
                } else {
                        src_page = pte_page(pteval);
                        copy_user_highpage(page, src_page, address, vma);
                        VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
                        release_pte_page(src_page);
                        /*
                         * ptl mostly unnecessary, but preempt has to
                         * be disabled to update the per-cpu stats
                         * inside page_remove_rmap().
                         */
                        spin_lock(ptl);
                        /*
                         * paravirt calls inside pte_clear here are
                         * superfluous.
                         */
                        pte_clear(vma->vm_mm, address, _pte);
                        page_remove_rmap(src_page, false);
                        spin_unlock(ptl);
                        free_page_and_swap_cache(src_page);
                }

                address += PAGE_SIZE;
                page++;
        }
}

static void khugepaged_alloc_sleep(void)
{
        DEFINE_WAIT(wait);

        add_wait_queue(&khugepaged_wait, &wait);
        freezable_schedule_timeout_interruptible(
                msecs_to_jiffies(khugepaged_alloc_sleep_millisecs));
        remove_wait_queue(&khugepaged_wait, &wait);
}

static int khugepaged_node_load[MAX_NUMNODES];

static bool khugepaged_scan_abort(int nid)
{
        int i;

        /*
         * If zone_reclaim_mode is disabled, then no extra effort is made to
         * allocate memory locally.
         */
        if (!zone_reclaim_mode)
                return false;

        /* If there is a count for this node already, it must be acceptable */
        if (khugepaged_node_load[nid])
                return false;

        for (i = 0; i < MAX_NUMNODES; i++) {
                if (!khugepaged_node_load[i])
                        continue;
                if (node_distance(nid, i) > RECLAIM_DISTANCE)
                        return true;
        }
        return false;
}

#ifdef CONFIG_NUMA
static int khugepaged_find_target_node(void)
{
        static int last_khugepaged_target_node = NUMA_NO_NODE;
        int nid, target_node = 0, max_value = 0;

        /* find first node with max normal pages hit */
        for (nid = 0; nid < MAX_NUMNODES; nid++)
                if (khugepaged_node_load[nid] > max_value) {
                        max_value = khugepaged_node_load[nid];
                        target_node = nid;
                }

        /* do some balance if several nodes have the same hit record */
        if (target_node <= last_khugepaged_target_node)
                for (nid = last_khugepaged_target_node + 1; nid < MAX_NUMNODES;
                                nid++)
                        if (max_value == khugepaged_node_load[nid]) {
                                target_node = nid;
                                break;
                        }

        last_khugepaged_target_node = target_node;
        return target_node;
}

static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
{
        if (IS_ERR(*hpage)) {
                if (!*wait)
                        return false;

                *wait = false;
                *hpage = NULL;
                khugepaged_alloc_sleep();
        } else if (*hpage) {
                put_page(*hpage);
                *hpage = NULL;
        }

        return true;
}

static struct page *
khugepaged_alloc_page(struct page **hpage, gfp_t gfp, struct mm_struct *mm,
                       unsigned long address, int node)
{
        VM_BUG_ON_PAGE(*hpage, *hpage);

        /*
         * Before allocating the hugepage, release the mmap_sem read lock.
         * The allocation can take potentially a long time if it involves
         * sync compaction, and we do not need to hold the mmap_sem during
         * that. We will recheck the vma after taking it again in write mode.
         */
        up_read(&mm->mmap_sem);

        *hpage = __alloc_pages_node(node, gfp, HPAGE_PMD_ORDER);
        if (unlikely(!*hpage)) {
                count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
                *hpage = ERR_PTR(-ENOMEM);
                return NULL;
        }

        prep_transhuge_page(*hpage);
        count_vm_event(THP_COLLAPSE_ALLOC);
        return *hpage;
}
#else
static int khugepaged_find_target_node(void)
{
        return 0;
}

static inline struct page *alloc_hugepage(int defrag)
{
        struct page *page;

        page = alloc_pages(alloc_hugepage_gfpmask(defrag, 0), HPAGE_PMD_ORDER);
        if (page)
                prep_transhuge_page(page);
        return page;
}

static struct page *khugepaged_alloc_hugepage(bool *wait)
{
        struct page *hpage;

        do {
                hpage = alloc_hugepage(khugepaged_defrag());
                if (!hpage) {
                        count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
                        if (!*wait)
                                return NULL;

                        *wait = false;
                        khugepaged_alloc_sleep();
                } else
                        count_vm_event(THP_COLLAPSE_ALLOC);
        } while (unlikely(!hpage) && likely(khugepaged_enabled()));

        return hpage;
}

static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
{
        if (!*hpage)
                *hpage = khugepaged_alloc_hugepage(wait);

        if (unlikely(!*hpage))
                return false;

        return true;
}

static struct page *
khugepaged_alloc_page(struct page **hpage, gfp_t gfp, struct mm_struct *mm,
                       unsigned long address, int node)
{
        up_read(&mm->mmap_sem);
        VM_BUG_ON(!*hpage);

        return  *hpage;
}
#endif

static bool hugepage_vma_check(struct vm_area_struct *vma)
{
        if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
            (vma->vm_flags & VM_NOHUGEPAGE))
                return false;
        if (!vma->anon_vma || vma->vm_ops)
                return false;
        if (is_vma_temporary_stack(vma))
                return false;
        VM_BUG_ON_VMA(vma->vm_flags & VM_NO_THP, vma);
        return true;
}

static void collapse_huge_page(struct mm_struct *mm,
                                   unsigned long address,
                                   struct page **hpage,
                                   struct vm_area_struct *vma,
                                   int node)
{
        pmd_t *pmd, _pmd;
        pte_t *pte;
        pgtable_t pgtable;
        struct page *new_page;
        spinlock_t *pmd_ptl, *pte_ptl;
        int isolated, result = 0;
        unsigned long hstart, hend;
        struct mem_cgroup *memcg;
        unsigned long mmun_start;       /* For mmu_notifiers */
        unsigned long mmun_end;         /* For mmu_notifiers */
        gfp_t gfp;

        VM_BUG_ON(address & ~HPAGE_PMD_MASK);

        /* Only allocate from the target node */
        gfp = alloc_hugepage_gfpmask(khugepaged_defrag(), __GFP_OTHER_NODE) |
                __GFP_THISNODE;

        /* release the mmap_sem read lock. */
        new_page = khugepaged_alloc_page(hpage, gfp, mm, address, node);
        if (!new_page) {
                result = SCAN_ALLOC_HUGE_PAGE_FAIL;
                goto out_nolock;
        }

        if (unlikely(mem_cgroup_try_charge(new_page, mm, gfp, &memcg, true))) {
                result = SCAN_CGROUP_CHARGE_FAIL;
                goto out_nolock;
        }

        /*
         * Prevent all access to pagetables with the exception of
         * gup_fast later hanlded by the ptep_clear_flush and the VM
         * handled by the anon_vma lock + PG_lock.
         */
        down_write(&mm->mmap_sem);
        if (unlikely(khugepaged_test_exit(mm))) {
                result = SCAN_ANY_PROCESS;
                goto out;
        }

        vma = find_vma(mm, address);
        if (!vma) {
                result = SCAN_VMA_NULL;
                goto out;
        }
        hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
        hend = vma->vm_end & HPAGE_PMD_MASK;
        if (address < hstart || address + HPAGE_PMD_SIZE > hend) {
                result = SCAN_ADDRESS_RANGE;
                goto out;
        }
        if (!hugepage_vma_check(vma)) {
                result = SCAN_VMA_CHECK;
                goto out;
        }
        pmd = mm_find_pmd(mm, address);
        if (!pmd) {
                result = SCAN_PMD_NULL;
                goto out;
        }

        anon_vma_lock_write(vma->anon_vma);

        pte = pte_offset_map(pmd, address);
        pte_ptl = pte_lockptr(mm, pmd);

        mmun_start = address;
        mmun_end   = address + HPAGE_PMD_SIZE;
        mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
        pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
        /*
         * After this gup_fast can't run anymore. This also removes
         * any huge TLB entry from the CPU so we won't allow
         * huge and small TLB entries for the same virtual address
         * to avoid the risk of CPU bugs in that area.
         */
        _pmd = pmdp_collapse_flush(vma, address, pmd);
        spin_unlock(pmd_ptl);
        mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);

        spin_lock(pte_ptl);
        isolated = __collapse_huge_page_isolate(vma, address, pte);
        spin_unlock(pte_ptl);

        if (unlikely(!isolated)) {
                pte_unmap(pte);
                spin_lock(pmd_ptl);
                BUG_ON(!pmd_none(*pmd));
                /*
                 * We can only use set_pmd_at when establishing
                 * hugepmds and never for establishing regular pmds that
                 * points to regular pagetables. Use pmd_populate for that
                 */
                pmd_populate(mm, pmd, pmd_pgtable(_pmd));
                spin_unlock(pmd_ptl);
                anon_vma_unlock_write(vma->anon_vma);
                result = SCAN_FAIL;
                goto out;
        }

        /*
         * All pages are isolated and locked so anon_vma rmap
         * can't run anymore.
         */
        anon_vma_unlock_write(vma->anon_vma);

        __collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl);
        pte_unmap(pte);
        __SetPageUptodate(new_page);
        pgtable = pmd_pgtable(_pmd);

        _pmd = mk_huge_pmd(new_page, vma->vm_page_prot);
        _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);

        /*
         * spin_lock() below is not the equivalent of smp_wmb(), so
         * this is needed to avoid the copy_huge_page writes to become
         * visible after the set_pmd_at() write.
         */
        smp_wmb();

        spin_lock(pmd_ptl);
        BUG_ON(!pmd_none(*pmd));
        page_add_new_anon_rmap(new_page, vma, address, true);
        mem_cgroup_commit_charge(new_page, memcg, false, true);
        lru_cache_add_active_or_unevictable(new_page, vma);
        pgtable_trans_huge_deposit(mm, pmd, pgtable);
        set_pmd_at(mm, address, pmd, _pmd);
        update_mmu_cache_pmd(vma, address, pmd);
        spin_unlock(pmd_ptl);

        *hpage = NULL;

        khugepaged_pages_collapsed++;
        result = SCAN_SUCCEED;
out_up_write:
        up_write(&mm->mmap_sem);
        trace_mm_collapse_huge_page(mm, isolated, result);
        return;

out_nolock:
        trace_mm_collapse_huge_page(mm, isolated, result);
        return;
out:
        mem_cgroup_cancel_charge(new_page, memcg, true);
        goto out_up_write;
}

static int khugepaged_scan_pmd(struct mm_struct *mm,
                               struct vm_area_struct *vma,
                               unsigned long address,
                               struct page **hpage)
{
        pmd_t *pmd;
        pte_t *pte, *_pte;
        int ret = 0, none_or_zero = 0, result = 0;
        struct page *page = NULL;
        unsigned long _address;
        spinlock_t *ptl;
        int node = NUMA_NO_NODE;
        bool writable = false, referenced = false;

        VM_BUG_ON(address & ~HPAGE_PMD_MASK);

        pmd = mm_find_pmd(mm, address);
        if (!pmd) {
                result = SCAN_PMD_NULL;
                goto out;
        }

        memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
        pte = pte_offset_map_lock(mm, pmd, address, &ptl);
        for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
             _pte++, _address += PAGE_SIZE) {
                pte_t pteval = *_pte;
                if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
                        if (!userfaultfd_armed(vma) &&
                            ++none_or_zero <= khugepaged_max_ptes_none) {
                                continue;
                        } else {
                                result = SCAN_EXCEED_NONE_PTE;
                                goto out_unmap;
                        }
                }
                if (!pte_present(pteval)) {
                        result = SCAN_PTE_NON_PRESENT;
                        goto out_unmap;
                }
                if (pte_write(pteval))
                        writable = true;

                page = vm_normal_page(vma, _address, pteval);
                if (unlikely(!page)) {
                        result = SCAN_PAGE_NULL;
                        goto out_unmap;
                }

                /* TODO: teach khugepaged to collapse THP mapped with pte */
                if (PageCompound(page)) {
                        result = SCAN_PAGE_COMPOUND;
                        goto out_unmap;
                }

                /*
                 * Record which node the original page is from and save this
                 * information to khugepaged_node_load[].
                 * Khupaged will allocate hugepage from the node has the max
                 * hit record.
                 */
                node = page_to_nid(page);
                if (khugepaged_scan_abort(node)) {
                        result = SCAN_SCAN_ABORT;
                        goto out_unmap;
                }
                khugepaged_node_load[node]++;
                if (!PageLRU(page)) {
                        result = SCAN_SCAN_ABORT;
                        goto out_unmap;
                }
                if (PageLocked(page)) {
                        result = SCAN_PAGE_LOCK;
                        goto out_unmap;
                }
                if (!PageAnon(page)) {
                        result = SCAN_PAGE_ANON;
                        goto out_unmap;
                }

                /*
                 * cannot use mapcount: can't collapse if there's a gup pin.
                 * The page must only be referenced by the scanned process
                 * and page swap cache.
                 */
                if (page_count(page) != 1 + !!PageSwapCache(page)) {
                        result = SCAN_PAGE_COUNT;
                        goto out_unmap;
                }
                if (pte_young(pteval) ||
                    page_is_young(page) || PageReferenced(page) ||
                    mmu_notifier_test_young(vma->vm_mm, address))
                        referenced = true;
        }
        if (writable) {
                if (referenced) {
                        result = SCAN_SUCCEED;
                        ret = 1;
                } else {
                        result = SCAN_NO_REFERENCED_PAGE;
                }
        } else {
                result = SCAN_PAGE_RO;
        }
out_unmap:
        pte_unmap_unlock(pte, ptl);
        if (ret) {
                node = khugepaged_find_target_node();
                /* collapse_huge_page will return with the mmap_sem released */
                collapse_huge_page(mm, address, hpage, vma, node);
        }
out:
        trace_mm_khugepaged_scan_pmd(mm, page_to_pfn(page), writable, referenced,
                                     none_or_zero, result);
        return ret;
}

static void collect_mm_slot(struct mm_slot *mm_slot)
{
        struct mm_struct *mm = mm_slot->mm;

        VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));

        if (khugepaged_test_exit(mm)) {
                /* free mm_slot */
                hash_del(&mm_slot->hash);
                list_del(&mm_slot->mm_node);

                /*
                 * Not strictly needed because the mm exited already.
                 *
                 * clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
                 */

                /* khugepaged_mm_lock actually not necessary for the below */
                free_mm_slot(mm_slot);
                mmdrop(mm);
        }
}

static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
                                            struct page **hpage)
        __releases(&khugepaged_mm_lock)
        __acquires(&khugepaged_mm_lock)
{
        struct mm_slot *mm_slot;
        struct mm_struct *mm;
        struct vm_area_struct *vma;
        int progress = 0;

        VM_BUG_ON(!pages);
        VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));

        if (khugepaged_scan.mm_slot)
                mm_slot = khugepaged_scan.mm_slot;
        else {
                mm_slot = list_entry(khugepaged_scan.mm_head.next,
                                     struct mm_slot, mm_node);
                khugepaged_scan.address = 0;
                khugepaged_scan.mm_slot = mm_slot;
        }
        spin_unlock(&khugepaged_mm_lock);

        mm = mm_slot->mm;
        down_read(&mm->mmap_sem);
        if (unlikely(khugepaged_test_exit(mm)))
                vma = NULL;
        else
                vma = find_vma(mm, khugepaged_scan.address);

        progress++;
        for (; vma; vma = vma->vm_next) {
                unsigned long hstart, hend;

                cond_resched();
                if (unlikely(khugepaged_test_exit(mm))) {
                        progress++;
                        break;
                }
                if (!hugepage_vma_check(vma)) {
skip:
                        progress++;
                        continue;
                }
                hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
                hend = vma->vm_end & HPAGE_PMD_MASK;
                if (hstart >= hend)
                        goto skip;
                if (khugepaged_scan.address > hend)
                        goto skip;
                if (khugepaged_scan.address < hstart)
                        khugepaged_scan.address = hstart;
                VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);

                while (khugepaged_scan.address < hend) {
                        int ret;
                        cond_resched();
                        if (unlikely(khugepaged_test_exit(mm)))
                                goto breakouterloop;

                        VM_BUG_ON(khugepaged_scan.address < hstart ||
                                  khugepaged_scan.address + HPAGE_PMD_SIZE >
                                  hend);
                        ret = khugepaged_scan_pmd(mm, vma,
                                                  khugepaged_scan.address,
                                                  hpage);
                        /* move to next address */
                        khugepaged_scan.address += HPAGE_PMD_SIZE;
                        progress += HPAGE_PMD_NR;
                        if (ret)
                                /* we released mmap_sem so break loop */
                                goto breakouterloop_mmap_sem;
                        if (progress >= pages)
                                goto breakouterloop;
                }
        }
breakouterloop:
        up_read(&mm->mmap_sem); /* exit_mmap will destroy ptes after this */
breakouterloop_mmap_sem:

        spin_lock(&khugepaged_mm_lock);
        VM_BUG_ON(khugepaged_scan.mm_slot != mm_slot);
        /*
         * Release the current mm_slot if this mm is about to die, or
         * if we scanned all vmas of this mm.
         */
        if (khugepaged_test_exit(mm) || !vma) {
                /*
                 * Make sure that if mm_users is reaching zero while
                 * khugepaged runs here, khugepaged_exit will find
                 * mm_slot not pointing to the exiting mm.
                 */
                if (mm_slot->mm_node.next != &khugepaged_scan.mm_head) {
                        khugepaged_scan.mm_slot = list_entry(
                                mm_slot->mm_node.next,
                                struct mm_slot, mm_node);
                        khugepaged_scan.address = 0;
                } else {
                        khugepaged_scan.mm_slot = NULL;
                        khugepaged_full_scans++;
                }

                collect_mm_slot(mm_slot);
        }

        return progress;
}

static int khugepaged_has_work(void)
{
        return !list_empty(&khugepaged_scan.mm_head) &&
                khugepaged_enabled();
}

static int khugepaged_wait_event(void)
{
        return !list_empty(&khugepaged_scan.mm_head) ||
                kthread_should_stop();
}

static void khugepaged_do_scan(void)
{
        struct page *hpage = NULL;
        unsigned int progress = 0, pass_through_head = 0;
        unsigned int pages = khugepaged_pages_to_scan;
        bool wait = true;

        barrier(); /* write khugepaged_pages_to_scan to local stack */

        while (progress < pages) {
                if (!khugepaged_prealloc_page(&hpage, &wait))
                        break;

                cond_resched();

                if (unlikely(kthread_should_stop() || try_to_freeze()))
                        break;

                spin_lock(&khugepaged_mm_lock);
                if (!khugepaged_scan.mm_slot)
                        pass_through_head++;
                if (khugepaged_has_work() &&
                    pass_through_head < 2)
                        progress += khugepaged_scan_mm_slot(pages - progress,
                                                            &hpage);
                else
                        progress = pages;
                spin_unlock(&khugepaged_mm_lock);
        }

        if (!IS_ERR_OR_NULL(hpage))
                put_page(hpage);
}

static void khugepaged_wait_work(void)
{
        if (khugepaged_has_work()) {
                if (!khugepaged_scan_sleep_millisecs)
                        return;

                wait_event_freezable_timeout(khugepaged_wait,
                                             kthread_should_stop(),
                        msecs_to_jiffies(khugepaged_scan_sleep_millisecs));
                return;
        }

        if (khugepaged_enabled())
                wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
}

static int khugepaged(void *none)
{
        struct mm_slot *mm_slot;

        set_freezable();
        set_user_nice(current, MAX_NICE);

        while (!kthread_should_stop()) {
                khugepaged_do_scan();
                khugepaged_wait_work();
        }

        spin_lock(&khugepaged_mm_lock);
        mm_slot = khugepaged_scan.mm_slot;
        khugepaged_scan.mm_slot = NULL;
        if (mm_slot)
                collect_mm_slot(mm_slot);
        spin_unlock(&khugepaged_mm_lock);
        return 0;
}

static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
                unsigned long haddr, pmd_t *pmd)
{
        struct mm_struct *mm = vma->vm_mm;
        pgtable_t pgtable;
        pmd_t _pmd;
        int i;

        /* leave pmd empty until pte is filled */
        pmdp_huge_clear_flush_notify(vma, haddr, pmd);

        pgtable = pgtable_trans_huge_withdraw(mm, pmd);
        pmd_populate(mm, &_pmd, pgtable);

        for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
                pte_t *pte, entry;
                entry = pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot);
                entry = pte_mkspecial(entry);
                pte = pte_offset_map(&_pmd, haddr);
                VM_BUG_ON(!pte_none(*pte));
                set_pte_at(mm, haddr, pte, entry);
                pte_unmap(pte);
        }
        smp_wmb(); /* make pte visible before pmd */
        pmd_populate(mm, pmd, pgtable);
        put_huge_zero_page();
}

static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
                unsigned long haddr, bool freeze)
{
        struct mm_struct *mm = vma->vm_mm;
        struct page *page;
        pgtable_t pgtable;
        pmd_t _pmd;
        bool young, write, dirty;
        int i;

        VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
        VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
        VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
        VM_BUG_ON(!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd));

        count_vm_event(THP_SPLIT_PMD);

        if (vma_is_dax(vma)) {
                pmd_t _pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
                if (is_huge_zero_pmd(_pmd))
                        put_huge_zero_page();
                return;
        } else if (is_huge_zero_pmd(*pmd)) {
                return __split_huge_zero_page_pmd(vma, haddr, pmd);
        }

        page = pmd_page(*pmd);
        VM_BUG_ON_PAGE(!page_count(page), page);
        atomic_add(HPAGE_PMD_NR - 1, &page->_count);
        write = pmd_write(*pmd);
        young = pmd_young(*pmd);
        dirty = pmd_dirty(*pmd);

        pgtable = pgtable_trans_huge_withdraw(mm, pmd);
        pmd_populate(mm, &_pmd, pgtable);

        for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
                pte_t entry, *pte;
                /*
                 * Note that NUMA hinting access restrictions are not
                 * transferred to avoid any possibility of altering
                 * permissions across VMAs.
                 */
                if (freeze) {
                        swp_entry_t swp_entry;
                        swp_entry = make_migration_entry(page + i, write);
                        entry = swp_entry_to_pte(swp_entry);
                } else {
                        entry = mk_pte(page + i, vma->vm_page_prot);
                        entry = maybe_mkwrite(entry, vma);
                        if (!write)
                                entry = pte_wrprotect(entry);
                        if (!young)
                                entry = pte_mkold(entry);
                }
                if (dirty)
                        SetPageDirty(page + i);
                pte = pte_offset_map(&_pmd, haddr);
                BUG_ON(!pte_none(*pte));
                set_pte_at(mm, haddr, pte, entry);
                atomic_inc(&page[i]._mapcount);
                pte_unmap(pte);
        }

        /*
         * Set PG_double_map before dropping compound_mapcount to avoid
         * false-negative page_mapped().
         */
        if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {
                for (i = 0; i < HPAGE_PMD_NR; i++)
                        atomic_inc(&page[i]._mapcount);
        }

        if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
                /* Last compound_mapcount is gone. */
                __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
                if (TestClearPageDoubleMap(page)) {
                        /* No need in mapcount reference anymore */
                        for (i = 0; i < HPAGE_PMD_NR; i++)
                                atomic_dec(&page[i]._mapcount);
                }
        }

        smp_wmb(); /* make pte visible before pmd */
        /*
         * Up to this point the pmd is present and huge and userland has the
         * whole access to the hugepage during the split (which happens in
         * place). If we overwrite the pmd with the not-huge version pointing
         * to the pte here (which of course we could if all CPUs were bug
         * free), userland could trigger a small page size TLB miss on the
         * small sized TLB while the hugepage TLB entry is still established in
         * the huge TLB. Some CPU doesn't like that.
         * See http://support.amd.com/us/Processor_TechDocs/41322.pdf, Erratum
         * 383 on page 93. Intel should be safe but is also warns that it's
         * only safe if the permission and cache attributes of the two entries
         * loaded in the two TLB is identical (which should be the case here).
         * But it is generally safer to never allow small and huge TLB entries
         * for the same virtual address to be loaded simultaneously. So instead
         * of doing "pmd_populate(); flush_pmd_tlb_range();" we first mark the
         * current pmd notpresent (atomically because here the pmd_trans_huge
         * and pmd_trans_splitting must remain set at all times on the pmd
         * until the split is complete for this pmd), then we flush the SMP TLB
         * and finally we write the non-huge version of the pmd entry with
         * pmd_populate.
         */
        pmdp_invalidate(vma, haddr, pmd);
        pmd_populate(mm, pmd, pgtable);

        if (freeze) {
                for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
                        page_remove_rmap(page + i, false);
                        put_page(page + i);
                }
        }
}

void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
                unsigned long address)
{
        spinlock_t *ptl;
        struct mm_struct *mm = vma->vm_mm;
        struct page *page = NULL;
        unsigned long haddr = address & HPAGE_PMD_MASK;

        mmu_notifier_invalidate_range_start(mm, haddr, haddr + HPAGE_PMD_SIZE);
        ptl = pmd_lock(mm, pmd);
        if (pmd_trans_huge(*pmd)) {
                page = pmd_page(*pmd);
                if (PageMlocked(page))
                        get_page(page);
                else
                        page = NULL;
        } else if (!pmd_devmap(*pmd))
                goto out;
        __split_huge_pmd_locked(vma, pmd, haddr, false);
out:
        spin_unlock(ptl);
        mmu_notifier_invalidate_range_end(mm, haddr, haddr + HPAGE_PMD_SIZE);
        if (page) {
                lock_page(page);
                munlock_vma_page(page);
                unlock_page(page);
                put_page(page);
        }
}

static void split_huge_pmd_address(struct vm_area_struct *vma,
                                    unsigned long address)
{
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;

        VM_BUG_ON(!(address & ~HPAGE_PMD_MASK));

        pgd = pgd_offset(vma->vm_mm, address);
        if (!pgd_present(*pgd))
                return;

        pud = pud_offset(pgd, address);
        if (!pud_present(*pud))
                return;

        pmd = pmd_offset(pud, address);
        if (!pmd_present(*pmd) || (!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)))
                return;
        /*
         * Caller holds the mmap_sem write mode, so a huge pmd cannot
         * materialize from under us.
         */
        split_huge_pmd(vma, pmd, address);
}

void vma_adjust_trans_huge(struct vm_area_struct *vma,
                             unsigned long start,
                             unsigned long end,
                             long adjust_next)
{
        /*
         * If the new start address isn't hpage aligned and it could
         * previously contain an hugepage: check if we need to split
         * an huge pmd.
         */
        if (start & ~HPAGE_PMD_MASK &&
            (start & HPAGE_PMD_MASK) >= vma->vm_start &&
            (start & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
                split_huge_pmd_address(vma, start);

        /*
         * If the new end address isn't hpage aligned and it could
         * previously contain an hugepage: check if we need to split
         * an huge pmd.
         */
        if (end & ~HPAGE_PMD_MASK &&
            (end & HPAGE_PMD_MASK) >= vma->vm_start &&
            (end & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
                split_huge_pmd_address(vma, end);

        /*
         * If we're also updating the vma->vm_next->vm_start, if the new
         * vm_next->vm_start isn't page aligned and it could previously
         * contain an hugepage: check if we need to split an huge pmd.
         */
        if (adjust_next > 0) {
                struct vm_area_struct *next = vma->vm_next;
                unsigned long nstart = next->vm_start;
                nstart += adjust_next << PAGE_SHIFT;
                if (nstart & ~HPAGE_PMD_MASK &&
                    (nstart & HPAGE_PMD_MASK) >= next->vm_start &&
                    (nstart & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= next->vm_end)
                        split_huge_pmd_address(next, nstart);
        }
}

static void freeze_page_vma(struct vm_area_struct *vma, struct page *page,
                unsigned long address)
{
        unsigned long haddr = address & HPAGE_PMD_MASK;
        spinlock_t *ptl;
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;
        int i, nr = HPAGE_PMD_NR;

        /* Skip pages which doesn't belong to the VMA */
        if (address < vma->vm_start) {
                int off = (vma->vm_start - address) >> PAGE_SHIFT;
                page += off;
                nr -= off;
                address = vma->vm_start;
        }

        pgd = pgd_offset(vma->vm_mm, address);
        if (!pgd_present(*pgd))
                return;
        pud = pud_offset(pgd, address);
        if (!pud_present(*pud))
                return;
        pmd = pmd_offset(pud, address);
        ptl = pmd_lock(vma->vm_mm, pmd);
        if (!pmd_present(*pmd)) {
                spin_unlock(ptl);
                return;
        }
        if (pmd_trans_huge(*pmd)) {
                if (page == pmd_page(*pmd))
                        __split_huge_pmd_locked(vma, pmd, haddr, true);
                spin_unlock(ptl);
                return;
        }
        spin_unlock(ptl);

        pte = pte_offset_map_lock(vma->vm_mm, pmd, address, &ptl);
        for (i = 0; i < nr; i++, address += PAGE_SIZE, page++, pte++) {
                pte_t entry, swp_pte;
                swp_entry_t swp_entry;

                /*
                 * We've just crossed page table boundary: need to map next one.
                 * It can happen if THP was mremaped to non PMD-aligned address.
                 */
                if (unlikely(address == haddr + HPAGE_PMD_SIZE)) {
                        pte_unmap_unlock(pte - 1, ptl);
                        pmd = mm_find_pmd(vma->vm_mm, address);
                        if (!pmd)
                                return;
                        pte = pte_offset_map_lock(vma->vm_mm, pmd,
                                        address, &ptl);
                }

                if (!pte_present(*pte))
                        continue;
                if (page_to_pfn(page) != pte_pfn(*pte))
                        continue;
                flush_cache_page(vma, address, page_to_pfn(page));
                entry = ptep_clear_flush(vma, address, pte);
                if (pte_dirty(entry))
                        SetPageDirty(page);
                swp_entry = make_migration_entry(page, pte_write(entry));
                swp_pte = swp_entry_to_pte(swp_entry);
                if (pte_soft_dirty(entry))
                        swp_pte = pte_swp_mksoft_dirty(swp_pte);
                set_pte_at(vma->vm_mm, address, pte, swp_pte);
                page_remove_rmap(page, false);
                put_page(page);
        }
        pte_unmap_unlock(pte - 1, ptl);
}

static void freeze_page(struct anon_vma *anon_vma, struct page *page)
{
        struct anon_vma_chain *avc;
        pgoff_t pgoff = page_to_pgoff(page);

        VM_BUG_ON_PAGE(!PageHead(page), page);

        anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff,
                        pgoff + HPAGE_PMD_NR - 1) {
                unsigned long address = __vma_address(page, avc->vma);

                mmu_notifier_invalidate_range_start(avc->vma->vm_mm,
                                address, address + HPAGE_PMD_SIZE);
                freeze_page_vma(avc->vma, page, address);
                mmu_notifier_invalidate_range_end(avc->vma->vm_mm,
                                address, address + HPAGE_PMD_SIZE);
        }
}

static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page,
                unsigned long address)
{
        spinlock_t *ptl;
        pmd_t *pmd;
        pte_t *pte, entry;
        swp_entry_t swp_entry;
        unsigned long haddr = address & HPAGE_PMD_MASK;
        int i, nr = HPAGE_PMD_NR;

        /* Skip pages which doesn't belong to the VMA */
        if (address < vma->vm_start) {
                int off = (vma->vm_start - address) >> PAGE_SHIFT;
                page += off;
                nr -= off;
                address = vma->vm_start;
        }

        pmd = mm_find_pmd(vma->vm_mm, address);
        if (!pmd)
                return;

        pte = pte_offset_map_lock(vma->vm_mm, pmd, address, &ptl);
        for (i = 0; i < nr; i++, address += PAGE_SIZE, page++, pte++) {
                /*
                 * We've just crossed page table boundary: need to map next one.
                 * It can happen if THP was mremaped to non-PMD aligned address.
                 */
                if (unlikely(address == haddr + HPAGE_PMD_SIZE)) {
                        pte_unmap_unlock(pte - 1, ptl);
                        pmd = mm_find_pmd(vma->vm_mm, address);
                        if (!pmd)
                                return;
                        pte = pte_offset_map_lock(vma->vm_mm, pmd,
                                        address, &ptl);
                }

                if (!is_swap_pte(*pte))
                        continue;

                swp_entry = pte_to_swp_entry(*pte);
                if (!is_migration_entry(swp_entry))
                        continue;
                if (migration_entry_to_page(swp_entry) != page)
                        continue;

                get_page(page);
                page_add_anon_rmap(page, vma, address, false);

                entry = pte_mkold(mk_pte(page, vma->vm_page_prot));
                if (PageDirty(page))
                        entry = pte_mkdirty(entry);
                if (is_write_migration_entry(swp_entry))
                        entry = maybe_mkwrite(entry, vma);

                flush_dcache_page(page);
                set_pte_at(vma->vm_mm, address, pte, entry);

                /* No need to invalidate - it was non-present before */
                update_mmu_cache(vma, address, pte);
        }
        pte_unmap_unlock(pte - 1, ptl);
}

static void unfreeze_page(struct anon_vma *anon_vma, struct page *page)
{
        struct anon_vma_chain *avc;
        pgoff_t pgoff = page_to_pgoff(page);

        anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root,
                        pgoff, pgoff + HPAGE_PMD_NR - 1) {
                unsigned long address = __vma_address(page, avc->vma);

                mmu_notifier_invalidate_range_start(avc->vma->vm_mm,
                                address, address + HPAGE_PMD_SIZE);
                unfreeze_page_vma(avc->vma, page, address);
                mmu_notifier_invalidate_range_end(avc->vma->vm_mm,
                                address, address + HPAGE_PMD_SIZE);
        }
}

static int __split_huge_page_tail(struct page *head, int tail,
                struct lruvec *lruvec, struct list_head *list)
{
        int mapcount;
        struct page *page_tail = head + tail;

        mapcount = atomic_read(&page_tail->_mapcount) + 1;
        VM_BUG_ON_PAGE(atomic_read(&page_tail->_count) != 0, page_tail);

        /*
         * tail_page->_count is zero and not changing from under us. But
         * get_page_unless_zero() may be running from under us on the
         * tail_page. If we used atomic_set() below instead of atomic_add(), we
         * would then run atomic_set() concurrently with
         * get_page_unless_zero(), and atomic_set() is implemented in C not
         * using locked ops. spin_unlock on x86 sometime uses locked ops
         * because of PPro errata 66, 92, so unless somebody can guarantee
         * atomic_set() here would be safe on all archs (and not only on x86),
         * it's safer to use atomic_add().
         */
        atomic_add(mapcount + 1, &page_tail->_count);


        page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
        page_tail->flags |= (head->flags &
                        ((1L << PG_referenced) |
                         (1L << PG_swapbacked) |
                         (1L << PG_mlocked) |
                         (1L << PG_uptodate) |
                         (1L << PG_active) |
                         (1L << PG_locked) |
                         (1L << PG_unevictable) |
                         (1L << PG_dirty)));

        /*
         * After clearing PageTail the gup refcount can be released.
         * Page flags also must be visible before we make the page non-compound.
         */
        smp_wmb();

        clear_compound_head(page_tail);

        if (page_is_young(head))
                set_page_young(page_tail);
        if (page_is_idle(head))
                set_page_idle(page_tail);

        /* ->mapping in first tail page is compound_mapcount */
        VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
                        page_tail);
        page_tail->mapping = head->mapping;

        page_tail->index = head->index + tail;
        page_cpupid_xchg_last(page_tail, page_cpupid_last(head));
        lru_add_page_tail(head, page_tail, lruvec, list);

        return mapcount;
}

static void __split_huge_page(struct page *page, struct list_head *list)
{
        struct page *head = compound_head(page);
        struct zone *zone = page_zone(head);
        struct lruvec *lruvec;
        int i, tail_mapcount;

        /* prevent PageLRU to go away from under us, and freeze lru stats */
        spin_lock_irq(&zone->lru_lock);
        lruvec = mem_cgroup_page_lruvec(head, zone);

        /* complete memcg works before add pages to LRU */
        mem_cgroup_split_huge_fixup(head);

        tail_mapcount = 0;
        for (i = HPAGE_PMD_NR - 1; i >= 1; i--)
                tail_mapcount += __split_huge_page_tail(head, i, lruvec, list);
        atomic_sub(tail_mapcount, &head->_count);

        ClearPageCompound(head);
        spin_unlock_irq(&zone->lru_lock);

        unfreeze_page(page_anon_vma(head), head);

        for (i = 0; i < HPAGE_PMD_NR; i++) {
                struct page *subpage = head + i;
                if (subpage == page)
                        continue;
                unlock_page(subpage);

                /*
                 * Subpages may be freed if there wasn't any mapping
                 * like if add_to_swap() is running on a lru page that
                 * had its mapping zapped. And freeing these pages
                 * requires taking the lru_lock so we do the put_page
                 * of the tail pages after the split is complete.
                 */
                put_page(subpage);
        }
}

int total_mapcount(struct page *page)
{
        int i, ret;

        VM_BUG_ON_PAGE(PageTail(page), page);

        if (likely(!PageCompound(page)))
                return atomic_read(&page->_mapcount) + 1;

        ret = compound_mapcount(page);
        if (PageHuge(page))
                return ret;
        for (i = 0; i < HPAGE_PMD_NR; i++)
                ret += atomic_read(&page[i]._mapcount) + 1;
        if (PageDoubleMap(page))
                ret -= HPAGE_PMD_NR;
        return ret;
}

/*
 * This function splits huge page into normal pages. @page can point to any
 * subpage of huge page to split. Split doesn't change the position of @page.
 *
 * Only caller must hold pin on the @page, otherwise split fails with -EBUSY.
 * The huge page must be locked.
 *
 * If @list is null, tail pages will be added to LRU list, otherwise, to @list.
 *
 * Both head page and tail pages will inherit mapping, flags, and so on from
 * the hugepage.
 *
 * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if
 * they are not mapped.
 *
 * Returns 0 if the hugepage is split successfully.
 * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
 * us.
 */
int split_huge_page_to_list(struct page *page, struct list_head *list)
{
        struct page *head = compound_head(page);
        struct anon_vma *anon_vma;
        int count, mapcount, ret;
        bool mlocked;

        VM_BUG_ON_PAGE(is_huge_zero_page(page), page);
        VM_BUG_ON_PAGE(!PageAnon(page), page);
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
        VM_BUG_ON_PAGE(!PageCompound(page), page);

        /*
         * The caller does not necessarily hold an mmap_sem that would prevent
         * the anon_vma disappearing so we first we take a reference to it
         * and then lock the anon_vma for write. This is similar to
         * page_lock_anon_vma_read except the write lock is taken to serialise
         * against parallel split or collapse operations.
         */
        anon_vma = page_get_anon_vma(head);
        if (!anon_vma) {
                ret = -EBUSY;
                goto out;
        }
        anon_vma_lock_write(anon_vma);

        /*
         * Racy check if we can split the page, before freeze_page() will
         * split PMDs
         */
        if (total_mapcount(head) != page_count(head) - 1) {
                ret = -EBUSY;
                goto out_unlock;
        }

        mlocked = PageMlocked(page);
        freeze_page(anon_vma, head);
        VM_BUG_ON_PAGE(compound_mapcount(head), head);

        /* Make sure the page is not on per-CPU pagevec as it takes pin */
        if (mlocked)
                lru_add_drain();

        /* Prevent deferred_split_scan() touching ->_count */
        spin_lock(&split_queue_lock);
        count = page_count(head);
        mapcount = total_mapcount(head);
        if (!mapcount && count == 1) {
                if (!list_empty(page_deferred_list(head))) {
                        split_queue_len--;
                        list_del(page_deferred_list(head));
                }
                spin_unlock(&split_queue_lock);
                __split_huge_page(page, list);
                ret = 0;
        } else if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
                spin_unlock(&split_queue_lock);
                pr_alert("total_mapcount: %u, page_count(): %u\n",
                                mapcount, count);
                if (PageTail(page))
                        dump_page(head, NULL);
                dump_page(page, "total_mapcount(head) > 0");
                BUG();
        } else {
                spin_unlock(&split_queue_lock);
                unfreeze_page(anon_vma, head);
                ret = -EBUSY;
        }

out_unlock:
        anon_vma_unlock_write(anon_vma);
        put_anon_vma(anon_vma);
out:
        count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
        return ret;
}

void free_transhuge_page(struct page *page)
{
        unsigned long flags;

        spin_lock_irqsave(&split_queue_lock, flags);
        if (!list_empty(page_deferred_list(page))) {
                split_queue_len--;
                list_del(page_deferred_list(page));
        }
        spin_unlock_irqrestore(&split_queue_lock, flags);
        free_compound_page(page);
}

void deferred_split_huge_page(struct page *page)
{
        unsigned long flags;

        VM_BUG_ON_PAGE(!PageTransHuge(page), page);

        spin_lock_irqsave(&split_queue_lock, flags);
        if (list_empty(page_deferred_list(page))) {
                list_add_tail(page_deferred_list(page), &split_queue);
                split_queue_len++;
        }
        spin_unlock_irqrestore(&split_queue_lock, flags);
}

static unsigned long deferred_split_count(struct shrinker *shrink,
                struct shrink_control *sc)
{
        /*
         * Split a page from split_queue will free up at least one page,
         * at most HPAGE_PMD_NR - 1. We don't track exact number.
         * Let's use HPAGE_PMD_NR / 2 as ballpark.
         */
        return ACCESS_ONCE(split_queue_len) * HPAGE_PMD_NR / 2;
}

static unsigned long deferred_split_scan(struct shrinker *shrink,
                struct shrink_control *sc)
{
        unsigned long flags;
        LIST_HEAD(list), *pos, *next;
        struct page *page;
        int split = 0;

        spin_lock_irqsave(&split_queue_lock, flags);
        list_splice_init(&split_queue, &list);

        /* Take pin on all head pages to avoid freeing them under us */
        list_for_each_safe(pos, next, &list) {
                page = list_entry((void *)pos, struct page, mapping);
                page = compound_head(page);
                /* race with put_compound_page() */
                if (!get_page_unless_zero(page)) {
                        list_del_init(page_deferred_list(page));
                        split_queue_len--;
                }
        }
        spin_unlock_irqrestore(&split_queue_lock, flags);

        list_for_each_safe(pos, next, &list) {
                page = list_entry((void *)pos, struct page, mapping);
                lock_page(page);
                /* split_huge_page() removes page from list on success */
                if (!split_huge_page(page))
                        split++;
                unlock_page(page);
                put_page(page);
        }

        spin_lock_irqsave(&split_queue_lock, flags);
        list_splice_tail(&list, &split_queue);
        spin_unlock_irqrestore(&split_queue_lock, flags);

        return split * HPAGE_PMD_NR / 2;
}

static struct shrinker deferred_split_shrinker = {
        .count_objects = deferred_split_count,
        .scan_objects = deferred_split_scan,
        .seeks = DEFAULT_SEEKS,
};

#ifdef CONFIG_DEBUG_FS
static int split_huge_pages_set(void *data, u64 val)
{
        struct zone *zone;
        struct page *page;
        unsigned long pfn, max_zone_pfn;
        unsigned long total = 0, split = 0;

        if (val != 1)
                return -EINVAL;

        for_each_populated_zone(zone) {
                max_zone_pfn = zone_end_pfn(zone);
                for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++) {
                        if (!pfn_valid(pfn))
                                continue;

                        page = pfn_to_page(pfn);
                        if (!get_page_unless_zero(page))
                                continue;

                        if (zone != page_zone(page))
                                goto next;

                        if (!PageHead(page) || !PageAnon(page) ||
                                        PageHuge(page))
                                goto next;

                        total++;
                        lock_page(page);
                        if (!split_huge_page(page))
                                split++;
                        unlock_page(page);
next:
                        put_page(page);
                }
        }

        pr_info("%lu of %lu THP split", split, total);

        return 0;
}
DEFINE_SIMPLE_ATTRIBUTE(split_huge_pages_fops, NULL, split_huge_pages_set,
                "%llu\n");

static int __init split_huge_pages_debugfs(void)
{
        void *ret;

        ret = debugfs_create_file("split_huge_pages", 0644, NULL, NULL,
                        &split_huge_pages_fops);
        if (!ret)
                pr_warn("Failed to create split_huge_pages in debugfs");
        return 0;
}
late_initcall(split_huge_pages_debugfs);
#endif