mm: add page_check_address_transhuge() helper

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

/*
 * mm/rmap.c - physical to virtual reverse mappings
 *
 * Copyright 2001, Rik van Riel <riel@conectiva.com.br>
 * Released under the General Public License (GPL).
 *
 * Simple, low overhead reverse mapping scheme.
 * Please try to keep this thing as modular as possible.
 *
 * Provides methods for unmapping each kind of mapped page:
 * the anon methods track anonymous pages, and
 * the file methods track pages belonging to an inode.
 *
 * Original design by Rik van Riel <riel@conectiva.com.br> 2001
 * File methods by Dave McCracken <dmccr@us.ibm.com> 2003, 2004
 * Anonymous methods by Andrea Arcangeli <andrea@suse.de> 2004
 * Contributions by Hugh Dickins 2003, 2004
 */

/*
 * Lock ordering in mm:
 *
 * inode->i_mutex       (while writing or truncating, not reading or faulting)
 *   mm->mmap_sem
 *     page->flags PG_locked (lock_page)
 *       mapping->i_mmap_rwsem
 *         anon_vma->rwsem
 *           mm->page_table_lock or pte_lock
 *             zone->lru_lock (in mark_page_accessed, isolate_lru_page)
 *             swap_lock (in swap_duplicate, swap_info_get)
 *               mmlist_lock (in mmput, drain_mmlist and others)
 *               mapping->private_lock (in __set_page_dirty_buffers)
 *                 mem_cgroup_{begin,end}_page_stat (memcg->move_lock)
 *                   mapping->tree_lock (widely used)
 *               inode->i_lock (in set_page_dirty's __mark_inode_dirty)
 *               bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty)
 *                 sb_lock (within inode_lock in fs/fs-writeback.c)
 *                 mapping->tree_lock (widely used, in set_page_dirty,
 *                           in arch-dependent flush_dcache_mmap_lock,
 *                           within bdi.wb->list_lock in __sync_single_inode)
 *
 * anon_vma->rwsem,mapping->i_mutex      (memory_failure, collect_procs_anon)
 *   ->tasklist_lock
 *     pte map lock
 */

#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/rcupdate.h>
#include <linux/export.h>
#include <linux/memcontrol.h>
#include <linux/mmu_notifier.h>
#include <linux/migrate.h>
#include <linux/hugetlb.h>
#include <linux/backing-dev.h>
#include <linux/page_idle.h>

#include <asm/tlbflush.h>

#include <trace/events/tlb.h>

#include "internal.h"

static struct kmem_cache *anon_vma_cachep;
static struct kmem_cache *anon_vma_chain_cachep;

static inline struct anon_vma *anon_vma_alloc(void)
{
        struct anon_vma *anon_vma;

        anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
        if (anon_vma) {
                atomic_set(&anon_vma->refcount, 1);
                anon_vma->degree = 1;   /* Reference for first vma */
                anon_vma->parent = anon_vma;
                /*
                 * Initialise the anon_vma root to point to itself. If called
                 * from fork, the root will be reset to the parents anon_vma.
                 */
                anon_vma->root = anon_vma;
        }

        return anon_vma;
}

static inline void anon_vma_free(struct anon_vma *anon_vma)
{
        VM_BUG_ON(atomic_read(&anon_vma->refcount));

        /*
         * Synchronize against page_lock_anon_vma_read() such that
         * we can safely hold the lock without the anon_vma getting
         * freed.
         *
         * Relies on the full mb implied by the atomic_dec_and_test() from
         * put_anon_vma() against the acquire barrier implied by
         * down_read_trylock() from page_lock_anon_vma_read(). This orders:
         *
         * page_lock_anon_vma_read()    VS      put_anon_vma()
         *   down_read_trylock()                  atomic_dec_and_test()
         *   LOCK                                 MB
         *   atomic_read()                        rwsem_is_locked()
         *
         * LOCK should suffice since the actual taking of the lock must
         * happen _before_ what follows.
         */
        might_sleep();
        if (rwsem_is_locked(&anon_vma->root->rwsem)) {
                anon_vma_lock_write(anon_vma);
                anon_vma_unlock_write(anon_vma);
        }

        kmem_cache_free(anon_vma_cachep, anon_vma);
}

static inline struct anon_vma_chain *anon_vma_chain_alloc(gfp_t gfp)
{
        return kmem_cache_alloc(anon_vma_chain_cachep, gfp);
}

static void anon_vma_chain_free(struct anon_vma_chain *anon_vma_chain)
{
        kmem_cache_free(anon_vma_chain_cachep, anon_vma_chain);
}

static void anon_vma_chain_link(struct vm_area_struct *vma,
                                struct anon_vma_chain *avc,
                                struct anon_vma *anon_vma)
{
        avc->vma = vma;
        avc->anon_vma = anon_vma;
        list_add(&avc->same_vma, &vma->anon_vma_chain);
        anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
}

/**
 * anon_vma_prepare - attach an anon_vma to a memory region
 * @vma: the memory region in question
 *
 * This makes sure the memory mapping described by 'vma' has
 * an 'anon_vma' attached to it, so that we can associate the
 * anonymous pages mapped into it with that anon_vma.
 *
 * The common case will be that we already have one, but if
 * not we either need to find an adjacent mapping that we
 * can re-use the anon_vma from (very common when the only
 * reason for splitting a vma has been mprotect()), or we
 * allocate a new one.
 *
 * Anon-vma allocations are very subtle, because we may have
 * optimistically looked up an anon_vma in page_lock_anon_vma_read()
 * and that may actually touch the spinlock even in the newly
 * allocated vma (it depends on RCU to make sure that the
 * anon_vma isn't actually destroyed).
 *
 * As a result, we need to do proper anon_vma locking even
 * for the new allocation. At the same time, we do not want
 * to do any locking for the common case of already having
 * an anon_vma.
 *
 * This must be called with the mmap_sem held for reading.
 */
int anon_vma_prepare(struct vm_area_struct *vma)
{
        struct anon_vma *anon_vma = vma->anon_vma;
        struct anon_vma_chain *avc;

        might_sleep();
        if (unlikely(!anon_vma)) {
                struct mm_struct *mm = vma->vm_mm;
                struct anon_vma *allocated;

                avc = anon_vma_chain_alloc(GFP_KERNEL);
                if (!avc)
                        goto out_enomem;

                anon_vma = find_mergeable_anon_vma(vma);
                allocated = NULL;
                if (!anon_vma) {
                        anon_vma = anon_vma_alloc();
                        if (unlikely(!anon_vma))
                                goto out_enomem_free_avc;
                        allocated = anon_vma;
                }

                anon_vma_lock_write(anon_vma);
                /* page_table_lock to protect against threads */
                spin_lock(&mm->page_table_lock);
                if (likely(!vma->anon_vma)) {
                        vma->anon_vma = anon_vma;
                        anon_vma_chain_link(vma, avc, anon_vma);
                        /* vma reference or self-parent link for new root */
                        anon_vma->degree++;
                        allocated = NULL;
                        avc = NULL;
                }
                spin_unlock(&mm->page_table_lock);
                anon_vma_unlock_write(anon_vma);

                if (unlikely(allocated))
                        put_anon_vma(allocated);
                if (unlikely(avc))
                        anon_vma_chain_free(avc);
        }
        return 0;

 out_enomem_free_avc:
        anon_vma_chain_free(avc);
 out_enomem:
        return -ENOMEM;
}

/*
 * This is a useful helper function for locking the anon_vma root as
 * we traverse the vma->anon_vma_chain, looping over anon_vma's that
 * have the same vma.
 *
 * Such anon_vma's should have the same root, so you'd expect to see
 * just a single mutex_lock for the whole traversal.
 */
static inline struct anon_vma *lock_anon_vma_root(struct anon_vma *root, struct anon_vma *anon_vma)
{
        struct anon_vma *new_root = anon_vma->root;
        if (new_root != root) {
                if (WARN_ON_ONCE(root))
                        up_write(&root->rwsem);
                root = new_root;
                down_write(&root->rwsem);
        }
        return root;
}

static inline void unlock_anon_vma_root(struct anon_vma *root)
{
        if (root)
                up_write(&root->rwsem);
}

/*
 * Attach the anon_vmas from src to dst.
 * Returns 0 on success, -ENOMEM on failure.
 *
 * If dst->anon_vma is NULL this function tries to find and reuse existing
 * anon_vma which has no vmas and only one child anon_vma. This prevents
 * degradation of anon_vma hierarchy to endless linear chain in case of
 * constantly forking task. On the other hand, an anon_vma with more than one
 * child isn't reused even if there was no alive vma, thus rmap walker has a
 * good chance of avoiding scanning the whole hierarchy when it searches where
 * page is mapped.
 */
int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
{
        struct anon_vma_chain *avc, *pavc;
        struct anon_vma *root = NULL;

        list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
                struct anon_vma *anon_vma;

                avc = anon_vma_chain_alloc(GFP_NOWAIT | __GFP_NOWARN);
                if (unlikely(!avc)) {
                        unlock_anon_vma_root(root);
                        root = NULL;
                        avc = anon_vma_chain_alloc(GFP_KERNEL);
                        if (!avc)
                                goto enomem_failure;
                }
                anon_vma = pavc->anon_vma;
                root = lock_anon_vma_root(root, anon_vma);
                anon_vma_chain_link(dst, avc, anon_vma);

                /*
                 * Reuse existing anon_vma if its degree lower than two,
                 * that means it has no vma and only one anon_vma child.
                 *
                 * Do not chose parent anon_vma, otherwise first child
                 * will always reuse it. Root anon_vma is never reused:
                 * it has self-parent reference and at least one child.
                 */
                if (!dst->anon_vma && anon_vma != src->anon_vma &&
                                anon_vma->degree < 2)
                        dst->anon_vma = anon_vma;
        }
        if (dst->anon_vma)
                dst->anon_vma->degree++;
        unlock_anon_vma_root(root);
        return 0;

 enomem_failure:
        /*
         * dst->anon_vma is dropped here otherwise its degree can be incorrectly
         * decremented in unlink_anon_vmas().
         * We can safely do this because callers of anon_vma_clone() don't care
         * about dst->anon_vma if anon_vma_clone() failed.
         */
        dst->anon_vma = NULL;
        unlink_anon_vmas(dst);
        return -ENOMEM;
}

/*
 * Attach vma to its own anon_vma, as well as to the anon_vmas that
 * the corresponding VMA in the parent process is attached to.
 * Returns 0 on success, non-zero on failure.
 */
int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
{
        struct anon_vma_chain *avc;
        struct anon_vma *anon_vma;
        int error;

        /* Don't bother if the parent process has no anon_vma here. */
        if (!pvma->anon_vma)
                return 0;

        /* Drop inherited anon_vma, we'll reuse existing or allocate new. */
        vma->anon_vma = NULL;

        /*
         * First, attach the new VMA to the parent VMA's anon_vmas,
         * so rmap can find non-COWed pages in child processes.
         */
        error = anon_vma_clone(vma, pvma);
        if (error)
                return error;

        /* An existing anon_vma has been reused, all done then. */
        if (vma->anon_vma)
                return 0;

        /* Then add our own anon_vma. */
        anon_vma = anon_vma_alloc();
        if (!anon_vma)
                goto out_error;
        avc = anon_vma_chain_alloc(GFP_KERNEL);
        if (!avc)
                goto out_error_free_anon_vma;

        /*
         * The root anon_vma's spinlock is the lock actually used when we
         * lock any of the anon_vmas in this anon_vma tree.
         */
        anon_vma->root = pvma->anon_vma->root;
        anon_vma->parent = pvma->anon_vma;
        /*
         * With refcounts, an anon_vma can stay around longer than the
         * process it belongs to. The root anon_vma needs to be pinned until
         * this anon_vma is freed, because the lock lives in the root.
         */
        get_anon_vma(anon_vma->root);
        /* Mark this anon_vma as the one where our new (COWed) pages go. */
        vma->anon_vma = anon_vma;
        anon_vma_lock_write(anon_vma);
        anon_vma_chain_link(vma, avc, anon_vma);
        anon_vma->parent->degree++;
        anon_vma_unlock_write(anon_vma);

        return 0;

 out_error_free_anon_vma:
        put_anon_vma(anon_vma);
 out_error:
        unlink_anon_vmas(vma);
        return -ENOMEM;
}

void unlink_anon_vmas(struct vm_area_struct *vma)
{
        struct anon_vma_chain *avc, *next;
        struct anon_vma *root = NULL;

        /*
         * Unlink each anon_vma chained to the VMA.  This list is ordered
         * from newest to oldest, ensuring the root anon_vma gets freed last.
         */
        list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
                struct anon_vma *anon_vma = avc->anon_vma;

                root = lock_anon_vma_root(root, anon_vma);
                anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);

                /*
                 * Leave empty anon_vmas on the list - we'll need
                 * to free them outside the lock.
                 */
                if (RB_EMPTY_ROOT(&anon_vma->rb_root)) {
                        anon_vma->parent->degree--;
                        continue;
                }

                list_del(&avc->same_vma);
                anon_vma_chain_free(avc);
        }
        if (vma->anon_vma)
                vma->anon_vma->degree--;
        unlock_anon_vma_root(root);

        /*
         * Iterate the list once more, it now only contains empty and unlinked
         * anon_vmas, destroy them. Could not do before due to __put_anon_vma()
         * needing to write-acquire the anon_vma->root->rwsem.
         */
        list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
                struct anon_vma *anon_vma = avc->anon_vma;

                BUG_ON(anon_vma->degree);
                put_anon_vma(anon_vma);

                list_del(&avc->same_vma);
                anon_vma_chain_free(avc);
        }
}

static void anon_vma_ctor(void *data)
{
        struct anon_vma *anon_vma = data;

        init_rwsem(&anon_vma->rwsem);
        atomic_set(&anon_vma->refcount, 0);
        anon_vma->rb_root = RB_ROOT;
}

void __init anon_vma_init(void)
{
        anon_vma_cachep = kmem_cache_create("anon_vma", sizeof(struct anon_vma),
                        0, SLAB_DESTROY_BY_RCU|SLAB_PANIC|SLAB_ACCOUNT,
                        anon_vma_ctor);
        anon_vma_chain_cachep = KMEM_CACHE(anon_vma_chain,
                        SLAB_PANIC|SLAB_ACCOUNT);
}

/*
 * Getting a lock on a stable anon_vma from a page off the LRU is tricky!
 *
 * Since there is no serialization what so ever against page_remove_rmap()
 * the best this function can do is return a locked anon_vma that might
 * have been relevant to this page.
 *
 * The page might have been remapped to a different anon_vma or the anon_vma
 * returned may already be freed (and even reused).
 *
 * In case it was remapped to a different anon_vma, the new anon_vma will be a
 * child of the old anon_vma, and the anon_vma lifetime rules will therefore
 * ensure that any anon_vma obtained from the page will still be valid for as
 * long as we observe page_mapped() [ hence all those page_mapped() tests ].
 *
 * All users of this function must be very careful when walking the anon_vma
 * chain and verify that the page in question is indeed mapped in it
 * [ something equivalent to page_mapped_in_vma() ].
 *
 * Since anon_vma's slab is DESTROY_BY_RCU and we know from page_remove_rmap()
 * that the anon_vma pointer from page->mapping is valid if there is a
 * mapcount, we can dereference the anon_vma after observing those.
 */
struct anon_vma *page_get_anon_vma(struct page *page)
{
        struct anon_vma *anon_vma = NULL;
        unsigned long anon_mapping;

        rcu_read_lock();
        anon_mapping = (unsigned long)READ_ONCE(page->mapping);
        if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
                goto out;
        if (!page_mapped(page))
                goto out;

        anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
        if (!atomic_inc_not_zero(&anon_vma->refcount)) {
                anon_vma = NULL;
                goto out;
        }

        /*
         * If this page is still mapped, then its anon_vma cannot have been
         * freed.  But if it has been unmapped, we have no security against the
         * anon_vma structure being freed and reused (for another anon_vma:
         * SLAB_DESTROY_BY_RCU guarantees that - so the atomic_inc_not_zero()
         * above cannot corrupt).
         */
        if (!page_mapped(page)) {
                rcu_read_unlock();
                put_anon_vma(anon_vma);
                return NULL;
        }
out:
        rcu_read_unlock();

        return anon_vma;
}

/*
 * Similar to page_get_anon_vma() except it locks the anon_vma.
 *
 * Its a little more complex as it tries to keep the fast path to a single
 * atomic op -- the trylock. If we fail the trylock, we fall back to getting a
 * reference like with page_get_anon_vma() and then block on the mutex.
 */
struct anon_vma *page_lock_anon_vma_read(struct page *page)
{
        struct anon_vma *anon_vma = NULL;
        struct anon_vma *root_anon_vma;
        unsigned long anon_mapping;

        rcu_read_lock();
        anon_mapping = (unsigned long)READ_ONCE(page->mapping);
        if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
                goto out;
        if (!page_mapped(page))
                goto out;

        anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
        root_anon_vma = READ_ONCE(anon_vma->root);
        if (down_read_trylock(&root_anon_vma->rwsem)) {
                /*
                 * If the page is still mapped, then this anon_vma is still
                 * its anon_vma, and holding the mutex ensures that it will
                 * not go away, see anon_vma_free().
                 */
                if (!page_mapped(page)) {
                        up_read(&root_anon_vma->rwsem);
                        anon_vma = NULL;
                }
                goto out;
        }

        /* trylock failed, we got to sleep */
        if (!atomic_inc_not_zero(&anon_vma->refcount)) {
                anon_vma = NULL;
                goto out;
        }

        if (!page_mapped(page)) {
                rcu_read_unlock();
                put_anon_vma(anon_vma);
                return NULL;
        }

        /* we pinned the anon_vma, its safe to sleep */
        rcu_read_unlock();
        anon_vma_lock_read(anon_vma);

        if (atomic_dec_and_test(&anon_vma->refcount)) {
                /*
                 * Oops, we held the last refcount, release the lock
                 * and bail -- can't simply use put_anon_vma() because
                 * we'll deadlock on the anon_vma_lock_write() recursion.
                 */
                anon_vma_unlock_read(anon_vma);
                __put_anon_vma(anon_vma);
                anon_vma = NULL;
        }

        return anon_vma;

out:
        rcu_read_unlock();
        return anon_vma;
}

void page_unlock_anon_vma_read(struct anon_vma *anon_vma)
{
        anon_vma_unlock_read(anon_vma);
}

#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
static void percpu_flush_tlb_batch_pages(void *data)
{
        /*
         * All TLB entries are flushed on the assumption that it is
         * cheaper to flush all TLBs and let them be refilled than
         * flushing individual PFNs. Note that we do not track mm's
         * to flush as that might simply be multiple full TLB flushes
         * for no gain.
         */
        count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED);
        flush_tlb_local();
}

/*
 * Flush TLB entries for recently unmapped pages from remote CPUs. It is
 * important if a PTE was dirty when it was unmapped that it's flushed
 * before any IO is initiated on the page to prevent lost writes. Similarly,
 * it must be flushed before freeing to prevent data leakage.
 */
void try_to_unmap_flush(void)
{
        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
        int cpu;

        if (!tlb_ubc->flush_required)
                return;

        cpu = get_cpu();

        trace_tlb_flush(TLB_REMOTE_SHOOTDOWN, -1UL);

        if (cpumask_test_cpu(cpu, &tlb_ubc->cpumask))
                percpu_flush_tlb_batch_pages(&tlb_ubc->cpumask);

        if (cpumask_any_but(&tlb_ubc->cpumask, cpu) < nr_cpu_ids) {
                smp_call_function_many(&tlb_ubc->cpumask,
                        percpu_flush_tlb_batch_pages, (void *)tlb_ubc, true);
        }
        cpumask_clear(&tlb_ubc->cpumask);
        tlb_ubc->flush_required = false;
        tlb_ubc->writable = false;
        put_cpu();
}

/* Flush iff there are potentially writable TLB entries that can race with IO */
void try_to_unmap_flush_dirty(void)
{
        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;

        if (tlb_ubc->writable)
                try_to_unmap_flush();
}

static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
                struct page *page, bool writable)
{
        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;

        cpumask_or(&tlb_ubc->cpumask, &tlb_ubc->cpumask, mm_cpumask(mm));
        tlb_ubc->flush_required = true;

        /*
         * If the PTE was dirty then it's best to assume it's writable. The
         * caller must use try_to_unmap_flush_dirty() or try_to_unmap_flush()
         * before the page is queued for IO.
         */
        if (writable)
                tlb_ubc->writable = true;
}

/*
 * Returns true if the TLB flush should be deferred to the end of a batch of
 * unmap operations to reduce IPIs.
 */
static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
{
        bool should_defer = false;

        if (!(flags & TTU_BATCH_FLUSH))
                return false;

        /* If remote CPUs need to be flushed then defer batch the flush */
        if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
                should_defer = true;
        put_cpu();

        return should_defer;
}
#else
static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
                struct page *page, bool writable)
{
}

static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
{
        return false;
}
#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */

/*
 * At what user virtual address is page expected in vma?
 * Caller should check the page is actually part of the vma.
 */
unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
{
        unsigned long address;
        if (PageAnon(page)) {
                struct anon_vma *page__anon_vma = page_anon_vma(page);
                /*
                 * Note: swapoff's unuse_vma() is more efficient with this
                 * check, and needs it to match anon_vma when KSM is active.
                 */
                if (!vma->anon_vma || !page__anon_vma ||
                    vma->anon_vma->root != page__anon_vma->root)
                        return -EFAULT;
        } else if (page->mapping) {
                if (!vma->vm_file || vma->vm_file->f_mapping != page->mapping)
                        return -EFAULT;
        } else
                return -EFAULT;
        address = __vma_address(page, vma);
        if (unlikely(address < vma->vm_start || address >= vma->vm_end))
                return -EFAULT;
        return address;
}

pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
{
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd = NULL;
        pmd_t pmde;

        pgd = pgd_offset(mm, address);
        if (!pgd_present(*pgd))
                goto out;

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

        pmd = pmd_offset(pud, address);
        /*
         * Some THP functions use the sequence pmdp_huge_clear_flush(), set_pmd_at()
         * without holding anon_vma lock for write.  So when looking for a
         * genuine pmde (in which to find pte), test present and !THP together.
         */
        pmde = *pmd;
        barrier();
        if (!pmd_present(pmde) || pmd_trans_huge(pmde))
                pmd = NULL;
out:
        return pmd;
}

/*
 * Check that @page is mapped at @address into @mm.
 *
 * If @sync is false, page_check_address may perform a racy check to avoid
 * the page table lock when the pte is not present (helpful when reclaiming
 * highly shared pages).
 *
 * On success returns with pte mapped and locked.
 */
pte_t *__page_check_address(struct page *page, struct mm_struct *mm,
                          unsigned long address, spinlock_t **ptlp, int sync)
{
        pmd_t *pmd;
        pte_t *pte;
        spinlock_t *ptl;

        if (unlikely(PageHuge(page))) {
                /* when pud is not present, pte will be NULL */
                pte = huge_pte_offset(mm, address);
                if (!pte)
                        return NULL;

                ptl = huge_pte_lockptr(page_hstate(page), mm, pte);
                goto check;
        }

        pmd = mm_find_pmd(mm, address);
        if (!pmd)
                return NULL;

        pte = pte_offset_map(pmd, address);
        /* Make a quick check before getting the lock */
        if (!sync && !pte_present(*pte)) {
                pte_unmap(pte);
                return NULL;
        }

        ptl = pte_lockptr(mm, pmd);
check:
        spin_lock(ptl);
        if (pte_present(*pte) && page_to_pfn(page) == pte_pfn(*pte)) {
                *ptlp = ptl;
                return pte;
        }
        pte_unmap_unlock(pte, ptl);
        return NULL;
}

/**
 * page_mapped_in_vma - check whether a page is really mapped in a VMA
 * @page: the page to test
 * @vma: the VMA to test
 *
 * Returns 1 if the page is mapped into the page tables of the VMA, 0
 * if the page is not mapped into the page tables of this VMA.  Only
 * valid for normal file or anonymous VMAs.
 */
int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma)
{
        unsigned long address;
        pte_t *pte;
        spinlock_t *ptl;

        address = __vma_address(page, vma);
        if (unlikely(address < vma->vm_start || address >= vma->vm_end))
                return 0;
        pte = page_check_address(page, vma->vm_mm, address, &ptl, 1);
        if (!pte)                       /* the page is not in this mm */
                return 0;
        pte_unmap_unlock(pte, ptl);

        return 1;
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
/*
 * Check that @page is mapped at @address into @mm. In contrast to
 * page_check_address(), this function can handle transparent huge pages.
 *
 * On success returns true with pte mapped and locked. For PMD-mapped
 * transparent huge pages *@ptep is set to NULL.
 */
bool page_check_address_transhuge(struct page *page, struct mm_struct *mm,
                                  unsigned long address, pmd_t **pmdp,
                                  pte_t **ptep, spinlock_t **ptlp)
{
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;
        spinlock_t *ptl;

        if (unlikely(PageHuge(page))) {
                /* when pud is not present, pte will be NULL */
                pte = huge_pte_offset(mm, address);
                if (!pte)
                        return false;

                ptl = huge_pte_lockptr(page_hstate(page), mm, pte);
                pmd = NULL;
                goto check_pte;
        }

        pgd = pgd_offset(mm, address);
        if (!pgd_present(*pgd))
                return false;
        pud = pud_offset(pgd, address);
        if (!pud_present(*pud))
                return false;
        pmd = pmd_offset(pud, address);

        if (pmd_trans_huge(*pmd)) {
                ptl = pmd_lock(mm, pmd);
                if (!pmd_present(*pmd))
                        goto unlock_pmd;
                if (unlikely(!pmd_trans_huge(*pmd))) {
                        spin_unlock(ptl);
                        goto map_pte;
                }

                if (pmd_page(*pmd) != page)
                        goto unlock_pmd;

                pte = NULL;
                goto found;
unlock_pmd:
                spin_unlock(ptl);
                return false;
        } else {
                pmd_t pmde = *pmd;

                barrier();
                if (!pmd_present(pmde) || pmd_trans_huge(pmde))
                        return false;
        }
map_pte:
        pte = pte_offset_map(pmd, address);
        if (!pte_present(*pte)) {
                pte_unmap(pte);
                return false;
        }

        ptl = pte_lockptr(mm, pmd);
check_pte:
        spin_lock(ptl);

        if (!pte_present(*pte)) {
                pte_unmap_unlock(pte, ptl);
                return false;
        }

        /* THP can be referenced by any subpage */
        if (pte_pfn(*pte) - page_to_pfn(page) >= hpage_nr_pages(page)) {
                pte_unmap_unlock(pte, ptl);
                return false;
        }
found:
        *ptep = pte;
        *pmdp = pmd;
        *ptlp = ptl;
        return true;
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

struct page_referenced_arg {
        int mapcount;
        int referenced;
        unsigned long vm_flags;
        struct mem_cgroup *memcg;
};
/*
 * arg: page_referenced_arg will be passed
 */
static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
                        unsigned long address, void *arg)
{
        struct mm_struct *mm = vma->vm_mm;
        struct page_referenced_arg *pra = arg;
        pmd_t *pmd;
        pte_t *pte;
        spinlock_t *ptl;
        int referenced = 0;

        if (!page_check_address_transhuge(page, mm, address, &pmd, &pte, &ptl))
                return SWAP_AGAIN;

        if (vma->vm_flags & VM_LOCKED) {
                if (pte)
                        pte_unmap(pte);
                spin_unlock(ptl);
                pra->vm_flags |= VM_LOCKED;
                return SWAP_FAIL; /* To break the loop */
        }

        if (pte) {
                if (ptep_clear_flush_young_notify(vma, address, pte)) {
                        /*
                         * Don't treat a reference through a sequentially read
                         * mapping as such.  If the page has been used in
                         * another mapping, we will catch it; if this other
                         * mapping is already gone, the unmap path will have
                         * set PG_referenced or activated the page.
                         */
                        if (likely(!(vma->vm_flags & VM_SEQ_READ)))
                                referenced++;
                }
                pte_unmap(pte);
        } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
                if (pmdp_clear_flush_young_notify(vma, address, pmd))
                        referenced++;
        } else {
                /* unexpected pmd-mapped page? */
                WARN_ON_ONCE(1);
        }
        spin_unlock(ptl);

        if (referenced)
                clear_page_idle(page);
        if (test_and_clear_page_young(page))
                referenced++;

        if (referenced) {
                pra->referenced++;
                pra->vm_flags |= vma->vm_flags;
        }

        pra->mapcount--;
        if (!pra->mapcount)
                return SWAP_SUCCESS; /* To break the loop */

        return SWAP_AGAIN;
}

static bool invalid_page_referenced_vma(struct vm_area_struct *vma, void *arg)
{
        struct page_referenced_arg *pra = arg;
        struct mem_cgroup *memcg = pra->memcg;

        if (!mm_match_cgroup(vma->vm_mm, memcg))
                return true;

        return false;
}

/**
 * page_referenced - test if the page was referenced
 * @page: the page to test
 * @is_locked: caller holds lock on the page
 * @memcg: target memory cgroup
 * @vm_flags: collect encountered vma->vm_flags who actually referenced the page
 *
 * Quick test_and_clear_referenced for all mappings to a page,
 * returns the number of ptes which referenced the page.
 */
int page_referenced(struct page *page,
                    int is_locked,
                    struct mem_cgroup *memcg,
                    unsigned long *vm_flags)
{
        int ret;
        int we_locked = 0;
        struct page_referenced_arg pra = {
                .mapcount = total_mapcount(page),
                .memcg = memcg,
        };
        struct rmap_walk_control rwc = {
                .rmap_one = page_referenced_one,
                .arg = (void *)&pra,
                .anon_lock = page_lock_anon_vma_read,
        };

        *vm_flags = 0;
        if (!page_mapped(page))
                return 0;

        if (!page_rmapping(page))
                return 0;

        if (!is_locked && (!PageAnon(page) || PageKsm(page))) {
                we_locked = trylock_page(page);
                if (!we_locked)
                        return 1;
        }

        /*
         * If we are reclaiming on behalf of a cgroup, skip
         * counting on behalf of references from different
         * cgroups
         */
        if (memcg) {
                rwc.invalid_vma = invalid_page_referenced_vma;
        }

        ret = rmap_walk(page, &rwc);
        *vm_flags = pra.vm_flags;

        if (we_locked)
                unlock_page(page);

        return pra.referenced;
}

static int page_mkclean_one(struct page *page, struct vm_area_struct *vma,
                            unsigned long address, void *arg)
{
        struct mm_struct *mm = vma->vm_mm;
        pte_t *pte;
        spinlock_t *ptl;
        int ret = 0;
        int *cleaned = arg;

        pte = page_check_address(page, mm, address, &ptl, 1);
        if (!pte)
                goto out;

        if (pte_dirty(*pte) || pte_write(*pte)) {
                pte_t entry;

                flush_cache_page(vma, address, pte_pfn(*pte));
                entry = ptep_clear_flush(vma, address, pte);
                entry = pte_wrprotect(entry);
                entry = pte_mkclean(entry);
                set_pte_at(mm, address, pte, entry);
                ret = 1;
        }

        pte_unmap_unlock(pte, ptl);

        if (ret) {
                mmu_notifier_invalidate_page(mm, address);
                (*cleaned)++;
        }
out:
        return SWAP_AGAIN;
}

static bool invalid_mkclean_vma(struct vm_area_struct *vma, void *arg)
{
        if (vma->vm_flags & VM_SHARED)
                return false;

        return true;
}

int page_mkclean(struct page *page)
{
        int cleaned = 0;
        struct address_space *mapping;
        struct rmap_walk_control rwc = {
                .arg = (void *)&cleaned,
                .rmap_one = page_mkclean_one,
                .invalid_vma = invalid_mkclean_vma,
        };

        BUG_ON(!PageLocked(page));

        if (!page_mapped(page))
                return 0;

        mapping = page_mapping(page);
        if (!mapping)
                return 0;

        rmap_walk(page, &rwc);

        return cleaned;
}
EXPORT_SYMBOL_GPL(page_mkclean);

/**
 * page_move_anon_rmap - move a page to our anon_vma
 * @page:       the page to move to our anon_vma
 * @vma:        the vma the page belongs to
 * @address:    the user virtual address mapped
 *
 * When a page belongs exclusively to one process after a COW event,
 * that page can be moved into the anon_vma that belongs to just that
 * process, so the rmap code will not search the parent or sibling
 * processes.
 */
void page_move_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_VMA(!anon_vma, vma);
        VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page);

        anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
        /*
         * Ensure that anon_vma and the PAGE_MAPPING_ANON bit are written
         * simultaneously, so a concurrent reader (eg page_referenced()'s
         * PageAnon()) will not see one without the other.
         */
        WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
}

/**
 * __page_set_anon_rmap - set up new anonymous rmap
 * @page:       Page to add to rmap     
 * @vma:        VM area to add page to.
 * @address:    User virtual address of the mapping     
 * @exclusive:  the page is exclusively owned by the current process
 */
static void __page_set_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address, int exclusive)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        BUG_ON(!anon_vma);

        if (PageAnon(page))
                return;

        /*
         * If the page isn't exclusively mapped into this vma,
         * we must use the _oldest_ possible anon_vma for the
         * page mapping!
         */
        if (!exclusive)
                anon_vma = anon_vma->root;

        anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
        page->mapping = (struct address_space *) anon_vma;
        page->index = linear_page_index(vma, address);
}

/**
 * __page_check_anon_rmap - sanity check anonymous rmap addition
 * @page:       the page to add the mapping to
 * @vma:        the vm area in which the mapping is added
 * @address:    the user virtual address mapped
 */
static void __page_check_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address)
{
#ifdef CONFIG_DEBUG_VM
        /*
         * The page's anon-rmap details (mapping and index) are guaranteed to
         * be set up correctly at this point.
         *
         * We have exclusion against page_add_anon_rmap because the caller
         * always holds the page locked, except if called from page_dup_rmap,
         * in which case the page is already known to be setup.
         *
         * We have exclusion against page_add_new_anon_rmap because those pages
         * are initially only visible via the pagetables, and the pte is locked
         * over the call to page_add_new_anon_rmap.
         */
        BUG_ON(page_anon_vma(page)->root != vma->anon_vma->root);
        BUG_ON(page_to_pgoff(page) != linear_page_index(vma, address));
#endif
}

/**
 * page_add_anon_rmap - add pte mapping to an anonymous page
 * @page:       the page to add the mapping to
 * @vma:        the vm area in which the mapping is added
 * @address:    the user virtual address mapped
 * @compound:   charge the page as compound or small page
 *
 * The caller needs to hold the pte lock, and the page must be locked in
 * the anon_vma case: to serialize mapping,index checking after setting,
 * and to ensure that PageAnon is not being upgraded racily to PageKsm
 * (but PageKsm is never downgraded to PageAnon).
 */
void page_add_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address, bool compound)
{
        do_page_add_anon_rmap(page, vma, address, compound ? RMAP_COMPOUND : 0);
}

/*
 * Special version of the above for do_swap_page, which often runs
 * into pages that are exclusively owned by the current process.
 * Everybody else should continue to use page_add_anon_rmap above.
 */
void do_page_add_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address, int flags)
{
        bool compound = flags & RMAP_COMPOUND;
        bool first;

        if (compound) {
                atomic_t *mapcount;
                VM_BUG_ON_PAGE(!PageLocked(page), page);
                VM_BUG_ON_PAGE(!PageTransHuge(page), page);
                mapcount = compound_mapcount_ptr(page);
                first = atomic_inc_and_test(mapcount);
        } else {
                first = atomic_inc_and_test(&page->_mapcount);
        }

        if (first) {
                int nr = compound ? hpage_nr_pages(page) : 1;
                /*
                 * We use the irq-unsafe __{inc|mod}_zone_page_stat because
                 * these counters are not modified in interrupt context, and
                 * pte lock(a spinlock) is held, which implies preemption
                 * disabled.
                 */
                if (compound) {
                        __inc_zone_page_state(page,
                                              NR_ANON_TRANSPARENT_HUGEPAGES);
                }
                __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);
        }
        if (unlikely(PageKsm(page)))
                return;

        VM_BUG_ON_PAGE(!PageLocked(page), page);

        /* address might be in next vma when migration races vma_adjust */
        if (first)
                __page_set_anon_rmap(page, vma, address,
                                flags & RMAP_EXCLUSIVE);
        else
                __page_check_anon_rmap(page, vma, address);
}

/**
 * page_add_new_anon_rmap - add pte mapping to a new anonymous page
 * @page:       the page to add the mapping to
 * @vma:        the vm area in which the mapping is added
 * @address:    the user virtual address mapped
 * @compound:   charge the page as compound or small page
 *
 * Same as page_add_anon_rmap but must only be called on *new* pages.
 * This means the inc-and-test can be bypassed.
 * Page does not have to be locked.
 */
void page_add_new_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address, bool compound)
{
        int nr = compound ? hpage_nr_pages(page) : 1;

        VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
        SetPageSwapBacked(page);
        if (compound) {
                VM_BUG_ON_PAGE(!PageTransHuge(page), page);
                /* increment count (starts at -1) */
                atomic_set(compound_mapcount_ptr(page), 0);
                __inc_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
        } else {
                /* Anon THP always mapped first with PMD */
                VM_BUG_ON_PAGE(PageTransCompound(page), page);
                /* increment count (starts at -1) */
                atomic_set(&page->_mapcount, 0);
        }
        __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);
        __page_set_anon_rmap(page, vma, address, 1);
}

/**
 * page_add_file_rmap - add pte mapping to a file page
 * @page: the page to add the mapping to
 *
 * The caller needs to hold the pte lock.
 */
void page_add_file_rmap(struct page *page)
{
        struct mem_cgroup *memcg;

        memcg = mem_cgroup_begin_page_stat(page);
        if (atomic_inc_and_test(&page->_mapcount)) {
                __inc_zone_page_state(page, NR_FILE_MAPPED);
                mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_FILE_MAPPED);
        }
        mem_cgroup_end_page_stat(memcg);
}

static void page_remove_file_rmap(struct page *page)
{
        struct mem_cgroup *memcg;

        memcg = mem_cgroup_begin_page_stat(page);

        /* Hugepages are not counted in NR_FILE_MAPPED for now. */
        if (unlikely(PageHuge(page))) {
                /* hugetlb pages are always mapped with pmds */
                atomic_dec(compound_mapcount_ptr(page));
                goto out;
        }

        /* page still mapped by someone else? */
        if (!atomic_add_negative(-1, &page->_mapcount))
                goto out;

        /*
         * We use the irq-unsafe __{inc|mod}_zone_page_stat because
         * these counters are not modified in interrupt context, and
         * pte lock(a spinlock) is held, which implies preemption disabled.
         */
        __dec_zone_page_state(page, NR_FILE_MAPPED);
        mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_FILE_MAPPED);

        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);
out:
        mem_cgroup_end_page_stat(memcg);
}

static void page_remove_anon_compound_rmap(struct page *page)
{
        int i, nr;

        if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
                return;

        /* Hugepages are not counted in NR_ANON_PAGES for now. */
        if (unlikely(PageHuge(page)))
                return;

        if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
                return;

        __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);

        if (TestClearPageDoubleMap(page)) {
                /*
                 * Subpages can be mapped with PTEs too. Check how many of
                 * themi are still mapped.
                 */
                for (i = 0, nr = 0; i < HPAGE_PMD_NR; i++) {
                        if (atomic_add_negative(-1, &page[i]._mapcount))
                                nr++;
                }
        } else {
                nr = HPAGE_PMD_NR;
        }

        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);

        if (nr) {
                __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
                deferred_split_huge_page(page);
        }
}

/**
 * page_remove_rmap - take down pte mapping from a page
 * @page:       page to remove mapping from
 * @compound:   uncharge the page as compound or small page
 *
 * The caller needs to hold the pte lock.
 */
void page_remove_rmap(struct page *page, bool compound)
{
        if (!PageAnon(page)) {
                VM_BUG_ON_PAGE(compound && !PageHuge(page), page);
                page_remove_file_rmap(page);
                return;
        }

        if (compound)
                return page_remove_anon_compound_rmap(page);

        /* page still mapped by someone else? */
        if (!atomic_add_negative(-1, &page->_mapcount))
                return;

        /*
         * We use the irq-unsafe __{inc|mod}_zone_page_stat because
         * these counters are not modified in interrupt context, and
         * pte lock(a spinlock) is held, which implies preemption disabled.
         */
        __dec_zone_page_state(page, NR_ANON_PAGES);

        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);

        if (PageTransCompound(page))
                deferred_split_huge_page(compound_head(page));

        /*
         * It would be tidy to reset the PageAnon mapping here,
         * but that might overwrite a racing page_add_anon_rmap
         * which increments mapcount after us but sets mapping
         * before us: so leave the reset to free_hot_cold_page,
         * and remember that it's only reliable while mapped.
         * Leaving it set also helps swapoff to reinstate ptes
         * faster for those pages still in swapcache.
         */
}

/*
 * @arg: enum ttu_flags will be passed to this argument
 */
static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                     unsigned long address, void *arg)
{
        struct mm_struct *mm = vma->vm_mm;
        pte_t *pte;
        pte_t pteval;
        spinlock_t *ptl;
        int ret = SWAP_AGAIN;
        enum ttu_flags flags = (enum ttu_flags)arg;

        /* munlock has nothing to gain from examining un-locked vmas */
        if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
                goto out;

        pte = page_check_address(page, mm, address, &ptl, 0);
        if (!pte)
                goto out;

        /*
         * If the page is mlock()d, we cannot swap it out.
         * If it's recently referenced (perhaps page_referenced
         * skipped over this mm) then we should reactivate it.
         */
        if (!(flags & TTU_IGNORE_MLOCK)) {
                if (vma->vm_flags & VM_LOCKED) {
                        /* Holding pte lock, we do *not* need mmap_sem here */
                        mlock_vma_page(page);
                        ret = SWAP_MLOCK;
                        goto out_unmap;
                }
                if (flags & TTU_MUNLOCK)
                        goto out_unmap;
        }
        if (!(flags & TTU_IGNORE_ACCESS)) {
                if (ptep_clear_flush_young_notify(vma, address, pte)) {
                        ret = SWAP_FAIL;
                        goto out_unmap;
                }
        }

        /* Nuke the page table entry. */
        flush_cache_page(vma, address, page_to_pfn(page));
        if (should_defer_flush(mm, flags)) {
                /*
                 * We clear the PTE but do not flush so potentially a remote
                 * CPU could still be writing to the page. If the entry was
                 * previously clean then the architecture must guarantee that
                 * a clear->dirty transition on a cached TLB entry is written
                 * through and traps if the PTE is unmapped.
                 */
                pteval = ptep_get_and_clear(mm, address, pte);

                set_tlb_ubc_flush_pending(mm, page, pte_dirty(pteval));
        } else {
                pteval = ptep_clear_flush(vma, address, pte);
        }

        /* Move the dirty bit to the physical page now the pte is gone. */
        if (pte_dirty(pteval))
                set_page_dirty(page);

        /* Update high watermark before we lower rss */
        update_hiwater_rss(mm);

        if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
                if (PageHuge(page)) {
                        hugetlb_count_sub(1 << compound_order(page), mm);
                } else {
                        dec_mm_counter(mm, mm_counter(page));
                }
                set_pte_at(mm, address, pte,
                           swp_entry_to_pte(make_hwpoison_entry(page)));
        } else if (pte_unused(pteval)) {
                /*
                 * The guest indicated that the page content is of no
                 * interest anymore. Simply discard the pte, vmscan
                 * will take care of the rest.
                 */
                dec_mm_counter(mm, mm_counter(page));
        } else if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION)) {
                swp_entry_t entry;
                pte_t swp_pte;
                /*
                 * Store the pfn of the page in a special migration
                 * pte. do_swap_page() will wait until the migration
                 * pte is removed and then restart fault handling.
                 */
                entry = make_migration_entry(page, pte_write(pteval));
                swp_pte = swp_entry_to_pte(entry);
                if (pte_soft_dirty(pteval))
                        swp_pte = pte_swp_mksoft_dirty(swp_pte);
                set_pte_at(mm, address, pte, swp_pte);
        } else if (PageAnon(page)) {
                swp_entry_t entry = { .val = page_private(page) };
                pte_t swp_pte;
                /*
                 * Store the swap location in the pte.
                 * See handle_pte_fault() ...
                 */
                VM_BUG_ON_PAGE(!PageSwapCache(page), page);
                if (swap_duplicate(entry) < 0) {
                        set_pte_at(mm, address, pte, pteval);
                        ret = SWAP_FAIL;
                        goto out_unmap;
                }
                if (list_empty(&mm->mmlist)) {
                        spin_lock(&mmlist_lock);
                        if (list_empty(&mm->mmlist))
                                list_add(&mm->mmlist, &init_mm.mmlist);
                        spin_unlock(&mmlist_lock);
                }
                dec_mm_counter(mm, MM_ANONPAGES);
                inc_mm_counter(mm, MM_SWAPENTS);
                swp_pte = swp_entry_to_pte(entry);
                if (pte_soft_dirty(pteval))
                        swp_pte = pte_swp_mksoft_dirty(swp_pte);
                set_pte_at(mm, address, pte, swp_pte);
        } else
                dec_mm_counter(mm, mm_counter_file(page));

        page_remove_rmap(page, PageHuge(page));
        page_cache_release(page);

out_unmap:
        pte_unmap_unlock(pte, ptl);
        if (ret != SWAP_FAIL && ret != SWAP_MLOCK && !(flags & TTU_MUNLOCK))
                mmu_notifier_invalidate_page(mm, address);
out:
        return ret;
}

bool is_vma_temporary_stack(struct vm_area_struct *vma)
{
        int maybe_stack = vma->vm_flags & (VM_GROWSDOWN | VM_GROWSUP);

        if (!maybe_stack)
                return false;

        if ((vma->vm_flags & VM_STACK_INCOMPLETE_SETUP) ==
                                                VM_STACK_INCOMPLETE_SETUP)
                return true;

        return false;
}

static bool invalid_migration_vma(struct vm_area_struct *vma, void *arg)
{
        return is_vma_temporary_stack(vma);
}

static int page_not_mapped(struct page *page)
{
        return !page_mapped(page);
};

/**
 * try_to_unmap - try to remove all page table mappings to a page
 * @page: the page to get unmapped
 * @flags: action and flags
 *
 * Tries to remove all the page table entries which are mapping this
 * page, used in the pageout path.  Caller must hold the page lock.
 * Return values are:
 *
 * SWAP_SUCCESS - we succeeded in removing all mappings
 * SWAP_AGAIN   - we missed a mapping, try again later
 * SWAP_FAIL    - the page is unswappable
 * SWAP_MLOCK   - page is mlocked.
 */
int try_to_unmap(struct page *page, enum ttu_flags flags)
{
        int ret;
        struct rmap_walk_control rwc = {
                .rmap_one = try_to_unmap_one,
                .arg = (void *)flags,
                .done = page_not_mapped,
                .anon_lock = page_lock_anon_vma_read,
        };

        VM_BUG_ON_PAGE(!PageHuge(page) && PageTransHuge(page), page);

        /*
         * During exec, a temporary VMA is setup and later moved.
         * The VMA is moved under the anon_vma lock but not the
         * page tables leading to a race where migration cannot
         * find the migration ptes. Rather than increasing the
         * locking requirements of exec(), migration skips
         * temporary VMAs until after exec() completes.
         */
        if ((flags & TTU_MIGRATION) && !PageKsm(page) && PageAnon(page))
                rwc.invalid_vma = invalid_migration_vma;

        ret = rmap_walk(page, &rwc);

        if (ret != SWAP_MLOCK && !page_mapped(page))
                ret = SWAP_SUCCESS;
        return ret;
}

/**
 * try_to_munlock - try to munlock a page
 * @page: the page to be munlocked
 *
 * Called from munlock code.  Checks all of the VMAs mapping the page
 * to make sure nobody else has this page mlocked. The page will be
 * returned with PG_mlocked cleared if no other vmas have it mlocked.
 *
 * Return values are:
 *
 * SWAP_AGAIN   - no vma is holding page mlocked, or,
 * SWAP_AGAIN   - page mapped in mlocked vma -- couldn't acquire mmap sem
 * SWAP_FAIL    - page cannot be located at present
 * SWAP_MLOCK   - page is now mlocked.
 */
int try_to_munlock(struct page *page)
{
        int ret;
        struct rmap_walk_control rwc = {
                .rmap_one = try_to_unmap_one,
                .arg = (void *)TTU_MUNLOCK,
                .done = page_not_mapped,
                .anon_lock = page_lock_anon_vma_read,

        };

        VM_BUG_ON_PAGE(!PageLocked(page) || PageLRU(page), page);

        ret = rmap_walk(page, &rwc);
        return ret;
}

void __put_anon_vma(struct anon_vma *anon_vma)
{
        struct anon_vma *root = anon_vma->root;

        anon_vma_free(anon_vma);
        if (root != anon_vma && atomic_dec_and_test(&root->refcount))
                anon_vma_free(root);
}

static struct anon_vma *rmap_walk_anon_lock(struct page *page,
                                        struct rmap_walk_control *rwc)
{
        struct anon_vma *anon_vma;

        if (rwc->anon_lock)
                return rwc->anon_lock(page);

        /*
         * Note: remove_migration_ptes() cannot use page_lock_anon_vma_read()
         * because that depends on page_mapped(); but not all its usages
         * are holding mmap_sem. Users without mmap_sem are required to
         * take a reference count to prevent the anon_vma disappearing
         */
        anon_vma = page_anon_vma(page);
        if (!anon_vma)
                return NULL;

        anon_vma_lock_read(anon_vma);
        return anon_vma;
}

/*
 * rmap_walk_anon - do something to anonymous page using the object-based
 * rmap method
 * @page: the page to be handled
 * @rwc: control variable according to each walk type
 *
 * Find all the mappings of a page using the mapping pointer and the vma chains
 * contained in the anon_vma struct it points to.
 *
 * When called from try_to_munlock(), the mmap_sem of the mm containing the vma
 * where the page was found will be held for write.  So, we won't recheck
 * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
 * LOCKED.
 */
static int rmap_walk_anon(struct page *page, struct rmap_walk_control *rwc)
{
        struct anon_vma *anon_vma;
        pgoff_t pgoff;
        struct anon_vma_chain *avc;
        int ret = SWAP_AGAIN;

        anon_vma = rmap_walk_anon_lock(page, rwc);
        if (!anon_vma)
                return ret;

        pgoff = page_to_pgoff(page);
        anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
                struct vm_area_struct *vma = avc->vma;
                unsigned long address = vma_address(page, vma);

                cond_resched();

                if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
                        continue;

                ret = rwc->rmap_one(page, vma, address, rwc->arg);
                if (ret != SWAP_AGAIN)
                        break;
                if (rwc->done && rwc->done(page))
                        break;
        }
        anon_vma_unlock_read(anon_vma);
        return ret;
}

/*
 * rmap_walk_file - do something to file page using the object-based rmap method
 * @page: the page to be handled
 * @rwc: control variable according to each walk type
 *
 * Find all the mappings of a page using the mapping pointer and the vma chains
 * contained in the address_space struct it points to.
 *
 * When called from try_to_munlock(), the mmap_sem of the mm containing the vma
 * where the page was found will be held for write.  So, we won't recheck
 * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
 * LOCKED.
 */
static int rmap_walk_file(struct page *page, struct rmap_walk_control *rwc)
{
        struct address_space *mapping = page->mapping;
        pgoff_t pgoff;
        struct vm_area_struct *vma;
        int ret = SWAP_AGAIN;

        /*
         * The page lock not only makes sure that page->mapping cannot
         * suddenly be NULLified by truncation, it makes sure that the
         * structure at mapping cannot be freed and reused yet,
         * so we can safely take mapping->i_mmap_rwsem.
         */
        VM_BUG_ON_PAGE(!PageLocked(page), page);

        if (!mapping)
                return ret;

        pgoff = page_to_pgoff(page);
        i_mmap_lock_read(mapping);
        vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
                unsigned long address = vma_address(page, vma);

                cond_resched();

                if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
                        continue;

                ret = rwc->rmap_one(page, vma, address, rwc->arg);
                if (ret != SWAP_AGAIN)
                        goto done;
                if (rwc->done && rwc->done(page))
                        goto done;
        }

done:
        i_mmap_unlock_read(mapping);
        return ret;
}

int rmap_walk(struct page *page, struct rmap_walk_control *rwc)
{
        if (unlikely(PageKsm(page)))
                return rmap_walk_ksm(page, rwc);
        else if (PageAnon(page))
                return rmap_walk_anon(page, rwc);
        else
                return rmap_walk_file(page, rwc);
}

#ifdef CONFIG_HUGETLB_PAGE
/*
 * The following three functions are for anonymous (private mapped) hugepages.
 * Unlike common anonymous pages, anonymous hugepages have no accounting code
 * and no lru code, because we handle hugepages differently from common pages.
 */
static void __hugepage_set_anon_rmap(struct page *page,
        struct vm_area_struct *vma, unsigned long address, int exclusive)
{
        struct anon_vma *anon_vma = vma->anon_vma;

        BUG_ON(!anon_vma);

        if (PageAnon(page))
                return;
        if (!exclusive)
                anon_vma = anon_vma->root;

        anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
        page->mapping = (struct address_space *) anon_vma;
        page->index = linear_page_index(vma, address);
}

void hugepage_add_anon_rmap(struct page *page,
                            struct vm_area_struct *vma, unsigned long address)
{
        struct anon_vma *anon_vma = vma->anon_vma;
        int first;

        BUG_ON(!PageLocked(page));
        BUG_ON(!anon_vma);
        /* address might be in next vma when migration races vma_adjust */
        first = atomic_inc_and_test(compound_mapcount_ptr(page));
        if (first)
                __hugepage_set_anon_rmap(page, vma, address, 0);
}

void hugepage_add_new_anon_rmap(struct page *page,
                        struct vm_area_struct *vma, unsigned long address)
{
        BUG_ON(address < vma->vm_start || address >= vma->vm_end);
        atomic_set(compound_mapcount_ptr(page), 0);
        __hugepage_set_anon_rmap(page, vma, address, 1);
}
#endif /* CONFIG_HUGETLB_PAGE */