mm: compaction: Update try_to_compact_pages()kerneldoc comment

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

/*
 * linux/mm/compaction.c
 *
 * Memory compaction for the reduction of external fragmentation. Note that
 * this heavily depends upon page migration to do all the real heavy
 * lifting
 *
 * Copyright IBM Corp. 2007-2010 Mel Gorman <mel@csn.ul.ie>
 */
#include <linux/swap.h>
#include <linux/migrate.h>
#include <linux/compaction.h>
#include <linux/mm_inline.h>
#include <linux/backing-dev.h>
#include <linux/sysctl.h>
#include <linux/sysfs.h>
#include "internal.h"

#if defined CONFIG_COMPACTION || defined CONFIG_CMA

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

static unsigned long release_freepages(struct list_head *freelist)
{
        struct page *page, *next;
        unsigned long count = 0;

        list_for_each_entry_safe(page, next, freelist, lru) {
                list_del(&page->lru);
                __free_page(page);
                count++;
        }

        return count;
}

static void map_pages(struct list_head *list)
{
        struct page *page;

        list_for_each_entry(page, list, lru) {
                arch_alloc_page(page, 0);
                kernel_map_pages(page, 1, 1);
        }
}

static inline bool migrate_async_suitable(int migratetype)
{
        return is_migrate_cma(migratetype) || migratetype == MIGRATE_MOVABLE;
}

/*
 * Compaction requires the taking of some coarse locks that are potentially
 * very heavily contended. Check if the process needs to be scheduled or
 * if the lock is contended. For async compaction, back out in the event
 * if contention is severe. For sync compaction, schedule.
 *
 * Returns true if the lock is held.
 * Returns false if the lock is released and compaction should abort
 */
static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
                                      bool locked, struct compact_control *cc)
{
        if (need_resched() || spin_is_contended(lock)) {
                if (locked) {
                        spin_unlock_irqrestore(lock, *flags);
                        locked = false;
                }

                /* async aborts if taking too long or contended */
                if (!cc->sync) {
                        cc->contended = true;
                        return false;
                }

                cond_resched();
        }

        if (!locked)
                spin_lock_irqsave(lock, *flags);
        return true;
}

static inline bool compact_trylock_irqsave(spinlock_t *lock,
                        unsigned long *flags, struct compact_control *cc)
{
        return compact_checklock_irqsave(lock, flags, false, cc);
}

static void compact_capture_page(struct compact_control *cc)
{
        unsigned long flags;
        int mtype, mtype_low, mtype_high;

        if (!cc->page || *cc->page)
                return;

        /*
         * For MIGRATE_MOVABLE allocations we capture a suitable page ASAP
         * regardless of the migratetype of the freelist is is captured from.
         * This is fine because the order for a high-order MIGRATE_MOVABLE
         * allocation is typically at least a pageblock size and overall
         * fragmentation is not impaired. Other allocation types must
         * capture pages from their own migratelist because otherwise they
         * could pollute other pageblocks like MIGRATE_MOVABLE with
         * difficult to move pages and making fragmentation worse overall.
         */
        if (cc->migratetype == MIGRATE_MOVABLE) {
                mtype_low = 0;
                mtype_high = MIGRATE_PCPTYPES;
        } else {
                mtype_low = cc->migratetype;
                mtype_high = cc->migratetype + 1;
        }

        /* Speculatively examine the free lists without zone lock */
        for (mtype = mtype_low; mtype < mtype_high; mtype++) {
                int order;
                for (order = cc->order; order < MAX_ORDER; order++) {
                        struct page *page;
                        struct free_area *area;
                        area = &(cc->zone->free_area[order]);
                        if (list_empty(&area->free_list[mtype]))
                                continue;

                        /* Take the lock and attempt capture of the page */
                        if (!compact_trylock_irqsave(&cc->zone->lock, &flags, cc))
                                return;
                        if (!list_empty(&area->free_list[mtype])) {
                                page = list_entry(area->free_list[mtype].next,
                                                        struct page, lru);
                                if (capture_free_page(page, cc->order, mtype)) {
                                        spin_unlock_irqrestore(&cc->zone->lock,
                                                                        flags);
                                        *cc->page = page;
                                        return;
                                }
                        }
                        spin_unlock_irqrestore(&cc->zone->lock, flags);
                }
        }
}

/*
 * Isolate free pages onto a private freelist. Caller must hold zone->lock.
 * If @strict is true, will abort returning 0 on any invalid PFNs or non-free
 * pages inside of the pageblock (even though it may still end up isolating
 * some pages).
 */
static unsigned long isolate_freepages_block(unsigned long blockpfn,
                                unsigned long end_pfn,
                                struct list_head *freelist,
                                bool strict)
{
        int nr_scanned = 0, total_isolated = 0;
        struct page *cursor;

        cursor = pfn_to_page(blockpfn);

        /* Isolate free pages. This assumes the block is valid */
        for (; blockpfn < end_pfn; blockpfn++, cursor++) {
                int isolated, i;
                struct page *page = cursor;

                if (!pfn_valid_within(blockpfn)) {
                        if (strict)
                                return 0;
                        continue;
                }
                nr_scanned++;

                if (!PageBuddy(page)) {
                        if (strict)
                                return 0;
                        continue;
                }

                /* Found a free page, break it into order-0 pages */
                isolated = split_free_page(page);
                if (!isolated && strict)
                        return 0;
                total_isolated += isolated;
                for (i = 0; i < isolated; i++) {
                        list_add(&page->lru, freelist);
                        page++;
                }

                /* If a page was split, advance to the end of it */
                if (isolated) {
                        blockpfn += isolated - 1;
                        cursor += isolated - 1;
                }
        }

        trace_mm_compaction_isolate_freepages(nr_scanned, total_isolated);
        return total_isolated;
}

/**
 * isolate_freepages_range() - isolate free pages.
 * @start_pfn: The first PFN to start isolating.
 * @end_pfn:   The one-past-last PFN.
 *
 * Non-free pages, invalid PFNs, or zone boundaries within the
 * [start_pfn, end_pfn) range are considered errors, cause function to
 * undo its actions and return zero.
 *
 * Otherwise, function returns one-past-the-last PFN of isolated page
 * (which may be greater then end_pfn if end fell in a middle of
 * a free page).
 */
unsigned long
isolate_freepages_range(unsigned long start_pfn, unsigned long end_pfn)
{
        unsigned long isolated, pfn, block_end_pfn, flags;
        struct zone *zone = NULL;
        LIST_HEAD(freelist);

        if (pfn_valid(start_pfn))
                zone = page_zone(pfn_to_page(start_pfn));

        for (pfn = start_pfn; pfn < end_pfn; pfn += isolated) {
                if (!pfn_valid(pfn) || zone != page_zone(pfn_to_page(pfn)))
                        break;

                /*
                 * On subsequent iterations ALIGN() is actually not needed,
                 * but we keep it that we not to complicate the code.
                 */
                block_end_pfn = ALIGN(pfn + 1, pageblock_nr_pages);
                block_end_pfn = min(block_end_pfn, end_pfn);

                spin_lock_irqsave(&zone->lock, flags);
                isolated = isolate_freepages_block(pfn, block_end_pfn,
                                                   &freelist, true);
                spin_unlock_irqrestore(&zone->lock, flags);

                /*
                 * In strict mode, isolate_freepages_block() returns 0 if
                 * there are any holes in the block (ie. invalid PFNs or
                 * non-free pages).
                 */
                if (!isolated)
                        break;

                /*
                 * If we managed to isolate pages, it is always (1 << n) *
                 * pageblock_nr_pages for some non-negative n.  (Max order
                 * page may span two pageblocks).
                 */
        }

        /* split_free_page does not map the pages */
        map_pages(&freelist);

        if (pfn < end_pfn) {
                /* Loop terminated early, cleanup. */
                release_freepages(&freelist);
                return 0;
        }

        /* We don't use freelists for anything. */
        return pfn;
}

/* Update the number of anon and file isolated pages in the zone */
static void acct_isolated(struct zone *zone, bool locked, struct compact_control *cc)
{
        struct page *page;
        unsigned int count[2] = { 0, };

        list_for_each_entry(page, &cc->migratepages, lru)
                count[!!page_is_file_cache(page)]++;

        /* If locked we can use the interrupt unsafe versions */
        if (locked) {
                __mod_zone_page_state(zone, NR_ISOLATED_ANON, count[0]);
                __mod_zone_page_state(zone, NR_ISOLATED_FILE, count[1]);
        } else {
                mod_zone_page_state(zone, NR_ISOLATED_ANON, count[0]);
                mod_zone_page_state(zone, NR_ISOLATED_FILE, count[1]);
        }
}

/* Similar to reclaim, but different enough that they don't share logic */
static bool too_many_isolated(struct zone *zone)
{
        unsigned long active, inactive, isolated;

        inactive = zone_page_state(zone, NR_INACTIVE_FILE) +
                                        zone_page_state(zone, NR_INACTIVE_ANON);
        active = zone_page_state(zone, NR_ACTIVE_FILE) +
                                        zone_page_state(zone, NR_ACTIVE_ANON);
        isolated = zone_page_state(zone, NR_ISOLATED_FILE) +
                                        zone_page_state(zone, NR_ISOLATED_ANON);

        return isolated > (inactive + active) / 2;
}

/**
 * isolate_migratepages_range() - isolate all migrate-able pages in range.
 * @zone:       Zone pages are in.
 * @cc:         Compaction control structure.
 * @low_pfn:    The first PFN of the range.
 * @end_pfn:    The one-past-the-last PFN of the range.
 *
 * Isolate all pages that can be migrated from the range specified by
 * [low_pfn, end_pfn).  Returns zero if there is a fatal signal
 * pending), otherwise PFN of the first page that was not scanned
 * (which may be both less, equal to or more then end_pfn).
 *
 * Assumes that cc->migratepages is empty and cc->nr_migratepages is
 * zero.
 *
 * Apart from cc->migratepages and cc->nr_migratetypes this function
 * does not modify any cc's fields, in particular it does not modify
 * (or read for that matter) cc->migrate_pfn.
 */
unsigned long
isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
                           unsigned long low_pfn, unsigned long end_pfn)
{
        unsigned long last_pageblock_nr = 0, pageblock_nr;
        unsigned long nr_scanned = 0, nr_isolated = 0;
        struct list_head *migratelist = &cc->migratepages;
        isolate_mode_t mode = 0;
        struct lruvec *lruvec;
        unsigned long flags;
        bool locked;

        /*
         * Ensure that there are not too many pages isolated from the LRU
         * list by either parallel reclaimers or compaction. If there are,
         * delay for some time until fewer pages are isolated
         */
        while (unlikely(too_many_isolated(zone))) {
                /* async migration should just abort */
                if (!cc->sync)
                        return 0;

                congestion_wait(BLK_RW_ASYNC, HZ/10);

                if (fatal_signal_pending(current))
                        return 0;
        }

        /* Time to isolate some pages for migration */
        cond_resched();
        spin_lock_irqsave(&zone->lru_lock, flags);
        locked = true;
        for (; low_pfn < end_pfn; low_pfn++) {
                struct page *page;

                /* give a chance to irqs before checking need_resched() */
                if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
                        spin_unlock_irqrestore(&zone->lru_lock, flags);
                        locked = false;
                }

                /* Check if it is ok to still hold the lock */
                locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
                                                                locked, cc);
                if (!locked || fatal_signal_pending(current))
                        break;

                /*
                 * migrate_pfn does not necessarily start aligned to a
                 * pageblock. Ensure that pfn_valid is called when moving
                 * into a new MAX_ORDER_NR_PAGES range in case of large
                 * memory holes within the zone
                 */
                if ((low_pfn & (MAX_ORDER_NR_PAGES - 1)) == 0) {
                        if (!pfn_valid(low_pfn)) {
                                low_pfn += MAX_ORDER_NR_PAGES - 1;
                                continue;
                        }
                }

                if (!pfn_valid_within(low_pfn))
                        continue;
                nr_scanned++;

                /*
                 * Get the page and ensure the page is within the same zone.
                 * See the comment in isolate_freepages about overlapping
                 * nodes. It is deliberate that the new zone lock is not taken
                 * as memory compaction should not move pages between nodes.
                 */
                page = pfn_to_page(low_pfn);
                if (page_zone(page) != zone)
                        continue;

                /* Skip if free */
                if (PageBuddy(page))
                        continue;

                /*
                 * For async migration, also only scan in MOVABLE blocks. Async
                 * migration is optimistic to see if the minimum amount of work
                 * satisfies the allocation
                 */
                pageblock_nr = low_pfn >> pageblock_order;
                if (!cc->sync && last_pageblock_nr != pageblock_nr &&
                    !migrate_async_suitable(get_pageblock_migratetype(page))) {
                        low_pfn += pageblock_nr_pages;
                        low_pfn = ALIGN(low_pfn, pageblock_nr_pages) - 1;
                        last_pageblock_nr = pageblock_nr;
                        continue;
                }

                if (!PageLRU(page))
                        continue;

                /*
                 * PageLRU is set, and lru_lock excludes isolation,
                 * splitting and collapsing (collapsing has already
                 * happened if PageLRU is set).
                 */
                if (PageTransHuge(page)) {
                        low_pfn += (1 << compound_order(page)) - 1;
                        continue;
                }

                if (!cc->sync)
                        mode |= ISOLATE_ASYNC_MIGRATE;

                lruvec = mem_cgroup_page_lruvec(page, zone);

                /* Try isolate the page */
                if (__isolate_lru_page(page, mode) != 0)
                        continue;

                VM_BUG_ON(PageTransCompound(page));

                /* Successfully isolated */
                del_page_from_lru_list(page, lruvec, page_lru(page));
                list_add(&page->lru, migratelist);
                cc->nr_migratepages++;
                nr_isolated++;

                /* Avoid isolating too much */
                if (cc->nr_migratepages == COMPACT_CLUSTER_MAX) {
                        ++low_pfn;
                        break;
                }
        }

        acct_isolated(zone, locked, cc);

        if (locked)
                spin_unlock_irqrestore(&zone->lru_lock, flags);

        trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);

        return low_pfn;
}

#endif /* CONFIG_COMPACTION || CONFIG_CMA */
#ifdef CONFIG_COMPACTION

/* Returns true if the page is within a block suitable for migration to */
static bool suitable_migration_target(struct page *page)
{

        int migratetype = get_pageblock_migratetype(page);

        /* Don't interfere with memory hot-remove or the min_free_kbytes blocks */
        if (migratetype == MIGRATE_ISOLATE || migratetype == MIGRATE_RESERVE)
                return false;

        /* If the page is a large free page, then allow migration */
        if (PageBuddy(page) && page_order(page) >= pageblock_order)
                return true;

        /* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
        if (migrate_async_suitable(migratetype))
                return true;

        /* Otherwise skip the block */
        return false;
}

/*
 * Returns the start pfn of the last page block in a zone.  This is the starting
 * point for full compaction of a zone.  Compaction searches for free pages from
 * the end of each zone, while isolate_freepages_block scans forward inside each
 * page block.
 */
static unsigned long start_free_pfn(struct zone *zone)
{
        unsigned long free_pfn;
        free_pfn = zone->zone_start_pfn + zone->spanned_pages;
        free_pfn &= ~(pageblock_nr_pages-1);
        return free_pfn;
}

/*
 * Based on information in the current compact_control, find blocks
 * suitable for isolating free pages from and then isolate them.
 */
static void isolate_freepages(struct zone *zone,
                                struct compact_control *cc)
{
        struct page *page;
        unsigned long high_pfn, low_pfn, pfn, zone_end_pfn, end_pfn;
        unsigned long flags;
        int nr_freepages = cc->nr_freepages;
        struct list_head *freelist = &cc->freepages;

        /*
         * Initialise the free scanner. The starting point is where we last
         * scanned from (or the end of the zone if starting). The low point
         * is the end of the pageblock the migration scanner is using.
         */
        pfn = cc->free_pfn;
        low_pfn = cc->migrate_pfn + pageblock_nr_pages;

        /*
         * Take care that if the migration scanner is at the end of the zone
         * that the free scanner does not accidentally move to the next zone
         * in the next isolation cycle.
         */
        high_pfn = min(low_pfn, pfn);

        zone_end_pfn = zone->zone_start_pfn + zone->spanned_pages;

        /*
         * Isolate free pages until enough are available to migrate the
         * pages on cc->migratepages. We stop searching if the migrate
         * and free page scanners meet or enough free pages are isolated.
         */
        for (; pfn > low_pfn && cc->nr_migratepages > nr_freepages;
                                        pfn -= pageblock_nr_pages) {
                unsigned long isolated;

                if (!pfn_valid(pfn))
                        continue;

                /*
                 * Check for overlapping nodes/zones. It's possible on some
                 * configurations to have a setup like
                 * node0 node1 node0
                 * i.e. it's possible that all pages within a zones range of
                 * pages do not belong to a single zone.
                 */
                page = pfn_to_page(pfn);
                if (page_zone(page) != zone)
                        continue;

                /* Check the block is suitable for migration */
                if (!suitable_migration_target(page))
                        continue;

                /*
                 * Found a block suitable for isolating free pages from. Now
                 * we disabled interrupts, double check things are ok and
                 * isolate the pages. This is to minimise the time IRQs
                 * are disabled
                 */
                isolated = 0;

                /*
                 * The zone lock must be held to isolate freepages. This
                 * unfortunately this is a very coarse lock and can be
                 * heavily contended if there are parallel allocations
                 * or parallel compactions. For async compaction do not
                 * spin on the lock
                 */
                if (!compact_trylock_irqsave(&zone->lock, &flags, cc))
                        break;
                if (suitable_migration_target(page)) {
                        end_pfn = min(pfn + pageblock_nr_pages, zone_end_pfn);
                        isolated = isolate_freepages_block(pfn, end_pfn,
                                                           freelist, false);
                        nr_freepages += isolated;
                }
                spin_unlock_irqrestore(&zone->lock, flags);

                /*
                 * Record the highest PFN we isolated pages from. When next
                 * looking for free pages, the search will restart here as
                 * page migration may have returned some pages to the allocator
                 */
                if (isolated) {
                        high_pfn = max(high_pfn, pfn);

                        /*
                         * If the free scanner has wrapped, update
                         * compact_cached_free_pfn to point to the highest
                         * pageblock with free pages. This reduces excessive
                         * scanning of full pageblocks near the end of the
                         * zone
                         */
                        if (cc->order > 0 && cc->wrapped)
                                zone->compact_cached_free_pfn = high_pfn;
                }
        }

        /* split_free_page does not map the pages */
        map_pages(freelist);

        cc->free_pfn = high_pfn;
        cc->nr_freepages = nr_freepages;

        /* If compact_cached_free_pfn is reset then set it now */
        if (cc->order > 0 && !cc->wrapped &&
                        zone->compact_cached_free_pfn == start_free_pfn(zone))
                zone->compact_cached_free_pfn = high_pfn;
}

/*
 * This is a migrate-callback that "allocates" freepages by taking pages
 * from the isolated freelists in the block we are migrating to.
 */
static struct page *compaction_alloc(struct page *migratepage,
                                        unsigned long data,
                                        int **result)
{
        struct compact_control *cc = (struct compact_control *)data;
        struct page *freepage;

        /* Isolate free pages if necessary */
        if (list_empty(&cc->freepages)) {
                isolate_freepages(cc->zone, cc);

                if (list_empty(&cc->freepages))
                        return NULL;
        }

        freepage = list_entry(cc->freepages.next, struct page, lru);
        list_del(&freepage->lru);
        cc->nr_freepages--;

        return freepage;
}

/*
 * We cannot control nr_migratepages and nr_freepages fully when migration is
 * running as migrate_pages() has no knowledge of compact_control. When
 * migration is complete, we count the number of pages on the lists by hand.
 */
static void update_nr_listpages(struct compact_control *cc)
{
        int nr_migratepages = 0;
        int nr_freepages = 0;
        struct page *page;

        list_for_each_entry(page, &cc->migratepages, lru)
                nr_migratepages++;
        list_for_each_entry(page, &cc->freepages, lru)
                nr_freepages++;

        cc->nr_migratepages = nr_migratepages;
        cc->nr_freepages = nr_freepages;
}

/* possible outcome of isolate_migratepages */
typedef enum {
        ISOLATE_ABORT,          /* Abort compaction now */
        ISOLATE_NONE,           /* No pages isolated, continue scanning */
        ISOLATE_SUCCESS,        /* Pages isolated, migrate */
} isolate_migrate_t;

/*
 * Isolate all pages that can be migrated from the block pointed to by
 * the migrate scanner within compact_control.
 */
static isolate_migrate_t isolate_migratepages(struct zone *zone,
                                        struct compact_control *cc)
{
        unsigned long low_pfn, end_pfn;

        /* Do not scan outside zone boundaries */
        low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);

        /* Only scan within a pageblock boundary */
        end_pfn = ALIGN(low_pfn + pageblock_nr_pages, pageblock_nr_pages);

        /* Do not cross the free scanner or scan within a memory hole */
        if (end_pfn > cc->free_pfn || !pfn_valid(low_pfn)) {
                cc->migrate_pfn = end_pfn;
                return ISOLATE_NONE;
        }

        /* Perform the isolation */
        low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn);
        if (!low_pfn || cc->contended)
                return ISOLATE_ABORT;

        cc->migrate_pfn = low_pfn;

        return ISOLATE_SUCCESS;
}

static int compact_finished(struct zone *zone,
                            struct compact_control *cc)
{
        unsigned long watermark;

        if (fatal_signal_pending(current))
                return COMPACT_PARTIAL;

        /*
         * A full (order == -1) compaction run starts at the beginning and
         * end of a zone; it completes when the migrate and free scanner meet.
         * A partial (order > 0) compaction can start with the free scanner
         * at a random point in the zone, and may have to restart.
         */
        if (cc->free_pfn <= cc->migrate_pfn) {
                if (cc->order > 0 && !cc->wrapped) {
                        /* We started partway through; restart at the end. */
                        unsigned long free_pfn = start_free_pfn(zone);
                        zone->compact_cached_free_pfn = free_pfn;
                        cc->free_pfn = free_pfn;
                        cc->wrapped = 1;
                        return COMPACT_CONTINUE;
                }
                return COMPACT_COMPLETE;
        }

        /* We wrapped around and ended up where we started. */
        if (cc->wrapped && cc->free_pfn <= cc->start_free_pfn)
                return COMPACT_COMPLETE;

        /*
         * order == -1 is expected when compacting via
         * /proc/sys/vm/compact_memory
         */
        if (cc->order == -1)
                return COMPACT_CONTINUE;

        /* Compaction run is not finished if the watermark is not met */
        watermark = low_wmark_pages(zone);
        watermark += (1 << cc->order);

        if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
                return COMPACT_CONTINUE;

        /* Direct compactor: Is a suitable page free? */
        if (cc->page) {
                /* Was a suitable page captured? */
                if (*cc->page)
                        return COMPACT_PARTIAL;
        } else {
                unsigned int order;
                for (order = cc->order; order < MAX_ORDER; order++) {
                        struct free_area *area = &zone->free_area[cc->order];
                        /* Job done if page is free of the right migratetype */
                        if (!list_empty(&area->free_list[cc->migratetype]))
                                return COMPACT_PARTIAL;

                        /* Job done if allocation would set block type */
                        if (cc->order >= pageblock_order && area->nr_free)
                                return COMPACT_PARTIAL;
                }
        }

        return COMPACT_CONTINUE;
}

/*
 * compaction_suitable: Is this suitable to run compaction on this zone now?
 * Returns
 *   COMPACT_SKIPPED  - If there are too few free pages for compaction
 *   COMPACT_PARTIAL  - If the allocation would succeed without compaction
 *   COMPACT_CONTINUE - If compaction should run now
 */
unsigned long compaction_suitable(struct zone *zone, int order)
{
        int fragindex;
        unsigned long watermark;

        /*
         * order == -1 is expected when compacting via
         * /proc/sys/vm/compact_memory
         */
        if (order == -1)
                return COMPACT_CONTINUE;

        /*
         * Watermarks for order-0 must be met for compaction. Note the 2UL.
         * This is because during migration, copies of pages need to be
         * allocated and for a short time, the footprint is higher
         */
        watermark = low_wmark_pages(zone) + (2UL << order);
        if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
                return COMPACT_SKIPPED;

        /*
         * fragmentation index determines if allocation failures are due to
         * low memory or external fragmentation
         *
         * index of -1000 implies allocations might succeed depending on
         * watermarks
         * index towards 0 implies failure is due to lack of memory
         * index towards 1000 implies failure is due to fragmentation
         *
         * Only compact if a failure would be due to fragmentation.
         */
        fragindex = fragmentation_index(zone, order);
        if (fragindex >= 0 && fragindex <= sysctl_extfrag_threshold)
                return COMPACT_SKIPPED;

        if (fragindex == -1000 && zone_watermark_ok(zone, order, watermark,
            0, 0))
                return COMPACT_PARTIAL;

        return COMPACT_CONTINUE;
}

static int compact_zone(struct zone *zone, struct compact_control *cc)
{
        int ret;

        ret = compaction_suitable(zone, cc->order);
        switch (ret) {
        case COMPACT_PARTIAL:
        case COMPACT_SKIPPED:
                /* Compaction is likely to fail */
                return ret;
        case COMPACT_CONTINUE:
                /* Fall through to compaction */
                ;
        }

        /* Setup to move all movable pages to the end of the zone */
        cc->migrate_pfn = zone->zone_start_pfn;

        if (cc->order > 0) {
                /* Incremental compaction. Start where the last one stopped. */
                cc->free_pfn = zone->compact_cached_free_pfn;
                cc->start_free_pfn = cc->free_pfn;
        } else {
                /* Order == -1 starts at the end of the zone. */
                cc->free_pfn = start_free_pfn(zone);
        }

        migrate_prep_local();

        while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
                unsigned long nr_migrate, nr_remaining;
                int err;

                switch (isolate_migratepages(zone, cc)) {
                case ISOLATE_ABORT:
                        ret = COMPACT_PARTIAL;
                        putback_lru_pages(&cc->migratepages);
                        cc->nr_migratepages = 0;
                        goto out;
                case ISOLATE_NONE:
                        continue;
                case ISOLATE_SUCCESS:
                        ;
                }

                nr_migrate = cc->nr_migratepages;
                err = migrate_pages(&cc->migratepages, compaction_alloc,
                                (unsigned long)cc, false,
                                cc->sync ? MIGRATE_SYNC_LIGHT : MIGRATE_ASYNC);
                update_nr_listpages(cc);
                nr_remaining = cc->nr_migratepages;

                count_vm_event(COMPACTBLOCKS);
                count_vm_events(COMPACTPAGES, nr_migrate - nr_remaining);
                if (nr_remaining)
                        count_vm_events(COMPACTPAGEFAILED, nr_remaining);
                trace_mm_compaction_migratepages(nr_migrate - nr_remaining,
                                                nr_remaining);

                /* Release LRU pages not migrated */
                if (err) {
                        putback_lru_pages(&cc->migratepages);
                        cc->nr_migratepages = 0;
                        if (err == -ENOMEM) {
                                ret = COMPACT_PARTIAL;
                                goto out;
                        }
                }

                /* Capture a page now if it is a suitable size */
                compact_capture_page(cc);
        }

out:
        /* Release free pages and check accounting */
        cc->nr_freepages -= release_freepages(&cc->freepages);
        VM_BUG_ON(cc->nr_freepages != 0);

        return ret;
}

static unsigned long compact_zone_order(struct zone *zone,
                                 int order, gfp_t gfp_mask,
                                 bool sync, bool *contended,
                                 struct page **page)
{
        unsigned long ret;
        struct compact_control cc = {
                .nr_freepages = 0,
                .nr_migratepages = 0,
                .order = order,
                .migratetype = allocflags_to_migratetype(gfp_mask),
                .zone = zone,
                .sync = sync,
                .page = page,
        };
        INIT_LIST_HEAD(&cc.freepages);
        INIT_LIST_HEAD(&cc.migratepages);

        ret = compact_zone(zone, &cc);

        VM_BUG_ON(!list_empty(&cc.freepages));
        VM_BUG_ON(!list_empty(&cc.migratepages));

        *contended = cc.contended;
        return ret;
}

int sysctl_extfrag_threshold = 500;

/**
 * try_to_compact_pages - Direct compact to satisfy a high-order allocation
 * @zonelist: The zonelist used for the current allocation
 * @order: The order of the current allocation
 * @gfp_mask: The GFP mask of the current allocation
 * @nodemask: The allowed nodes to allocate from
 * @sync: Whether migration is synchronous or not
 * @contended: Return value that is true if compaction was aborted due to lock contention
 * @page: Optionally capture a free page of the requested order during compaction
 *
 * This is the main entry point for direct page compaction.
 */
unsigned long try_to_compact_pages(struct zonelist *zonelist,
                        int order, gfp_t gfp_mask, nodemask_t *nodemask,
                        bool sync, bool *contended, struct page **page)
{
        enum zone_type high_zoneidx = gfp_zone(gfp_mask);
        int may_enter_fs = gfp_mask & __GFP_FS;
        int may_perform_io = gfp_mask & __GFP_IO;
        struct zoneref *z;
        struct zone *zone;
        int rc = COMPACT_SKIPPED;
        int alloc_flags = 0;

        /* Check if the GFP flags allow compaction */
        if (!order || !may_enter_fs || !may_perform_io)
                return rc;

        count_vm_event(COMPACTSTALL);

#ifdef CONFIG_CMA
        if (allocflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
                alloc_flags |= ALLOC_CMA;
#endif
        /* Compact each zone in the list */
        for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx,
                                                                nodemask) {
                int status;

                status = compact_zone_order(zone, order, gfp_mask, sync,
                                                contended, page);
                rc = max(status, rc);

                /* If a normal allocation would succeed, stop compacting */
                if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0,
                                      alloc_flags))
                        break;
        }

        return rc;
}


/* Compact all zones within a node */
static int __compact_pgdat(pg_data_t *pgdat, struct compact_control *cc)
{
        int zoneid;
        struct zone *zone;

        for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {

                zone = &pgdat->node_zones[zoneid];
                if (!populated_zone(zone))
                        continue;

                cc->nr_freepages = 0;
                cc->nr_migratepages = 0;
                cc->zone = zone;
                INIT_LIST_HEAD(&cc->freepages);
                INIT_LIST_HEAD(&cc->migratepages);

                if (cc->order == -1 || !compaction_deferred(zone, cc->order))
                        compact_zone(zone, cc);

                if (cc->order > 0) {
                        int ok = zone_watermark_ok(zone, cc->order,
                                                low_wmark_pages(zone), 0, 0);
                        if (ok && cc->order >= zone->compact_order_failed)
                                zone->compact_order_failed = cc->order + 1;
                        /* Currently async compaction is never deferred. */
                        else if (!ok && cc->sync)
                                defer_compaction(zone, cc->order);
                }

                VM_BUG_ON(!list_empty(&cc->freepages));
                VM_BUG_ON(!list_empty(&cc->migratepages));
        }

        return 0;
}

int compact_pgdat(pg_data_t *pgdat, int order)
{
        struct compact_control cc = {
                .order = order,
                .sync = false,
                .page = NULL,
        };

        return __compact_pgdat(pgdat, &cc);
}

static int compact_node(int nid)
{
        struct compact_control cc = {
                .order = -1,
                .sync = true,
                .page = NULL,
        };

        return __compact_pgdat(NODE_DATA(nid), &cc);
}

/* Compact all nodes in the system */
static int compact_nodes(void)
{
        int nid;

        /* Flush pending updates to the LRU lists */
        lru_add_drain_all();

        for_each_online_node(nid)
                compact_node(nid);

        return COMPACT_COMPLETE;
}

/* The written value is actually unused, all memory is compacted */
int sysctl_compact_memory;

/* This is the entry point for compacting all nodes via /proc/sys/vm */
int sysctl_compaction_handler(struct ctl_table *table, int write,
                        void __user *buffer, size_t *length, loff_t *ppos)
{
        if (write)
                return compact_nodes();

        return 0;
}

int sysctl_extfrag_handler(struct ctl_table *table, int write,
                        void __user *buffer, size_t *length, loff_t *ppos)
{
        proc_dointvec_minmax(table, write, buffer, length, ppos);

        return 0;
}

#if defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
ssize_t sysfs_compact_node(struct device *dev,
                        struct device_attribute *attr,
                        const char *buf, size_t count)
{
        int nid = dev->id;

        if (nid >= 0 && nid < nr_node_ids && node_online(nid)) {
                /* Flush pending updates to the LRU lists */
                lru_add_drain_all();

                compact_node(nid);
        }

        return count;
}
static DEVICE_ATTR(compact, S_IWUSR, NULL, sysfs_compact_node);

int compaction_register_node(struct node *node)
{
        return device_create_file(&node->dev, &dev_attr_compact);
}

void compaction_unregister_node(struct node *node)
{
        return device_remove_file(&node->dev, &dev_attr_compact);
}
#endif /* CONFIG_SYSFS && CONFIG_NUMA */

#endif /* CONFIG_COMPACTION */