s390: TASK_SIZE for kernel threads

[linux/fpc-iii.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/cpu.h>
#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 <linux/page-isolation.h>
#include <linux/kasan.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/page_owner.h>
#include "internal.h"

#ifdef CONFIG_COMPACTION
static inline void count_compact_event(enum vm_event_item item)
{
        count_vm_event(item);
}

static inline void count_compact_events(enum vm_event_item item, long delta)
{
        count_vm_events(item, delta);
}
#else
#define count_compact_event(item) do { } while (0)
#define count_compact_events(item, delta) do { } while (0)
#endif

#if defined CONFIG_COMPACTION || defined CONFIG_CMA

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

#define block_start_pfn(pfn, order)     round_down(pfn, 1UL << (order))
#define block_end_pfn(pfn, order)       ALIGN((pfn) + 1, 1UL << (order))
#define pageblock_start_pfn(pfn)        block_start_pfn(pfn, pageblock_order)
#define pageblock_end_pfn(pfn)          block_end_pfn(pfn, pageblock_order)

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

        list_for_each_entry_safe(page, next, freelist, lru) {
                unsigned long pfn = page_to_pfn(page);
                list_del(&page->lru);
                __free_page(page);
                if (pfn > high_pfn)
                        high_pfn = pfn;
        }

        return high_pfn;
}

static void map_pages(struct list_head *list)
{
        unsigned int i, order, nr_pages;
        struct page *page, *next;
        LIST_HEAD(tmp_list);

        list_for_each_entry_safe(page, next, list, lru) {
                list_del(&page->lru);

                order = page_private(page);
                nr_pages = 1 << order;

                post_alloc_hook(page, order, __GFP_MOVABLE);
                if (order)
                        split_page(page, order);

                for (i = 0; i < nr_pages; i++) {
                        list_add(&page->lru, &tmp_list);
                        page++;
                }
        }

        list_splice(&tmp_list, list);
}

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

#ifdef CONFIG_COMPACTION

int PageMovable(struct page *page)
{
        struct address_space *mapping;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        if (!__PageMovable(page))
                return 0;

        mapping = page_mapping(page);
        if (mapping && mapping->a_ops && mapping->a_ops->isolate_page)
                return 1;

        return 0;
}
EXPORT_SYMBOL(PageMovable);

void __SetPageMovable(struct page *page, struct address_space *mapping)
{
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE((unsigned long)mapping & PAGE_MAPPING_MOVABLE, page);
        page->mapping = (void *)((unsigned long)mapping | PAGE_MAPPING_MOVABLE);
}
EXPORT_SYMBOL(__SetPageMovable);

void __ClearPageMovable(struct page *page)
{
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageMovable(page), page);
        /*
         * Clear registered address_space val with keeping PAGE_MAPPING_MOVABLE
         * flag so that VM can catch up released page by driver after isolation.
         * With it, VM migration doesn't try to put it back.
         */
        page->mapping = (void *)((unsigned long)page->mapping &
                                PAGE_MAPPING_MOVABLE);
}
EXPORT_SYMBOL(__ClearPageMovable);

/* Do not skip compaction more than 64 times */
#define COMPACT_MAX_DEFER_SHIFT 6

/*
 * Compaction is deferred when compaction fails to result in a page
 * allocation success. 1 << compact_defer_limit compactions are skipped up
 * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT
 */
void defer_compaction(struct zone *zone, int order)
{
        zone->compact_considered = 0;
        zone->compact_defer_shift++;

        if (order < zone->compact_order_failed)
                zone->compact_order_failed = order;

        if (zone->compact_defer_shift > COMPACT_MAX_DEFER_SHIFT)
                zone->compact_defer_shift = COMPACT_MAX_DEFER_SHIFT;

        trace_mm_compaction_defer_compaction(zone, order);
}

/* Returns true if compaction should be skipped this time */
bool compaction_deferred(struct zone *zone, int order)
{
        unsigned long defer_limit = 1UL << zone->compact_defer_shift;

        if (order < zone->compact_order_failed)
                return false;

        /* Avoid possible overflow */
        if (++zone->compact_considered > defer_limit)
                zone->compact_considered = defer_limit;

        if (zone->compact_considered >= defer_limit)
                return false;

        trace_mm_compaction_deferred(zone, order);

        return true;
}

/*
 * Update defer tracking counters after successful compaction of given order,
 * which means an allocation either succeeded (alloc_success == true) or is
 * expected to succeed.
 */
void compaction_defer_reset(struct zone *zone, int order,
                bool alloc_success)
{
        if (alloc_success) {
                zone->compact_considered = 0;
                zone->compact_defer_shift = 0;
        }
        if (order >= zone->compact_order_failed)
                zone->compact_order_failed = order + 1;

        trace_mm_compaction_defer_reset(zone, order);
}

/* Returns true if restarting compaction after many failures */
bool compaction_restarting(struct zone *zone, int order)
{
        if (order < zone->compact_order_failed)
                return false;

        return zone->compact_defer_shift == COMPACT_MAX_DEFER_SHIFT &&
                zone->compact_considered >= 1UL << zone->compact_defer_shift;
}

/* Returns true if the pageblock should be scanned for pages to isolate. */
static inline bool isolation_suitable(struct compact_control *cc,
                                        struct page *page)
{
        if (cc->ignore_skip_hint)
                return true;

        return !get_pageblock_skip(page);
}

static void reset_cached_positions(struct zone *zone)
{
        zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
        zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
        zone->compact_cached_free_pfn =
                                pageblock_start_pfn(zone_end_pfn(zone) - 1);
}

/*
 * This function is called to clear all cached information on pageblocks that
 * should be skipped for page isolation when the migrate and free page scanner
 * meet.
 */
static void __reset_isolation_suitable(struct zone *zone)
{
        unsigned long start_pfn = zone->zone_start_pfn;
        unsigned long end_pfn = zone_end_pfn(zone);
        unsigned long pfn;

        zone->compact_blockskip_flush = false;

        /* Walk the zone and mark every pageblock as suitable for isolation */
        for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
                struct page *page;

                cond_resched();

                if (!pfn_valid(pfn))
                        continue;

                page = pfn_to_page(pfn);
                if (zone != page_zone(page))
                        continue;

                clear_pageblock_skip(page);
        }

        reset_cached_positions(zone);
}

void reset_isolation_suitable(pg_data_t *pgdat)
{
        int zoneid;

        for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
                struct zone *zone = &pgdat->node_zones[zoneid];
                if (!populated_zone(zone))
                        continue;

                /* Only flush if a full compaction finished recently */
                if (zone->compact_blockskip_flush)
                        __reset_isolation_suitable(zone);
        }
}

/*
 * If no pages were isolated then mark this pageblock to be skipped in the
 * future. The information is later cleared by __reset_isolation_suitable().
 */
static void update_pageblock_skip(struct compact_control *cc,
                        struct page *page, unsigned long nr_isolated,
                        bool migrate_scanner)
{
        struct zone *zone = cc->zone;
        unsigned long pfn;

        if (cc->ignore_skip_hint)
                return;

        if (!page)
                return;

        if (nr_isolated)
                return;

        set_pageblock_skip(page);

        pfn = page_to_pfn(page);

        /* Update where async and sync compaction should restart */
        if (migrate_scanner) {
                if (pfn > zone->compact_cached_migrate_pfn[0])
                        zone->compact_cached_migrate_pfn[0] = pfn;
                if (cc->mode != MIGRATE_ASYNC &&
                    pfn > zone->compact_cached_migrate_pfn[1])
                        zone->compact_cached_migrate_pfn[1] = pfn;
        } else {
                if (pfn < zone->compact_cached_free_pfn)
                        zone->compact_cached_free_pfn = pfn;
        }
}
#else
static inline bool isolation_suitable(struct compact_control *cc,
                                        struct page *page)
{
        return true;
}

static void update_pageblock_skip(struct compact_control *cc,
                        struct page *page, unsigned long nr_isolated,
                        bool migrate_scanner)
{
}
#endif /* CONFIG_COMPACTION */

/*
 * Compaction requires the taking of some coarse locks that are potentially
 * very heavily contended. For async compaction, back out if the lock cannot
 * be taken immediately. For sync compaction, spin on the lock if needed.
 *
 * Returns true if the lock is held
 * Returns false if the lock is not held and compaction should abort
 */
static bool compact_trylock_irqsave(spinlock_t *lock, unsigned long *flags,
                                                struct compact_control *cc)
{
        if (cc->mode == MIGRATE_ASYNC) {
                if (!spin_trylock_irqsave(lock, *flags)) {
                        cc->contended = true;
                        return false;
                }
        } else {
                spin_lock_irqsave(lock, *flags);
        }

        return true;
}

/*
 * Compaction requires the taking of some coarse locks that are potentially
 * very heavily contended. The lock should be periodically unlocked to avoid
 * having disabled IRQs for a long time, even when there is nobody waiting on
 * the lock. It might also be that allowing the IRQs will result in
 * need_resched() becoming true. If scheduling is needed, async compaction
 * aborts. Sync compaction schedules.
 * Either compaction type will also abort if a fatal signal is pending.
 * In either case if the lock was locked, it is dropped and not regained.
 *
 * Returns true if compaction should abort due to fatal signal pending, or
 *              async compaction due to need_resched()
 * Returns false when compaction can continue (sync compaction might have
 *              scheduled)
 */
static bool compact_unlock_should_abort(spinlock_t *lock,
                unsigned long flags, bool *locked, struct compact_control *cc)
{
        if (*locked) {
                spin_unlock_irqrestore(lock, flags);
                *locked = false;
        }

        if (fatal_signal_pending(current)) {
                cc->contended = true;
                return true;
        }

        if (need_resched()) {
                if (cc->mode == MIGRATE_ASYNC) {
                        cc->contended = true;
                        return true;
                }
                cond_resched();
        }

        return false;
}

/*
 * Aside from avoiding lock contention, compaction also periodically checks
 * need_resched() and either schedules in sync compaction or aborts async
 * compaction. This is similar to what compact_unlock_should_abort() does, but
 * is used where no lock is concerned.
 *
 * Returns false when no scheduling was needed, or sync compaction scheduled.
 * Returns true when async compaction should abort.
 */
static inline bool compact_should_abort(struct compact_control *cc)
{
        /* async compaction aborts if contended */
        if (need_resched()) {
                if (cc->mode == MIGRATE_ASYNC) {
                        cc->contended = true;
                        return true;
                }

                cond_resched();
        }

        return false;
}

/*
 * Isolate free pages onto a private freelist. 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(struct compact_control *cc,
                                unsigned long *start_pfn,
                                unsigned long end_pfn,
                                struct list_head *freelist,
                                bool strict)
{
        int nr_scanned = 0, total_isolated = 0;
        struct page *cursor, *valid_page = NULL;
        unsigned long flags = 0;
        bool locked = false;
        unsigned long blockpfn = *start_pfn;
        unsigned int order;

        cursor = pfn_to_page(blockpfn);

        /* Isolate free pages. */
        for (; blockpfn < end_pfn; blockpfn++, cursor++) {
                int isolated;
                struct page *page = cursor;

                /*
                 * Periodically drop the lock (if held) regardless of its
                 * contention, to give chance to IRQs. Abort if fatal signal
                 * pending or async compaction detects need_resched()
                 */
                if (!(blockpfn % SWAP_CLUSTER_MAX)
                    && compact_unlock_should_abort(&cc->zone->lock, flags,
                                                                &locked, cc))
                        break;

                nr_scanned++;
                if (!pfn_valid_within(blockpfn))
                        goto isolate_fail;

                if (!valid_page)
                        valid_page = page;

                /*
                 * For compound pages such as THP and hugetlbfs, we can save
                 * potentially a lot of iterations if we skip them at once.
                 * The check is racy, but we can consider only valid values
                 * and the only danger is skipping too much.
                 */
                if (PageCompound(page)) {
                        unsigned int comp_order = compound_order(page);

                        if (likely(comp_order < MAX_ORDER)) {
                                blockpfn += (1UL << comp_order) - 1;
                                cursor += (1UL << comp_order) - 1;
                        }

                        goto isolate_fail;
                }

                if (!PageBuddy(page))
                        goto isolate_fail;

                /*
                 * If we already hold the lock, we can skip some rechecking.
                 * Note that if we hold the lock now, checked_pageblock was
                 * already set in some previous iteration (or strict is true),
                 * so it is correct to skip the suitable migration target
                 * recheck as well.
                 */
                if (!locked) {
                        /*
                         * The zone lock must be held to isolate freepages.
                         * 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 and we acquire the lock as late as
                         * possible.
                         */
                        locked = compact_trylock_irqsave(&cc->zone->lock,
                                                                &flags, cc);
                        if (!locked)
                                break;

                        /* Recheck this is a buddy page under lock */
                        if (!PageBuddy(page))
                                goto isolate_fail;
                }

                /* Found a free page, will break it into order-0 pages */
                order = page_order(page);
                isolated = __isolate_free_page(page, order);
                if (!isolated)
                        break;
                set_page_private(page, order);

                total_isolated += isolated;
                cc->nr_freepages += isolated;
                list_add_tail(&page->lru, freelist);

                if (!strict && cc->nr_migratepages <= cc->nr_freepages) {
                        blockpfn += isolated;
                        break;
                }
                /* Advance to the end of split page */
                blockpfn += isolated - 1;
                cursor += isolated - 1;
                continue;

isolate_fail:
                if (strict)
                        break;
                else
                        continue;

        }

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

        /*
         * There is a tiny chance that we have read bogus compound_order(),
         * so be careful to not go outside of the pageblock.
         */
        if (unlikely(blockpfn > end_pfn))
                blockpfn = end_pfn;

        trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
                                        nr_scanned, total_isolated);

        /* Record how far we have got within the block */
        *start_pfn = blockpfn;

        /*
         * If strict isolation is requested by CMA then check that all the
         * pages requested were isolated. If there were any failures, 0 is
         * returned and CMA will fail.
         */
        if (strict && blockpfn < end_pfn)
                total_isolated = 0;

        /* Update the pageblock-skip if the whole pageblock was scanned */
        if (blockpfn == end_pfn)
                update_pageblock_skip(cc, valid_page, total_isolated, false);

        count_compact_events(COMPACTFREE_SCANNED, nr_scanned);
        if (total_isolated)
                count_compact_events(COMPACTISOLATED, 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(struct compact_control *cc,
                        unsigned long start_pfn, unsigned long end_pfn)
{
        unsigned long isolated, pfn, block_start_pfn, block_end_pfn;
        LIST_HEAD(freelist);

        pfn = start_pfn;
        block_start_pfn = pageblock_start_pfn(pfn);
        if (block_start_pfn < cc->zone->zone_start_pfn)
                block_start_pfn = cc->zone->zone_start_pfn;
        block_end_pfn = pageblock_end_pfn(pfn);

        for (; pfn < end_pfn; pfn += isolated,
                                block_start_pfn = block_end_pfn,
                                block_end_pfn += pageblock_nr_pages) {
                /* Protect pfn from changing by isolate_freepages_block */
                unsigned long isolate_start_pfn = pfn;

                block_end_pfn = min(block_end_pfn, end_pfn);

                /*
                 * pfn could pass the block_end_pfn if isolated freepage
                 * is more than pageblock order. In this case, we adjust
                 * scanning range to right one.
                 */
                if (pfn >= block_end_pfn) {
                        block_start_pfn = pageblock_start_pfn(pfn);
                        block_end_pfn = pageblock_end_pfn(pfn);
                        block_end_pfn = min(block_end_pfn, end_pfn);
                }

                if (!pageblock_pfn_to_page(block_start_pfn,
                                        block_end_pfn, cc->zone))
                        break;

                isolated = isolate_freepages_block(cc, &isolate_start_pfn,
                                                block_end_pfn, &freelist, true);

                /*
                 * 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).
                 */
        }

        /* __isolate_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;
}

/* 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 = node_page_state(zone->zone_pgdat, NR_INACTIVE_FILE) +
                        node_page_state(zone->zone_pgdat, NR_INACTIVE_ANON);
        active = node_page_state(zone->zone_pgdat, NR_ACTIVE_FILE) +
                        node_page_state(zone->zone_pgdat, NR_ACTIVE_ANON);
        isolated = node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE) +
                        node_page_state(zone->zone_pgdat, NR_ISOLATED_ANON);

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

/**
 * isolate_migratepages_block() - isolate all migrate-able pages within
 *                                a single pageblock
 * @cc:         Compaction control structure.
 * @low_pfn:    The first PFN to isolate
 * @end_pfn:    The one-past-the-last PFN to isolate, within same pageblock
 * @isolate_mode: Isolation mode to be used.
 *
 * Isolate all pages that can be migrated from the range specified by
 * [low_pfn, end_pfn). The range is expected to be within same pageblock.
 * 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
 * than end_pfn).
 *
 * The pages are isolated on cc->migratepages list (not required to be empty),
 * and cc->nr_migratepages is updated accordingly. The cc->migrate_pfn field
 * is neither read nor updated.
 */
static unsigned long
isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
                        unsigned long end_pfn, isolate_mode_t isolate_mode)
{
        struct zone *zone = cc->zone;
        unsigned long nr_scanned = 0, nr_isolated = 0;
        struct lruvec *lruvec;
        unsigned long flags = 0;
        bool locked = false;
        struct page *page = NULL, *valid_page = NULL;
        unsigned long start_pfn = low_pfn;
        bool skip_on_failure = false;
        unsigned long next_skip_pfn = 0;

        /*
         * 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->mode == MIGRATE_ASYNC)
                        return 0;

                congestion_wait(BLK_RW_ASYNC, HZ/10);

                if (fatal_signal_pending(current))
                        return 0;
        }

        if (compact_should_abort(cc))
                return 0;

        if (cc->direct_compaction && (cc->mode == MIGRATE_ASYNC)) {
                skip_on_failure = true;
                next_skip_pfn = block_end_pfn(low_pfn, cc->order);
        }

        /* Time to isolate some pages for migration */
        for (; low_pfn < end_pfn; low_pfn++) {

                if (skip_on_failure && low_pfn >= next_skip_pfn) {
                        /*
                         * We have isolated all migration candidates in the
                         * previous order-aligned block, and did not skip it due
                         * to failure. We should migrate the pages now and
                         * hopefully succeed compaction.
                         */
                        if (nr_isolated)
                                break;

                        /*
                         * We failed to isolate in the previous order-aligned
                         * block. Set the new boundary to the end of the
                         * current block. Note we can't simply increase
                         * next_skip_pfn by 1 << order, as low_pfn might have
                         * been incremented by a higher number due to skipping
                         * a compound or a high-order buddy page in the
                         * previous loop iteration.
                         */
                        next_skip_pfn = block_end_pfn(low_pfn, cc->order);
                }

                /*
                 * Periodically drop the lock (if held) regardless of its
                 * contention, to give chance to IRQs. Abort async compaction
                 * if contended.
                 */
                if (!(low_pfn % SWAP_CLUSTER_MAX)
                    && compact_unlock_should_abort(zone_lru_lock(zone), flags,
                                                                &locked, cc))
                        break;

                if (!pfn_valid_within(low_pfn))
                        goto isolate_fail;
                nr_scanned++;

                page = pfn_to_page(low_pfn);

                if (!valid_page)
                        valid_page = page;

                /*
                 * Skip if free. We read page order here without zone lock
                 * which is generally unsafe, but the race window is small and
                 * the worst thing that can happen is that we skip some
                 * potential isolation targets.
                 */
                if (PageBuddy(page)) {
                        unsigned long freepage_order = page_order_unsafe(page);

                        /*
                         * Without lock, we cannot be sure that what we got is
                         * a valid page order. Consider only values in the
                         * valid order range to prevent low_pfn overflow.
                         */
                        if (freepage_order > 0 && freepage_order < MAX_ORDER)
                                low_pfn += (1UL << freepage_order) - 1;
                        continue;
                }

                /*
                 * Regardless of being on LRU, compound pages such as THP and
                 * hugetlbfs are not to be compacted. We can potentially save
                 * a lot of iterations if we skip them at once. The check is
                 * racy, but we can consider only valid values and the only
                 * danger is skipping too much.
                 */
                if (PageCompound(page)) {
                        unsigned int comp_order = compound_order(page);

                        if (likely(comp_order < MAX_ORDER))
                                low_pfn += (1UL << comp_order) - 1;

                        goto isolate_fail;
                }

                /*
                 * Check may be lockless but that's ok as we recheck later.
                 * It's possible to migrate LRU and non-lru movable pages.
                 * Skip any other type of page
                 */
                if (!PageLRU(page)) {
                        /*
                         * __PageMovable can return false positive so we need
                         * to verify it under page_lock.
                         */
                        if (unlikely(__PageMovable(page)) &&
                                        !PageIsolated(page)) {
                                if (locked) {
                                        spin_unlock_irqrestore(zone_lru_lock(zone),
                                                                        flags);
                                        locked = false;
                                }

                                if (isolate_movable_page(page, isolate_mode))
                                        goto isolate_success;
                        }

                        goto isolate_fail;
                }

                /*
                 * Migration will fail if an anonymous page is pinned in memory,
                 * so avoid taking lru_lock and isolating it unnecessarily in an
                 * admittedly racy check.
                 */
                if (!page_mapping(page) &&
                    page_count(page) > page_mapcount(page))
                        goto isolate_fail;

                /* If we already hold the lock, we can skip some rechecking */
                if (!locked) {
                        locked = compact_trylock_irqsave(zone_lru_lock(zone),
                                                                &flags, cc);
                        if (!locked)
                                break;

                        /* Recheck PageLRU and PageCompound under lock */
                        if (!PageLRU(page))
                                goto isolate_fail;

                        /*
                         * Page become compound since the non-locked check,
                         * and it's on LRU. It can only be a THP so the order
                         * is safe to read and it's 0 for tail pages.
                         */
                        if (unlikely(PageCompound(page))) {
                                low_pfn += (1UL << compound_order(page)) - 1;
                                goto isolate_fail;
                        }
                }

                lruvec = mem_cgroup_page_lruvec(page, zone->zone_pgdat);

                /* Try isolate the page */
                if (__isolate_lru_page(page, isolate_mode) != 0)
                        goto isolate_fail;

                VM_BUG_ON_PAGE(PageCompound(page), page);

                /* Successfully isolated */
                del_page_from_lru_list(page, lruvec, page_lru(page));
                inc_node_page_state(page,
                                NR_ISOLATED_ANON + page_is_file_cache(page));

isolate_success:
                list_add(&page->lru, &cc->migratepages);
                cc->nr_migratepages++;
                nr_isolated++;

                /*
                 * Record where we could have freed pages by migration and not
                 * yet flushed them to buddy allocator.
                 * - this is the lowest page that was isolated and likely be
                 * then freed by migration.
                 */
                if (!cc->last_migrated_pfn)
                        cc->last_migrated_pfn = low_pfn;

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

                continue;
isolate_fail:
                if (!skip_on_failure)
                        continue;

                /*
                 * We have isolated some pages, but then failed. Release them
                 * instead of migrating, as we cannot form the cc->order buddy
                 * page anyway.
                 */
                if (nr_isolated) {
                        if (locked) {
                                spin_unlock_irqrestore(zone_lru_lock(zone), flags);
                                locked = false;
                        }
                        putback_movable_pages(&cc->migratepages);
                        cc->nr_migratepages = 0;
                        cc->last_migrated_pfn = 0;
                        nr_isolated = 0;
                }

                if (low_pfn < next_skip_pfn) {
                        low_pfn = next_skip_pfn - 1;
                        /*
                         * The check near the loop beginning would have updated
                         * next_skip_pfn too, but this is a bit simpler.
                         */
                        next_skip_pfn += 1UL << cc->order;
                }
        }

        /*
         * The PageBuddy() check could have potentially brought us outside
         * the range to be scanned.
         */
        if (unlikely(low_pfn > end_pfn))
                low_pfn = end_pfn;

        if (locked)
                spin_unlock_irqrestore(zone_lru_lock(zone), flags);

        /*
         * Update the pageblock-skip information and cached scanner pfn,
         * if the whole pageblock was scanned without isolating any page.
         */
        if (low_pfn == end_pfn)
                update_pageblock_skip(cc, valid_page, nr_isolated, true);

        trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn,
                                                nr_scanned, nr_isolated);

        count_compact_events(COMPACTMIGRATE_SCANNED, nr_scanned);
        if (nr_isolated)
                count_compact_events(COMPACTISOLATED, nr_isolated);

        return low_pfn;
}

/**
 * isolate_migratepages_range() - isolate migrate-able pages in a PFN range
 * @cc:        Compaction control structure.
 * @start_pfn: The first PFN to start isolating.
 * @end_pfn:   The one-past-last PFN.
 *
 * Returns zero if isolation fails fatally due to e.g. pending signal.
 * Otherwise, function returns one-past-the-last PFN of isolated page
 * (which may be greater than end_pfn if end fell in a middle of a THP page).
 */
unsigned long
isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
                                                        unsigned long end_pfn)
{
        unsigned long pfn, block_start_pfn, block_end_pfn;

        /* Scan block by block. First and last block may be incomplete */
        pfn = start_pfn;
        block_start_pfn = pageblock_start_pfn(pfn);
        if (block_start_pfn < cc->zone->zone_start_pfn)
                block_start_pfn = cc->zone->zone_start_pfn;
        block_end_pfn = pageblock_end_pfn(pfn);

        for (; pfn < end_pfn; pfn = block_end_pfn,
                                block_start_pfn = block_end_pfn,
                                block_end_pfn += pageblock_nr_pages) {

                block_end_pfn = min(block_end_pfn, end_pfn);

                if (!pageblock_pfn_to_page(block_start_pfn,
                                        block_end_pfn, cc->zone))
                        continue;

                pfn = isolate_migratepages_block(cc, pfn, block_end_pfn,
                                                        ISOLATE_UNEVICTABLE);

                if (!pfn)
                        break;

                if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
                        break;
        }

        return 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 compact_control *cc,
                                                        struct page *page)
{
        if (cc->ignore_block_suitable)
                return true;

        /* If the page is a large free page, then disallow migration */
        if (PageBuddy(page)) {
                /*
                 * We are checking page_order without zone->lock taken. But
                 * the only small danger is that we skip a potentially suitable
                 * pageblock, so it's not worth to check order for valid range.
                 */
                if (page_order_unsafe(page) >= pageblock_order)
                        return false;
        }

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

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

/*
 * Test whether the free scanner has reached the same or lower pageblock than
 * the migration scanner, and compaction should thus terminate.
 */
static inline bool compact_scanners_met(struct compact_control *cc)
{
        return (cc->free_pfn >> pageblock_order)
                <= (cc->migrate_pfn >> pageblock_order);
}

/*
 * 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 compact_control *cc)
{
        struct zone *zone = cc->zone;
        struct page *page;
        unsigned long block_start_pfn;  /* start of current pageblock */
        unsigned long isolate_start_pfn; /* exact pfn we start at */
        unsigned long block_end_pfn;    /* end of current pageblock */
        unsigned long low_pfn;       /* lowest pfn scanner is able to scan */
        struct list_head *freelist = &cc->freepages;

        /*
         * Initialise the free scanner. The starting point is where we last
         * successfully isolated from, zone-cached value, or the end of the
         * zone when isolating for the first time. For looping we also need
         * this pfn aligned down to the pageblock boundary, because we do
         * block_start_pfn -= pageblock_nr_pages in the for loop.
         * For ending point, take care when isolating in last pageblock of a
         * a zone which ends in the middle of a pageblock.
         * The low boundary is the end of the pageblock the migration scanner
         * is using.
         */
        isolate_start_pfn = cc->free_pfn;
        block_start_pfn = pageblock_start_pfn(cc->free_pfn);
        block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
                                                zone_end_pfn(zone));
        low_pfn = pageblock_end_pfn(cc->migrate_pfn);

        /*
         * 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 (; block_start_pfn >= low_pfn;
                                block_end_pfn = block_start_pfn,
                                block_start_pfn -= pageblock_nr_pages,
                                isolate_start_pfn = block_start_pfn) {
                /*
                 * This can iterate a massively long zone without finding any
                 * suitable migration targets, so periodically check if we need
                 * to schedule, or even abort async compaction.
                 */
                if (!(block_start_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
                                                && compact_should_abort(cc))
                        break;

                page = pageblock_pfn_to_page(block_start_pfn, block_end_pfn,
                                                                        zone);
                if (!page)
                        continue;

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

                /* If isolation recently failed, do not retry */
                if (!isolation_suitable(cc, page))
                        continue;

                /* Found a block suitable for isolating free pages from. */
                isolate_freepages_block(cc, &isolate_start_pfn, block_end_pfn,
                                        freelist, false);

                /*
                 * If we isolated enough freepages, or aborted due to lock
                 * contention, terminate.
                 */
                if ((cc->nr_freepages >= cc->nr_migratepages)
                                                        || cc->contended) {
                        if (isolate_start_pfn >= block_end_pfn) {
                                /*
                                 * Restart at previous pageblock if more
                                 * freepages can be isolated next time.
                                 */
                                isolate_start_pfn =
                                        block_start_pfn - pageblock_nr_pages;
                        }
                        break;
                } else if (isolate_start_pfn < block_end_pfn) {
                        /*
                         * If isolation failed early, do not continue
                         * needlessly.
                         */
                        break;
                }
        }

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

        /*
         * Record where the free scanner will restart next time. Either we
         * broke from the loop and set isolate_start_pfn based on the last
         * call to isolate_freepages_block(), or we met the migration scanner
         * and the loop terminated due to isolate_start_pfn < low_pfn
         */
        cc->free_pfn = isolate_start_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, and if we are not aborting due to
         * contention.
         */
        if (list_empty(&cc->freepages)) {
                if (!cc->contended)
                        isolate_freepages(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;
}

/*
 * This is a migrate-callback that "frees" freepages back to the isolated
 * freelist.  All pages on the freelist are from the same zone, so there is no
 * special handling needed for NUMA.
 */
static void compaction_free(struct page *page, unsigned long data)
{
        struct compact_control *cc = (struct compact_control *)data;

        list_add(&page->lru, &cc->freepages);
        cc->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;

/*
 * Allow userspace to control policy on scanning the unevictable LRU for
 * compactable pages.
 */
int sysctl_compact_unevictable_allowed __read_mostly = 1;

/*
 * Isolate all pages that can be migrated from the first suitable block,
 * starting at the block pointed to by the migrate scanner pfn within
 * compact_control.
 */
static isolate_migrate_t isolate_migratepages(struct zone *zone,
                                        struct compact_control *cc)
{
        unsigned long block_start_pfn;
        unsigned long block_end_pfn;
        unsigned long low_pfn;
        struct page *page;
        const isolate_mode_t isolate_mode =
                (sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
                (cc->mode != MIGRATE_SYNC ? ISOLATE_ASYNC_MIGRATE : 0);

        /*
         * Start at where we last stopped, or beginning of the zone as
         * initialized by compact_zone()
         */
        low_pfn = cc->migrate_pfn;
        block_start_pfn = pageblock_start_pfn(low_pfn);
        if (block_start_pfn < zone->zone_start_pfn)
                block_start_pfn = zone->zone_start_pfn;

        /* Only scan within a pageblock boundary */
        block_end_pfn = pageblock_end_pfn(low_pfn);

        /*
         * Iterate over whole pageblocks until we find the first suitable.
         * Do not cross the free scanner.
         */
        for (; block_end_pfn <= cc->free_pfn;
                        low_pfn = block_end_pfn,
                        block_start_pfn = block_end_pfn,
                        block_end_pfn += pageblock_nr_pages) {

                /*
                 * This can potentially iterate a massively long zone with
                 * many pageblocks unsuitable, so periodically check if we
                 * need to schedule, or even abort async compaction.
                 */
                if (!(low_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages))
                                                && compact_should_abort(cc))
                        break;

                page = pageblock_pfn_to_page(block_start_pfn, block_end_pfn,
                                                                        zone);
                if (!page)
                        continue;

                /* If isolation recently failed, do not retry */
                if (!isolation_suitable(cc, page))
                        continue;

                /*
                 * For async compaction, also only scan in MOVABLE blocks.
                 * Async compaction is optimistic to see if the minimum amount
                 * of work satisfies the allocation.
                 */
                if (cc->mode == MIGRATE_ASYNC &&
                    !migrate_async_suitable(get_pageblock_migratetype(page)))
                        continue;

                /* Perform the isolation */
                low_pfn = isolate_migratepages_block(cc, low_pfn,
                                                block_end_pfn, isolate_mode);

                if (!low_pfn || cc->contended)
                        return ISOLATE_ABORT;

                /*
                 * Either we isolated something and proceed with migration. Or
                 * we failed and compact_zone should decide if we should
                 * continue or not.
                 */
                break;
        }

        /* Record where migration scanner will be restarted. */
        cc->migrate_pfn = low_pfn;

        return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
}

/*
 * order == -1 is expected when compacting via
 * /proc/sys/vm/compact_memory
 */
static inline bool is_via_compact_memory(int order)
{
        return order == -1;
}

static enum compact_result __compact_finished(struct zone *zone, struct compact_control *cc,
                            const int migratetype)
{
        unsigned int order;
        unsigned long watermark;

        if (cc->contended || fatal_signal_pending(current))
                return COMPACT_CONTENDED;

        /* Compaction run completes if the migrate and free scanner meet */
        if (compact_scanners_met(cc)) {
                /* Let the next compaction start anew. */
                reset_cached_positions(zone);

                /*
                 * Mark that the PG_migrate_skip information should be cleared
                 * by kswapd when it goes to sleep. kcompactd does not set the
                 * flag itself as the decision to be clear should be directly
                 * based on an allocation request.
                 */
                if (cc->direct_compaction)
                        zone->compact_blockskip_flush = true;

                if (cc->whole_zone)
                        return COMPACT_COMPLETE;
                else
                        return COMPACT_PARTIAL_SKIPPED;
        }

        if (is_via_compact_memory(cc->order))
                return COMPACT_CONTINUE;

        /* Compaction run is not finished if the watermark is not met */
        watermark = zone->watermark[cc->alloc_flags & ALLOC_WMARK_MASK];

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

        /* Direct compactor: Is a suitable page free? */
        for (order = cc->order; order < MAX_ORDER; order++) {
                struct free_area *area = &zone->free_area[order];
                bool can_steal;

                /* Job done if page is free of the right migratetype */
                if (!list_empty(&area->free_list[migratetype]))
                        return COMPACT_SUCCESS;

#ifdef CONFIG_CMA
                /* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
                if (migratetype == MIGRATE_MOVABLE &&
                        !list_empty(&area->free_list[MIGRATE_CMA]))
                        return COMPACT_SUCCESS;
#endif
                /*
                 * Job done if allocation would steal freepages from
                 * other migratetype buddy lists.
                 */
                if (find_suitable_fallback(area, order, migratetype,
                                                true, &can_steal) != -1)
                        return COMPACT_SUCCESS;
        }

        return COMPACT_NO_SUITABLE_PAGE;
}

static enum compact_result compact_finished(struct zone *zone,
                        struct compact_control *cc,
                        const int migratetype)
{
        int ret;

        ret = __compact_finished(zone, cc, migratetype);
        trace_mm_compaction_finished(zone, cc->order, ret);
        if (ret == COMPACT_NO_SUITABLE_PAGE)
                ret = COMPACT_CONTINUE;

        return ret;
}

/*
 * 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_SUCCESS  - If the allocation would succeed without compaction
 *   COMPACT_CONTINUE - If compaction should run now
 */
static enum compact_result __compaction_suitable(struct zone *zone, int order,
                                        unsigned int alloc_flags,
                                        int classzone_idx,
                                        unsigned long wmark_target)
{
        unsigned long watermark;

        if (is_via_compact_memory(order))
                return COMPACT_CONTINUE;

        watermark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
        /*
         * If watermarks for high-order allocation are already met, there
         * should be no need for compaction at all.
         */
        if (zone_watermark_ok(zone, order, watermark, classzone_idx,
                                                                alloc_flags))
                return COMPACT_SUCCESS;

        /*
         * Watermarks for order-0 must be met for compaction to be able to
         * isolate free pages for migration targets. This means that the
         * watermark and alloc_flags have to match, or be more pessimistic than
         * the check in __isolate_free_page(). We don't use the direct
         * compactor's alloc_flags, as they are not relevant for freepage
         * isolation. We however do use the direct compactor's classzone_idx to
         * skip over zones where lowmem reserves would prevent allocation even
         * if compaction succeeds.
         * For costly orders, we require low watermark instead of min for
         * compaction to proceed to increase its chances.
         * ALLOC_CMA is used, as pages in CMA pageblocks are considered
         * suitable migration targets
         */
        watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
                                low_wmark_pages(zone) : min_wmark_pages(zone);
        watermark += compact_gap(order);
        if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
                                                ALLOC_CMA, wmark_target))
                return COMPACT_SKIPPED;

        return COMPACT_CONTINUE;
}

enum compact_result compaction_suitable(struct zone *zone, int order,
                                        unsigned int alloc_flags,
                                        int classzone_idx)
{
        enum compact_result ret;
        int fragindex;

        ret = __compaction_suitable(zone, order, alloc_flags, classzone_idx,
                                    zone_page_state(zone, NR_FREE_PAGES));
        /*
         * fragmentation index determines if allocation failures are due to
         * low memory or external fragmentation
         *
         * index of -1000 would imply allocations might succeed depending on
         * watermarks, but we already failed the high-order watermark check
         * 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. Also
         * ignore fragindex for non-costly orders where the alternative to
         * a successful reclaim/compaction is OOM. Fragindex and the
         * vm.extfrag_threshold sysctl is meant as a heuristic to prevent
         * excessive compaction for costly orders, but it should not be at the
         * expense of system stability.
         */
        if (ret == COMPACT_CONTINUE && (order > PAGE_ALLOC_COSTLY_ORDER)) {
                fragindex = fragmentation_index(zone, order);
                if (fragindex >= 0 && fragindex <= sysctl_extfrag_threshold)
                        ret = COMPACT_NOT_SUITABLE_ZONE;
        }

        trace_mm_compaction_suitable(zone, order, ret);
        if (ret == COMPACT_NOT_SUITABLE_ZONE)
                ret = COMPACT_SKIPPED;

        return ret;
}

bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
                int alloc_flags)
{
        struct zone *zone;
        struct zoneref *z;

        /*
         * Make sure at least one zone would pass __compaction_suitable if we continue
         * retrying the reclaim.
         */
        for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx,
                                        ac->nodemask) {
                unsigned long available;
                enum compact_result compact_result;

                /*
                 * Do not consider all the reclaimable memory because we do not
                 * want to trash just for a single high order allocation which
                 * is even not guaranteed to appear even if __compaction_suitable
                 * is happy about the watermark check.
                 */
                available = zone_reclaimable_pages(zone) / order;
                available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
                compact_result = __compaction_suitable(zone, order, alloc_flags,
                                ac_classzone_idx(ac), available);
                if (compact_result != COMPACT_SKIPPED)
                        return true;
        }

        return false;
}

static enum compact_result compact_zone(struct zone *zone, struct compact_control *cc)
{
        enum compact_result ret;
        unsigned long start_pfn = zone->zone_start_pfn;
        unsigned long end_pfn = zone_end_pfn(zone);
        const int migratetype = gfpflags_to_migratetype(cc->gfp_mask);
        const bool sync = cc->mode != MIGRATE_ASYNC;

        ret = compaction_suitable(zone, cc->order, cc->alloc_flags,
                                                        cc->classzone_idx);
        /* Compaction is likely to fail */
        if (ret == COMPACT_SUCCESS || ret == COMPACT_SKIPPED)
                return ret;

        /* huh, compaction_suitable is returning something unexpected */
        VM_BUG_ON(ret != COMPACT_CONTINUE);

        /*
         * Clear pageblock skip if there were failures recently and compaction
         * is about to be retried after being deferred.
         */
        if (compaction_restarting(zone, cc->order))
                __reset_isolation_suitable(zone);

        /*
         * Setup to move all movable pages to the end of the zone. Used cached
         * information on where the scanners should start (unless we explicitly
         * want to compact the whole zone), but check that it is initialised
         * by ensuring the values are within zone boundaries.
         */
        if (cc->whole_zone) {
                cc->migrate_pfn = start_pfn;
                cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
        } else {
                cc->migrate_pfn = zone->compact_cached_migrate_pfn[sync];
                cc->free_pfn = zone->compact_cached_free_pfn;
                if (cc->free_pfn < start_pfn || cc->free_pfn >= end_pfn) {
                        cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
                        zone->compact_cached_free_pfn = cc->free_pfn;
                }
                if (cc->migrate_pfn < start_pfn || cc->migrate_pfn >= end_pfn) {
                        cc->migrate_pfn = start_pfn;
                        zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
                        zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
                }

                if (cc->migrate_pfn == start_pfn)
                        cc->whole_zone = true;
        }

        cc->last_migrated_pfn = 0;

        trace_mm_compaction_begin(start_pfn, cc->migrate_pfn,
                                cc->free_pfn, end_pfn, sync);

        migrate_prep_local();

        while ((ret = compact_finished(zone, cc, migratetype)) ==
                                                COMPACT_CONTINUE) {
                int err;

                switch (isolate_migratepages(zone, cc)) {
                case ISOLATE_ABORT:
                        ret = COMPACT_CONTENDED;
                        putback_movable_pages(&cc->migratepages);
                        cc->nr_migratepages = 0;
                        goto out;
                case ISOLATE_NONE:
                        /*
                         * We haven't isolated and migrated anything, but
                         * there might still be unflushed migrations from
                         * previous cc->order aligned block.
                         */
                        goto check_drain;
                case ISOLATE_SUCCESS:
                        ;
                }

                err = migrate_pages(&cc->migratepages, compaction_alloc,
                                compaction_free, (unsigned long)cc, cc->mode,
                                MR_COMPACTION);

                trace_mm_compaction_migratepages(cc->nr_migratepages, err,
                                                        &cc->migratepages);

                /* All pages were either migrated or will be released */
                cc->nr_migratepages = 0;
                if (err) {
                        putback_movable_pages(&cc->migratepages);
                        /*
                         * migrate_pages() may return -ENOMEM when scanners meet
                         * and we want compact_finished() to detect it
                         */
                        if (err == -ENOMEM && !compact_scanners_met(cc)) {
                                ret = COMPACT_CONTENDED;
                                goto out;
                        }
                        /*
                         * We failed to migrate at least one page in the current
                         * order-aligned block, so skip the rest of it.
                         */
                        if (cc->direct_compaction &&
                                                (cc->mode == MIGRATE_ASYNC)) {
                                cc->migrate_pfn = block_end_pfn(
                                                cc->migrate_pfn - 1, cc->order);
                                /* Draining pcplists is useless in this case */
                                cc->last_migrated_pfn = 0;

                        }
                }

check_drain:
                /*
                 * Has the migration scanner moved away from the previous
                 * cc->order aligned block where we migrated from? If yes,
                 * flush the pages that were freed, so that they can merge and
                 * compact_finished() can detect immediately if allocation
                 * would succeed.
                 */
                if (cc->order > 0 && cc->last_migrated_pfn) {
                        int cpu;
                        unsigned long current_block_start =
                                block_start_pfn(cc->migrate_pfn, cc->order);

                        if (cc->last_migrated_pfn < current_block_start) {
                                cpu = get_cpu();
                                lru_add_drain_cpu(cpu);
                                drain_local_pages(zone);
                                put_cpu();
                                /* No more flushing until we migrate again */
                                cc->last_migrated_pfn = 0;
                        }
                }

        }

out:
        /*
         * Release free pages and update where the free scanner should restart,
         * so we don't leave any returned pages behind in the next attempt.
         */
        if (cc->nr_freepages > 0) {
                unsigned long free_pfn = release_freepages(&cc->freepages);

                cc->nr_freepages = 0;
                VM_BUG_ON(free_pfn == 0);
                /* The cached pfn is always the first in a pageblock */
                free_pfn = pageblock_start_pfn(free_pfn);
                /*
                 * Only go back, not forward. The cached pfn might have been
                 * already reset to zone end in compact_finished()
                 */
                if (free_pfn > zone->compact_cached_free_pfn)
                        zone->compact_cached_free_pfn = free_pfn;
        }

        trace_mm_compaction_end(start_pfn, cc->migrate_pfn,
                                cc->free_pfn, end_pfn, sync, ret);

        return ret;
}

static enum compact_result compact_zone_order(struct zone *zone, int order,
                gfp_t gfp_mask, enum compact_priority prio,
                unsigned int alloc_flags, int classzone_idx)
{
        enum compact_result ret;
        struct compact_control cc = {
                .nr_freepages = 0,
                .nr_migratepages = 0,
                .order = order,
                .gfp_mask = gfp_mask,
                .zone = zone,
                .mode = (prio == COMPACT_PRIO_ASYNC) ?
                                        MIGRATE_ASYNC : MIGRATE_SYNC_LIGHT,
                .alloc_flags = alloc_flags,
                .classzone_idx = classzone_idx,
                .direct_compaction = true,
                .whole_zone = (prio == MIN_COMPACT_PRIORITY),
                .ignore_skip_hint = (prio == MIN_COMPACT_PRIORITY),
                .ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY)
        };
        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));

        return ret;
}

int sysctl_extfrag_threshold = 500;

/**
 * try_to_compact_pages - Direct compact to satisfy a high-order allocation
 * @gfp_mask: The GFP mask of the current allocation
 * @order: The order of the current allocation
 * @alloc_flags: The allocation flags of the current allocation
 * @ac: The context of current allocation
 * @mode: The migration mode for async, sync light, or sync migration
 *
 * This is the main entry point for direct page compaction.
 */
enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
                unsigned int alloc_flags, const struct alloc_context *ac,
                enum compact_priority prio)
{
        int may_enter_fs = gfp_mask & __GFP_FS;
        int may_perform_io = gfp_mask & __GFP_IO;
        struct zoneref *z;
        struct zone *zone;
        enum compact_result rc = COMPACT_SKIPPED;

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

        trace_mm_compaction_try_to_compact_pages(order, gfp_mask, prio);

        /* Compact each zone in the list */
        for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx,
                                                                ac->nodemask) {
                enum compact_result status;

                if (prio > MIN_COMPACT_PRIORITY
                                        && compaction_deferred(zone, order)) {
                        rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
                        continue;
                }

                status = compact_zone_order(zone, order, gfp_mask, prio,
                                        alloc_flags, ac_classzone_idx(ac));
                rc = max(status, rc);

                /* The allocation should succeed, stop compacting */
                if (status == COMPACT_SUCCESS) {
                        /*
                         * We think the allocation will succeed in this zone,
                         * but it is not certain, hence the false. The caller
                         * will repeat this with true if allocation indeed
                         * succeeds in this zone.
                         */
                        compaction_defer_reset(zone, order, false);

                        break;
                }

                if (prio != COMPACT_PRIO_ASYNC && (status == COMPACT_COMPLETE ||
                                        status == COMPACT_PARTIAL_SKIPPED))
                        /*
                         * We think that allocation won't succeed in this zone
                         * so we defer compaction there. If it ends up
                         * succeeding after all, it will be reset.
                         */
                        defer_compaction(zone, order);

                /*
                 * We might have stopped compacting due to need_resched() in
                 * async compaction, or due to a fatal signal detected. In that
                 * case do not try further zones
                 */
                if ((prio == COMPACT_PRIO_ASYNC && need_resched())
                                        || fatal_signal_pending(current))
                        break;
        }

        return rc;
}


/* Compact all zones within a node */
static void compact_node(int nid)
{
        pg_data_t *pgdat = NODE_DATA(nid);
        int zoneid;
        struct zone *zone;
        struct compact_control cc = {
                .order = -1,
                .mode = MIGRATE_SYNC,
                .ignore_skip_hint = true,
                .whole_zone = true,
        };


        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);

                compact_zone(zone, &cc);

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

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

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

        for_each_online_node(nid)
                compact_node(nid);
}

/* 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/compact_memory
 */
int sysctl_compaction_handler(struct ctl_table *table, int write,
                        void __user *buffer, size_t *length, loff_t *ppos)
{
        if (write)
                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)
static 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 */

static inline bool kcompactd_work_requested(pg_data_t *pgdat)
{
        return pgdat->kcompactd_max_order > 0 || kthread_should_stop();
}

static bool kcompactd_node_suitable(pg_data_t *pgdat)
{
        int zoneid;
        struct zone *zone;
        enum zone_type classzone_idx = pgdat->kcompactd_classzone_idx;

        for (zoneid = 0; zoneid <= classzone_idx; zoneid++) {
                zone = &pgdat->node_zones[zoneid];

                if (!populated_zone(zone))
                        continue;

                if (compaction_suitable(zone, pgdat->kcompactd_max_order, 0,
                                        classzone_idx) == COMPACT_CONTINUE)
                        return true;
        }

        return false;
}

static void kcompactd_do_work(pg_data_t *pgdat)
{
        /*
         * With no special task, compact all zones so that a page of requested
         * order is allocatable.
         */
        int zoneid;
        struct zone *zone;
        struct compact_control cc = {
                .order = pgdat->kcompactd_max_order,
                .classzone_idx = pgdat->kcompactd_classzone_idx,
                .mode = MIGRATE_SYNC_LIGHT,
                .ignore_skip_hint = true,

        };
        trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
                                                        cc.classzone_idx);
        count_vm_event(KCOMPACTD_WAKE);

        for (zoneid = 0; zoneid <= cc.classzone_idx; zoneid++) {
                int status;

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

                if (compaction_deferred(zone, cc.order))
                        continue;

                if (compaction_suitable(zone, cc.order, 0, zoneid) !=
                                                        COMPACT_CONTINUE)
                        continue;

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

                if (kthread_should_stop())
                        return;
                status = compact_zone(zone, &cc);

                if (status == COMPACT_SUCCESS) {
                        compaction_defer_reset(zone, cc.order, false);
                } else if (status == COMPACT_PARTIAL_SKIPPED || status == COMPACT_COMPLETE) {
                        /*
                         * We use sync migration mode here, so we defer like
                         * sync direct compaction does.
                         */
                        defer_compaction(zone, cc.order);
                }

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

        /*
         * Regardless of success, we are done until woken up next. But remember
         * the requested order/classzone_idx in case it was higher/tighter than
         * our current ones
         */
        if (pgdat->kcompactd_max_order <= cc.order)
                pgdat->kcompactd_max_order = 0;
        if (pgdat->kcompactd_classzone_idx >= cc.classzone_idx)
                pgdat->kcompactd_classzone_idx = pgdat->nr_zones - 1;
}

void wakeup_kcompactd(pg_data_t *pgdat, int order, int classzone_idx)
{
        if (!order)
                return;

        if (pgdat->kcompactd_max_order < order)
                pgdat->kcompactd_max_order = order;

        if (pgdat->kcompactd_classzone_idx > classzone_idx)
                pgdat->kcompactd_classzone_idx = classzone_idx;

        if (!waitqueue_active(&pgdat->kcompactd_wait))
                return;

        if (!kcompactd_node_suitable(pgdat))
                return;

        trace_mm_compaction_wakeup_kcompactd(pgdat->node_id, order,
                                                        classzone_idx);
        wake_up_interruptible(&pgdat->kcompactd_wait);
}

/*
 * The background compaction daemon, started as a kernel thread
 * from the init process.
 */
static int kcompactd(void *p)
{
        pg_data_t *pgdat = (pg_data_t*)p;
        struct task_struct *tsk = current;

        const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);

        if (!cpumask_empty(cpumask))
                set_cpus_allowed_ptr(tsk, cpumask);

        set_freezable();

        pgdat->kcompactd_max_order = 0;
        pgdat->kcompactd_classzone_idx = pgdat->nr_zones - 1;

        while (!kthread_should_stop()) {
                trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
                wait_event_freezable(pgdat->kcompactd_wait,
                                kcompactd_work_requested(pgdat));

                kcompactd_do_work(pgdat);
        }

        return 0;
}

/*
 * This kcompactd start function will be called by init and node-hot-add.
 * On node-hot-add, kcompactd will moved to proper cpus if cpus are hot-added.
 */
int kcompactd_run(int nid)
{
        pg_data_t *pgdat = NODE_DATA(nid);
        int ret = 0;

        if (pgdat->kcompactd)
                return 0;

        pgdat->kcompactd = kthread_run(kcompactd, pgdat, "kcompactd%d", nid);
        if (IS_ERR(pgdat->kcompactd)) {
                pr_err("Failed to start kcompactd on node %d\n", nid);
                ret = PTR_ERR(pgdat->kcompactd);
                pgdat->kcompactd = NULL;
        }
        return ret;
}

/*
 * Called by memory hotplug when all memory in a node is offlined. Caller must
 * hold mem_hotplug_begin/end().
 */
void kcompactd_stop(int nid)
{
        struct task_struct *kcompactd = NODE_DATA(nid)->kcompactd;

        if (kcompactd) {
                kthread_stop(kcompactd);
                NODE_DATA(nid)->kcompactd = NULL;
        }
}

/*
 * It's optimal to keep kcompactd on the same CPUs as their memory, but
 * not required for correctness. So if the last cpu in a node goes
 * away, we get changed to run anywhere: as the first one comes back,
 * restore their cpu bindings.
 */
static int cpu_callback(struct notifier_block *nfb, unsigned long action,
                        void *hcpu)
{
        int nid;

        if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN) {
                for_each_node_state(nid, N_MEMORY) {
                        pg_data_t *pgdat = NODE_DATA(nid);
                        const struct cpumask *mask;

                        mask = cpumask_of_node(pgdat->node_id);

                        if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
                                /* One of our CPUs online: restore mask */
                                set_cpus_allowed_ptr(pgdat->kcompactd, mask);
                }
        }
        return NOTIFY_OK;
}

static int __init kcompactd_init(void)
{
        int nid;

        for_each_node_state(nid, N_MEMORY)
                kcompactd_run(nid);
        hotcpu_notifier(cpu_callback, 0);
        return 0;
}
subsys_initcall(kcompactd_init)

#endif /* CONFIG_COMPACTION */