// SPDX-License-Identifier: GPL-2.0
/*
 * 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/sched/signal.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 <linux/psi.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)

/*
 * Fragmentation score check interval for proactive compaction purposes.
 */
static const unsigned int HPAGE_FRAG_CHECK_INTERVAL_MSEC = 500;

/*
 * Page order with-respect-to which proactive compaction
 * calculates external fragmentation, which is used as
 * the "fragmentation score" of a node/zone.
 */
#if defined CONFIG_TRANSPARENT_HUGEPAGE
#define COMPACTION_HPAGE_ORDER  HPAGE_PMD_ORDER
#elif defined CONFIG_HUGETLBFS
#define COMPACTION_HPAGE_ORDER  HUGETLB_PAGE_ORDER
#else
#define COMPACTION_HPAGE_ORDER  (PMD_SHIFT - PAGE_SHIFT)
#endif

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 split_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);
}

#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(!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_shift, compactions are skipped up
 * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT
 */
static 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 */
static 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;
                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 */
static 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);
}

/*
 * Compound pages of >= pageblock_order should consistently be skipped until
 * released. It is always pointless to compact pages of such order (if they are
 * migratable), and the pageblocks they occupy cannot contain any free pages.
 */
static bool pageblock_skip_persistent(struct page *page)
{
        if (!PageCompound(page))
                return false;

        page = compound_head(page);

        if (compound_order(page) >= pageblock_order)
                return true;

        return false;
}

static bool
__reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
                                                        bool check_target)
{
        struct page *page = pfn_to_online_page(pfn);
        struct page *block_page;
        struct page *end_page;
        unsigned long block_pfn;

        if (!page)
                return false;
        if (zone != page_zone(page))
                return false;
        if (pageblock_skip_persistent(page))
                return false;

        /*
         * If skip is already cleared do no further checking once the
         * restart points have been set.
         */
        if (check_source && check_target && !get_pageblock_skip(page))
                return true;

        /*
         * If clearing skip for the target scanner, do not select a
         * non-movable pageblock as the starting point.
         */
        if (!check_source && check_target &&
            get_pageblock_migratetype(page) != MIGRATE_MOVABLE)
                return false;

        /* Ensure the start of the pageblock or zone is online and valid */
        block_pfn = pageblock_start_pfn(pfn);
        block_pfn = max(block_pfn, zone->zone_start_pfn);
        block_page = pfn_to_online_page(block_pfn);
        if (block_page) {
                page = block_page;
                pfn = block_pfn;
        }

        /* Ensure the end of the pageblock or zone is online and valid */
        block_pfn = pageblock_end_pfn(pfn) - 1;
        block_pfn = min(block_pfn, zone_end_pfn(zone) - 1);
        end_page = pfn_to_online_page(block_pfn);
        if (!end_page)
                return false;

        /*
         * Only clear the hint if a sample indicates there is either a
         * free page or an LRU page in the block. One or other condition
         * is necessary for the block to be a migration source/target.
         */
        do {
                if (pfn_valid_within(pfn)) {
                        if (check_source && PageLRU(page)) {
                                clear_pageblock_skip(page);
                                return true;
                        }

                        if (check_target && PageBuddy(page)) {
                                clear_pageblock_skip(page);
                                return true;
                        }
                }

                page += (1 << PAGE_ALLOC_COSTLY_ORDER);
                pfn += (1 << PAGE_ALLOC_COSTLY_ORDER);
        } while (page <= end_page);

        return false;
}

/*
 * 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 migrate_pfn = zone->zone_start_pfn;
        unsigned long free_pfn = zone_end_pfn(zone) - 1;
        unsigned long reset_migrate = free_pfn;
        unsigned long reset_free = migrate_pfn;
        bool source_set = false;
        bool free_set = false;

        if (!zone->compact_blockskip_flush)
                return;

        zone->compact_blockskip_flush = false;

        /*
         * Walk the zone and update pageblock skip information. Source looks
         * for PageLRU while target looks for PageBuddy. When the scanner
         * is found, both PageBuddy and PageLRU are checked as the pageblock
         * is suitable as both source and target.
         */
        for (; migrate_pfn < free_pfn; migrate_pfn += pageblock_nr_pages,
                                        free_pfn -= pageblock_nr_pages) {
                cond_resched();

                /* Update the migrate PFN */
                if (__reset_isolation_pfn(zone, migrate_pfn, true, source_set) &&
                    migrate_pfn < reset_migrate) {
                        source_set = true;
                        reset_migrate = migrate_pfn;
                        zone->compact_init_migrate_pfn = reset_migrate;
                        zone->compact_cached_migrate_pfn[0] = reset_migrate;
                        zone->compact_cached_migrate_pfn[1] = reset_migrate;
                }

                /* Update the free PFN */
                if (__reset_isolation_pfn(zone, free_pfn, free_set, true) &&
                    free_pfn > reset_free) {
                        free_set = true;
                        reset_free = free_pfn;
                        zone->compact_init_free_pfn = reset_free;
                        zone->compact_cached_free_pfn = reset_free;
                }
        }

        /* Leave no distance if no suitable block was reset */
        if (reset_migrate >= reset_free) {
                zone->compact_cached_migrate_pfn[0] = migrate_pfn;
                zone->compact_cached_migrate_pfn[1] = migrate_pfn;
                zone->compact_cached_free_pfn = free_pfn;
        }
}

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);
        }
}

/*
 * Sets the pageblock skip bit if it was clear. Note that this is a hint as
 * locks are not required for read/writers. Returns true if it was already set.
 */
static bool test_and_set_skip(struct compact_control *cc, struct page *page,
                                                        unsigned long pfn)
{
        bool skip;

        /* Do no update if skip hint is being ignored */
        if (cc->ignore_skip_hint)
                return false;

        if (!IS_ALIGNED(pfn, pageblock_nr_pages))
                return false;

        skip = get_pageblock_skip(page);
        if (!skip && !cc->no_set_skip_hint)
                set_pageblock_skip(page);

        return skip;
}

static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
{
        struct zone *zone = cc->zone;

        pfn = pageblock_end_pfn(pfn);

        /* Set for isolation rather than compaction */
        if (cc->no_set_skip_hint)
                return;

        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;
}

/*
 * 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 pfn)
{
        struct zone *zone = cc->zone;

        if (cc->no_set_skip_hint)
                return;

        if (!page)
                return;

        set_pageblock_skip(page);

        /* Update where async and sync compaction should restart */
        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 inline bool pageblock_skip_persistent(struct page *page)
{
        return false;
}

static inline void update_pageblock_skip(struct compact_control *cc,
                        struct page *page, unsigned long pfn)
{
}

static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
{
}

static bool test_and_set_skip(struct compact_control *cc, struct page *page,
                                                        unsigned long pfn)
{
        return false;
}
#endif /* CONFIG_COMPACTION */

/*
 * Compaction requires the taking of some coarse locks that are potentially
 * very heavily contended. For async compaction, trylock and record if the
 * lock is contended. The lock will still be acquired but compaction will
 * abort when the current block is finished regardless of success rate.
 * Sync compaction acquires the lock.
 *
 * Always returns true which makes it easier to track lock state in callers.
 */
static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
                                                struct compact_control *cc)
        __acquires(lock)
{
        /* Track if the lock is contended in async mode */
        if (cc->mode == MIGRATE_ASYNC && !cc->contended) {
                if (spin_trylock_irqsave(lock, *flags))
                        return true;

                cc->contended = true;
        }

        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;
        }

        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,
                                unsigned int stride,
                                bool strict)
{
        int nr_scanned = 0, total_isolated = 0;
        struct page *cursor;
        unsigned long flags = 0;
        bool locked = false;
        unsigned long blockpfn = *start_pfn;
        unsigned int order;

        /* Strict mode is for isolation, speed is secondary */
        if (strict)
                stride = 1;

        cursor = pfn_to_page(blockpfn);

        /* Isolate free pages. */
        for (; blockpfn < end_pfn; blockpfn += stride, cursor += stride) {
                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;

                /*
                 * 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)) {
                        const unsigned int order = compound_order(page);

                        if (likely(order < MAX_ORDER)) {
                                blockpfn += (1UL << order) - 1;
                                cursor += (1UL << 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) {
                        locked = compact_lock_irqsave(&cc->zone->lock,
                                                                &flags, cc);

                        /* 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 = buddy_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;

        cc->total_free_scanned += nr_scanned;
        if (total_isolated)
                count_compact_events(COMPACTISOLATED, total_isolated);
        return total_isolated;
}

/**
 * isolate_freepages_range() - isolate free pages.
 * @cc:        Compaction control structure.
 * @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, 0, 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 */
        split_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(pg_data_t *pgdat)
{
        unsigned long active, inactive, isolated;

        inactive = node_page_state(pgdat, NR_INACTIVE_FILE) +
                        node_page_state(pgdat, NR_INACTIVE_ANON);
        active = node_page_state(pgdat, NR_ACTIVE_FILE) +
                        node_page_state(pgdat, NR_ACTIVE_ANON);
        isolated = node_page_state(pgdat, NR_ISOLATED_FILE) +
                        node_page_state(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 errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
 * -ENOMEM in case we could not allocate a page, or 0.
 * cc->migrate_pfn will contain the next pfn to scan.
 *
 * The pages are isolated on cc->migratepages list (not required to be empty),
 * and cc->nr_migratepages is updated accordingly.
 */
static int
isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
                        unsigned long end_pfn, isolate_mode_t isolate_mode)
{
        pg_data_t *pgdat = cc->zone->zone_pgdat;
        unsigned long nr_scanned = 0, nr_isolated = 0;
        struct lruvec *lruvec;
        unsigned long flags = 0;
        struct lruvec *locked = NULL;
        struct page *page = NULL, *valid_page = NULL;
        unsigned long start_pfn = low_pfn;
        bool skip_on_failure = false;
        unsigned long next_skip_pfn = 0;
        bool skip_updated = false;
        int ret = 0;

        cc->migrate_pfn = low_pfn;

        /*
         * 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(pgdat))) {
                /* stop isolation if there are still pages not migrated */
                if (cc->nr_migratepages)
                        return -EAGAIN;

                /* async migration should just abort */
                if (cc->mode == MIGRATE_ASYNC)
                        return -EAGAIN;

                congestion_wait(BLK_RW_ASYNC, HZ/10);

                if (fatal_signal_pending(current))
                        return -EINTR;
        }

        cond_resched();

        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 completely if
                 * a fatal signal is pending.
                 */
                if (!(low_pfn % SWAP_CLUSTER_MAX)) {
                        if (locked) {
                                unlock_page_lruvec_irqrestore(locked, flags);
                                locked = NULL;
                        }

                        if (fatal_signal_pending(current)) {
                                cc->contended = true;
                                ret = -EINTR;

                                goto fatal_pending;
                        }

                        cond_resched();
                }

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

                page = pfn_to_page(low_pfn);

                /*
                 * Check if the pageblock has already been marked skipped.
                 * Only the aligned PFN is checked as the caller isolates
                 * COMPACT_CLUSTER_MAX at a time so the second call must
                 * not falsely conclude that the block should be skipped.
                 */
                if (!valid_page && IS_ALIGNED(low_pfn, pageblock_nr_pages)) {
                        if (!cc->ignore_skip_hint && get_pageblock_skip(page)) {
                                low_pfn = end_pfn;
                                page = NULL;
                                goto isolate_abort;
                        }
                        valid_page = page;
                }

                if (PageHuge(page) && cc->alloc_contig) {
                        ret = isolate_or_dissolve_huge_page(page, &cc->migratepages);

                        /*
                         * Fail isolation in case isolate_or_dissolve_huge_page()
                         * reports an error. In case of -ENOMEM, abort right away.
                         */
                        if (ret < 0) {
                                 /* Do not report -EBUSY down the chain */
                                if (ret == -EBUSY)
                                        ret = 0;
                                low_pfn += (1UL << compound_order(page)) - 1;
                                goto isolate_fail;
                        }

                        if (PageHuge(page)) {
                                /*
                                 * Hugepage was successfully isolated and placed
                                 * on the cc->migratepages list.
                                 */
                                low_pfn += compound_nr(page) - 1;
                                goto isolate_success_no_list;
                        }

                        /*
                         * Ok, the hugepage was dissolved. Now these pages are
                         * Buddy and cannot be re-allocated because they are
                         * isolated. Fall-through as the check below handles
                         * Buddy pages.
                         */
                }

                /*
                 * 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 = buddy_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 unless we are attempting
                 * an allocation much larger than the huge page size (eg CMA).
                 * 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) && !cc->alloc_contig) {
                        const unsigned int order = compound_order(page);

                        if (likely(order < MAX_ORDER))
                                low_pfn += (1UL << 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) {
                                        unlock_page_lruvec_irqrestore(locked, flags);
                                        locked = NULL;
                                }

                                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;

                /*
                 * Only allow to migrate anonymous pages in GFP_NOFS context
                 * because those do not depend on fs locks.
                 */
                if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
                        goto isolate_fail;

                /*
                 * Be careful not to clear PageLRU until after we're
                 * sure the page is not being freed elsewhere -- the
                 * page release code relies on it.
                 */
                if (unlikely(!get_page_unless_zero(page)))
                        goto isolate_fail;

                if (!__isolate_lru_page_prepare(page, isolate_mode))
                        goto isolate_fail_put;

                /* Try isolate the page */
                if (!TestClearPageLRU(page))
                        goto isolate_fail_put;

                lruvec = mem_cgroup_page_lruvec(page);

                /* If we already hold the lock, we can skip some rechecking */
                if (lruvec != locked) {
                        if (locked)
                                unlock_page_lruvec_irqrestore(locked, flags);

                        compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);
                        locked = lruvec;

                        lruvec_memcg_debug(lruvec, page);

                        /* Try get exclusive access under lock */
                        if (!skip_updated) {
                                skip_updated = true;
                                if (test_and_set_skip(cc, page, low_pfn))
                                        goto isolate_abort;
                        }

                        /*
                         * 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) && !cc->alloc_contig)) {
                                low_pfn += compound_nr(page) - 1;
                                SetPageLRU(page);
                                goto isolate_fail_put;
                        }
                }

                /* The whole page is taken off the LRU; skip the tail pages. */
                if (PageCompound(page))
                        low_pfn += compound_nr(page) - 1;

                /* Successfully isolated */
                del_page_from_lru_list(page, lruvec);
                mod_node_page_state(page_pgdat(page),
                                NR_ISOLATED_ANON + page_is_file_lru(page),
                                thp_nr_pages(page));

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

                /*
                 * Avoid isolating too much unless this block is being
                 * rescanned (e.g. dirty/writeback pages, parallel allocation)
                 * or a lock is contended. For contention, isolate quickly to
                 * potentially remove one source of contention.
                 */
                if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX &&
                    !cc->rescan && !cc->contended) {
                        ++low_pfn;
                        break;
                }

                continue;

isolate_fail_put:
                /* Avoid potential deadlock in freeing page under lru_lock */
                if (locked) {
                        unlock_page_lruvec_irqrestore(locked, flags);
                        locked = NULL;
                }
                put_page(page);

isolate_fail:
                if (!skip_on_failure && ret != -ENOMEM)
                        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) {
                                unlock_page_lruvec_irqrestore(locked, flags);
                                locked = NULL;
                        }
                        putback_movable_pages(&cc->migratepages);
                        cc->nr_migratepages = 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;
                }

                if (ret == -ENOMEM)
                        break;
        }

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

        page = NULL;

isolate_abort:
        if (locked)
                unlock_page_lruvec_irqrestore(locked, flags);
        if (page) {
                SetPageLRU(page);
                put_page(page);
        }

        /*
         * Updated the cached scanner pfn once the pageblock has been scanned
         * Pages will either be migrated in which case there is no point
         * scanning in the near future or migration failed in which case the
         * failure reason may persist. The block is marked for skipping if
         * there were no pages isolated in the block or if the block is
         * rescanned twice in a row.
         */
        if (low_pfn == end_pfn && (!nr_isolated || cc->rescan)) {
                if (valid_page && !skip_updated)
                        set_pageblock_skip(valid_page);
                update_cached_migrate(cc, low_pfn);
        }

        trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn,
                                                nr_scanned, nr_isolated);

fatal_pending:
        cc->total_migrate_scanned += nr_scanned;
        if (nr_isolated)
                count_compact_events(COMPACTISOLATED, nr_isolated);

        cc->migrate_pfn = low_pfn;

        return ret;
}

/**
 * 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 -EAGAIN when contented, -EINTR in case of a signal pending, -ENOMEM
 * in case we could not allocate a page, or 0.
 */
int
isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
                                                        unsigned long end_pfn)
{
        unsigned long pfn, block_start_pfn, block_end_pfn;
        int ret = 0;

        /* 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;

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

                if (ret)
                        break;

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

        return ret;
}

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

static bool suitable_migration_source(struct compact_control *cc,
                                                        struct page *page)
{
        int block_mt;

        if (pageblock_skip_persistent(page))
                return false;

        if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
                return true;

        block_mt = get_pageblock_migratetype(page);

        if (cc->migratetype == MIGRATE_MOVABLE)
                return is_migrate_movable(block_mt);
        else
                return block_mt == cc->migratetype;
}

/* 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 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 (buddy_order_unsafe(page) >= pageblock_order)
                        return false;
        }

        if (cc->ignore_block_suitable)
                return true;

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

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

static inline unsigned int
freelist_scan_limit(struct compact_control *cc)
{
        unsigned short shift = BITS_PER_LONG - 1;

        return (COMPACT_CLUSTER_MAX >> min(shift, cc->fast_search_fail)) + 1;
}

/*
 * 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);
}

/*
 * Used when scanning for a suitable migration target which scans freelists
 * in reverse. Reorders the list such as the unscanned pages are scanned
 * first on the next iteration of the free scanner
 */
static void
move_freelist_head(struct list_head *freelist, struct page *freepage)
{
        LIST_HEAD(sublist);

        if (!list_is_last(freelist, &freepage->lru)) {
                list_cut_before(&sublist, freelist, &freepage->lru);
                list_splice_tail(&sublist, freelist);
        }
}

/*
 * Similar to move_freelist_head except used by the migration scanner
 * when scanning forward. It's possible for these list operations to
 * move against each other if they search the free list exactly in
 * lockstep.
 */
static void
move_freelist_tail(struct list_head *freelist, struct page *freepage)
{
        LIST_HEAD(sublist);

        if (!list_is_first(freelist, &freepage->lru)) {
                list_cut_position(&sublist, freelist, &freepage->lru);
                list_splice_tail(&sublist, freelist);
        }
}

static void
fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long nr_isolated)
{
        unsigned long start_pfn, end_pfn;
        struct page *page;

        /* Do not search around if there are enough pages already */
        if (cc->nr_freepages >= cc->nr_migratepages)
                return;

        /* Minimise scanning during async compaction */
        if (cc->direct_compaction && cc->mode == MIGRATE_ASYNC)
                return;

        /* Pageblock boundaries */
        start_pfn = max(pageblock_start_pfn(pfn), cc->zone->zone_start_pfn);
        end_pfn = min(pageblock_end_pfn(pfn), zone_end_pfn(cc->zone));

        page = pageblock_pfn_to_page(start_pfn, end_pfn, cc->zone);
        if (!page)
                return;

        /* Scan before */
        if (start_pfn != pfn) {
                isolate_freepages_block(cc, &start_pfn, pfn, &cc->freepages, 1, false);
                if (cc->nr_freepages >= cc->nr_migratepages)
                        return;
        }

        /* Scan after */
        start_pfn = pfn + nr_isolated;
        if (start_pfn < end_pfn)
                isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);

        /* Skip this pageblock in the future as it's full or nearly full */
        if (cc->nr_freepages < cc->nr_migratepages)
                set_pageblock_skip(page);
}

/* Search orders in round-robin fashion */
static int next_search_order(struct compact_control *cc, int order)
{
        order--;
        if (order < 0)
                order = cc->order - 1;

        /* Search wrapped around? */
        if (order == cc->search_order) {
                cc->search_order--;
                if (cc->search_order < 0)
                        cc->search_order = cc->order - 1;
                return -1;
        }

        return order;
}

static unsigned long
fast_isolate_freepages(struct compact_control *cc)
{
        unsigned int limit = max(1U, freelist_scan_limit(cc) >> 1);
        unsigned int nr_scanned = 0;
        unsigned long low_pfn, min_pfn, highest = 0;
        unsigned long nr_isolated = 0;
        unsigned long distance;
        struct page *page = NULL;
        bool scan_start = false;
        int order;

        /* Full compaction passes in a negative order */
        if (cc->order <= 0)
                return cc->free_pfn;

        /*
         * If starting the scan, use a deeper search and use the highest
         * PFN found if a suitable one is not found.
         */
        if (cc->free_pfn >= cc->zone->compact_init_free_pfn) {
                limit = pageblock_nr_pages >> 1;
                scan_start = true;
        }

        /*
         * Preferred point is in the top quarter of the scan space but take
         * a pfn from the top half if the search is problematic.
         */
        distance = (cc->free_pfn - cc->migrate_pfn);
        low_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 2));
        min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1));

        if (WARN_ON_ONCE(min_pfn > low_pfn))
                low_pfn = min_pfn;

        /*
         * Search starts from the last successful isolation order or the next
         * order to search after a previous failure
         */
        cc->search_order = min_t(unsigned int, cc->order - 1, cc->search_order);

        for (order = cc->search_order;
             !page && order >= 0;
             order = next_search_order(cc, order)) {
                struct free_area *area = &cc->zone->free_area[order];
                struct list_head *freelist;
                struct page *freepage;
                unsigned long flags;
                unsigned int order_scanned = 0;
                unsigned long high_pfn = 0;

                if (!area->nr_free)
                        continue;

                spin_lock_irqsave(&cc->zone->lock, flags);
                freelist = &area->free_list[MIGRATE_MOVABLE];
                list_for_each_entry_reverse(freepage, freelist, lru) {
                        unsigned long pfn;

                        order_scanned++;
                        nr_scanned++;
                        pfn = page_to_pfn(freepage);

                        if (pfn >= highest)
                                highest = max(pageblock_start_pfn(pfn),
                                              cc->zone->zone_start_pfn);

                        if (pfn >= low_pfn) {
                                cc->fast_search_fail = 0;
                                cc->search_order = order;
                                page = freepage;
                                break;
                        }

                        if (pfn >= min_pfn && pfn > high_pfn) {
                                high_pfn = pfn;

                                /* Shorten the scan if a candidate is found */
                                limit >>= 1;
                        }

                        if (order_scanned >= limit)
                                break;
                }

                /* Use a minimum pfn if a preferred one was not found */
                if (!page && high_pfn) {
                        page = pfn_to_page(high_pfn);

                        /* Update freepage for the list reorder below */
                        freepage = page;
                }

                /* Reorder to so a future search skips recent pages */
                move_freelist_head(freelist, freepage);

                /* Isolate the page if available */
                if (page) {
                        if (__isolate_free_page(page, order)) {
                                set_page_private(page, order);
                                nr_isolated = 1 << order;
                                cc->nr_freepages += nr_isolated;
                                list_add_tail(&page->lru, &cc->freepages);
                                count_compact_events(COMPACTISOLATED, nr_isolated);
                        } else {
                                /* If isolation fails, abort the search */
                                order = cc->search_order + 1;
                                page = NULL;
                        }
                }

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

                /*
                 * Smaller scan on next order so the total scan is related
                 * to freelist_scan_limit.
                 */
                if (order_scanned >= limit)
                        limit = max(1U, limit >> 1);
        }

        if (!page) {
                cc->fast_search_fail++;
                if (scan_start) {
                        /*
                         * Use the highest PFN found above min. If one was
                         * not found, be pessimistic for direct compaction
                         * and use the min mark.
                         */
                        if (highest) {
                                page = pfn_to_page(highest);
                                cc->free_pfn = highest;
                        } else {
                                if (cc->direct_compaction && pfn_valid(min_pfn)) {
                                        page = pageblock_pfn_to_page(min_pfn,
                                                min(pageblock_end_pfn(min_pfn),
                                                    zone_end_pfn(cc->zone)),
                                                cc->zone);
                                        cc->free_pfn = min_pfn;
                                }
                        }
                }
        }

        if (highest && highest >= cc->zone->compact_cached_free_pfn) {
                highest -= pageblock_nr_pages;
                cc->zone->compact_cached_free_pfn = highest;
        }

        cc->total_free_scanned += nr_scanned;
        if (!page)
                return cc->free_pfn;

        low_pfn = page_to_pfn(page);
        fast_isolate_around(cc, low_pfn, nr_isolated);
        return low_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 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;
        unsigned int stride;

        /* Try a small search of the free lists for a candidate */
        isolate_start_pfn = fast_isolate_freepages(cc);
        if (cc->nr_freepages)
                goto splitmap;

        /*
         * 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
         * 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(isolate_start_pfn);
        block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
                                                zone_end_pfn(zone));
        low_pfn = pageblock_end_pfn(cc->migrate_pfn);
        stride = cc->mode == MIGRATE_ASYNC ? COMPACT_CLUSTER_MAX : 1;

        /*
         * 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) {
                unsigned long nr_isolated;

                /*
                 * This can iterate a massively long zone without finding any
                 * suitable migration targets, so periodically check resched.
                 */
                if (!(block_start_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages)))
                        cond_resched();

                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. */
                nr_isolated = isolate_freepages_block(cc, &isolate_start_pfn,
                                        block_end_pfn, freelist, stride, false);

                /* Update the skip hint if the full pageblock was scanned */
                if (isolate_start_pfn == block_end_pfn)
                        update_pageblock_skip(cc, page, block_start_pfn);

                /* Are enough freepages isolated? */
                if (cc->nr_freepages >= cc->nr_migratepages) {
                        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;
                }

                /* Adjust stride depending on isolation */
                if (nr_isolated) {
                        stride = 1;
                        continue;
                }
                stride = min_t(unsigned int, COMPACT_CLUSTER_MAX, stride << 1);
        }

        /*
         * 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;

splitmap:
        /* __isolate_free_page() does not map the pages */
        split_map_pages(freelist);
}

/*
 * 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)
{
        struct compact_control *cc = (struct compact_control *)data;
        struct page *freepage;

        if (list_empty(&cc->freepages)) {
                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.
 */
#ifdef CONFIG_PREEMPT_RT
int sysctl_compact_unevictable_allowed __read_mostly = 0;
#else
int sysctl_compact_unevictable_allowed __read_mostly = 1;
#endif

static inline void
update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
{
        if (cc->fast_start_pfn == ULONG_MAX)
                return;

        if (!cc->fast_start_pfn)
                cc->fast_start_pfn = pfn;

        cc->fast_start_pfn = min(cc->fast_start_pfn, pfn);
}

static inline unsigned long
reinit_migrate_pfn(struct compact_control *cc)
{
        if (!cc->fast_start_pfn || cc->fast_start_pfn == ULONG_MAX)
                return cc->migrate_pfn;

        cc->migrate_pfn = cc->fast_start_pfn;
        cc->fast_start_pfn = ULONG_MAX;

        return cc->migrate_pfn;
}

/*
 * Briefly search the free lists for a migration source that already has
 * some free pages to reduce the number of pages that need migration
 * before a pageblock is free.
 */
static unsigned long fast_find_migrateblock(struct compact_control *cc)
{
        unsigned int limit = freelist_scan_limit(cc);
        unsigned int nr_scanned = 0;
        unsigned long distance;
        unsigned long pfn = cc->migrate_pfn;
        unsigned long high_pfn;
        int order;
        bool found_block = false;

        /* Skip hints are relied on to avoid repeats on the fast search */
        if (cc->ignore_skip_hint)
                return pfn;

        /*
         * If the migrate_pfn is not at the start of a zone or the start
         * of a pageblock then assume this is a continuation of a previous
         * scan restarted due to COMPACT_CLUSTER_MAX.
         */
        if (pfn != cc->zone->zone_start_pfn && pfn != pageblock_start_pfn(pfn))
                return pfn;

        /*
         * For smaller orders, just linearly scan as the number of pages
         * to migrate should be relatively small and does not necessarily
         * justify freeing up a large block for a small allocation.
         */
        if (cc->order <= PAGE_ALLOC_COSTLY_ORDER)
                return pfn;

        /*
         * Only allow kcompactd and direct requests for movable pages to
         * quickly clear out a MOVABLE pageblock for allocation. This
         * reduces the risk that a large movable pageblock is freed for
         * an unmovable/reclaimable small allocation.
         */
        if (cc->direct_compaction && cc->migratetype != MIGRATE_MOVABLE)
                return pfn;

        /*
         * When starting the migration scanner, pick any pageblock within the
         * first half of the search space. Otherwise try and pick a pageblock
         * within the first eighth to reduce the chances that a migration
         * target later becomes a source.
         */
        distance = (cc->free_pfn - cc->migrate_pfn) >> 1;
        if (cc->migrate_pfn != cc->zone->zone_start_pfn)
                distance >>= 2;
        high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance);

        for (order = cc->order - 1;
             order >= PAGE_ALLOC_COSTLY_ORDER && !found_block && nr_scanned < limit;
             order--) {
                struct free_area *area = &cc->zone->free_area[order];
                struct list_head *freelist;
                unsigned long flags;
                struct page *freepage;

                if (!area->nr_free)
                        continue;

                spin_lock_irqsave(&cc->zone->lock, flags);
                freelist = &area->free_list[MIGRATE_MOVABLE];
                list_for_each_entry(freepage, freelist, lru) {
                        unsigned long free_pfn;

                        if (nr_scanned++ >= limit) {
                                move_freelist_tail(freelist, freepage);
                                break;
                        }

                        free_pfn = page_to_pfn(freepage);
                        if (free_pfn < high_pfn) {
                                /*
                                 * Avoid if skipped recently. Ideally it would
                                 * move to the tail but even safe iteration of
                                 * the list assumes an entry is deleted, not
                                 * reordered.
                                 */
                                if (get_pageblock_skip(freepage))
                                        continue;

                                /* Reorder to so a future search skips recent pages */
                                move_freelist_tail(freelist, freepage);

                                update_fast_start_pfn(cc, free_pfn);
                                pfn = pageblock_start_pfn(free_pfn);
                                cc->fast_search_fail = 0;
                                found_block = true;
                                set_pageblock_skip(freepage);
                                break;
                        }
                }
                spin_unlock_irqrestore(&cc->zone->lock, flags);
        }

        cc->total_migrate_scanned += nr_scanned;

        /*
         * If fast scanning failed then use a cached entry for a page block
         * that had free pages as the basis for starting a linear scan.
         */
        if (!found_block) {
                cc->fast_search_fail++;
                pfn = reinit_migrate_pfn(cc);
        }
        return pfn;
}

/*
 * 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 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);
        bool fast_find_block;

        /*
         * Start at where we last stopped, or beginning of the zone as
         * initialized by compact_zone(). The first failure will use
         * the lowest PFN as the starting point for linear scanning.
         */
        low_pfn = fast_find_migrateblock(cc);
        block_start_pfn = pageblock_start_pfn(low_pfn);
        if (block_start_pfn < cc->zone->zone_start_pfn)
                block_start_pfn = cc->zone->zone_start_pfn;

        /*
         * fast_find_migrateblock marks a pageblock skipped so to avoid
         * the isolation_suitable check below, check whether the fast
         * search was successful.
         */
        fast_find_block = low_pfn != cc->migrate_pfn && !cc->fast_search_fail;

        /* 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;
                        fast_find_block = false,
                        cc->migrate_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.
                 */
                if (!(low_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages)))
                        cond_resched();

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

                /*
                 * If isolation recently failed, do not retry. Only check the
                 * pageblock once. COMPACT_CLUSTER_MAX causes a pageblock
                 * to be visited multiple times. Assume skip was checked
                 * before making it "skip" so other compaction instances do
                 * not scan the same block.
                 */
                if (IS_ALIGNED(low_pfn, pageblock_nr_pages) &&
                    !fast_find_block && !isolation_suitable(cc, page))
                        continue;

                /*
                 * For async compaction, also only scan in MOVABLE blocks
                 * without huge pages. Async compaction is optimistic to see
                 * if the minimum amount of work satisfies the allocation.
                 * The cached PFN is updated as it's possible that all
                 * remaining blocks between source and target are unsuitable
                 * and the compaction scanners fail to meet.
                 */
                if (!suitable_migration_source(cc, page)) {
                        update_cached_migrate(cc, block_end_pfn);
                        continue;
                }

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

        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 bool kswapd_is_running(pg_data_t *pgdat)
{
        return pgdat->kswapd && task_is_running(pgdat->kswapd);
}

/*
 * A zone's fragmentation score is the external fragmentation wrt to the
 * COMPACTION_HPAGE_ORDER. It returns a value in the range [0, 100].
 */
static unsigned int fragmentation_score_zone(struct zone *zone)
{
        return extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
}

/*
 * A weighted zone's fragmentation score is the external fragmentation
 * wrt to the COMPACTION_HPAGE_ORDER scaled by the zone's size. It
 * returns a value in the range [0, 100].
 *
 * The scaling factor ensures that proactive compaction focuses on larger
 * zones like ZONE_NORMAL, rather than smaller, specialized zones like
 * ZONE_DMA32. For smaller zones, the score value remains close to zero,
 * and thus never exceeds the high threshold for proactive compaction.
 */
static unsigned int fragmentation_score_zone_weighted(struct zone *zone)
{
        unsigned long score;

        score = zone->present_pages * fragmentation_score_zone(zone);
        return div64_ul(score, zone->zone_pgdat->node_present_pages + 1);
}

/*
 * The per-node proactive (background) compaction process is started by its
 * corresponding kcompactd thread when the node's fragmentation score
 * exceeds the high threshold. The compaction process remains active till
 * the node's score falls below the low threshold, or one of the back-off
 * conditions is met.
 */
static unsigned int fragmentation_score_node(pg_data_t *pgdat)
{
        unsigned int score = 0;
        int zoneid;

        for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
                struct zone *zone;