vmscan.c

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/*
 *  linux/mm/vmscan.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *
 *  Swap reorganised 29.12.95, Stephen Tweedie.
 *  kswapd added: 7.1.96  sct
 *  Removed kswapd_ctl limits, and swap out as many pages as needed
 *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
 *  Zone aware kswapd started 02/00, Kanoj Sarcar ([email protected]).
 *  Multiqueue VM started 5.8.00, Rik van Riel.
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/gfp.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/pagemap.h>
#include <linux/init.h>
#include <linux/highmem.h>
#include <linux/vmstat.h>
#include <linux/file.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>  /* for try_to_release_page(),
                    buffer_heads_over_limit */
#include <linux/mm_inline.h>
#include <linux/backing-dev.h>
#include <linux/rmap.h>
#include <linux/topology.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/compaction.h>
#include <linux/notifier.h>
#include <linux/rwsem.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/memcontrol.h>
#include <linux/delayacct.h>
#include <linux/sysctl.h>
#include <linux/oom.h>
#include <linux/prefetch.h>

#include <asm/tlbflush.h>
#include <asm/div64.h>

#include <linux/swapops.h>

#include "internal.h"

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

struct scan_control {
    /* Incremented by the number of inactive pages that were scanned */
    unsigned long nr_scanned;

    /* Number of pages freed so far during a call to shrink_zones() */
    unsigned long nr_reclaimed;

    /* How many pages shrink_list() should reclaim */
    unsigned long nr_to_reclaim;

    unsigned long hibernation_mode;

    /* This context's GFP mask */
    gfp_t gfp_mask;

    int may_writepage;

    /* Can mapped pages be reclaimed? */
    int may_unmap;

    /* Can pages be swapped as part of reclaim? */
    int may_swap;

    int order;

    /* Scan (total_size >> priority) pages at once */
    int priority;

    /*
     * The memory cgroup that hit its limit and as a result is the
     * primary target of this reclaim invocation.
     */
    struct mem_cgroup *target_mem_cgroup;

    /*
     * Nodemask of nodes allowed by the caller. If NULL, all nodes
     * are scanned.
     */
    nodemask_t  *nodemask;
};

#define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))

#ifdef ARCH_HAS_PREFETCH
#define prefetch_prev_lru_page(_page, _base, _field)            \
    do {                                \
        if ((_page)->lru.prev != _base) {           \
            struct page *prev;              \
                                    \
            prev = lru_to_page(&(_page->lru));      \
            prefetch(&prev->_field);            \
        }                           \
    } while (0)
#else
#define prefetch_prev_lru_page(_page, _base, _field) do { } while (0)
#endif

#ifdef ARCH_HAS_PREFETCHW
#define prefetchw_prev_lru_page(_page, _base, _field)           \
    do {                                \
        if ((_page)->lru.prev != _base) {           \
            struct page *prev;              \
                                    \
            prev = lru_to_page(&(_page->lru));      \
            prefetchw(&prev->_field);           \
        }                           \
    } while (0)
#else
#define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
#endif

/*
 * From 0 .. 100.  Higher means more swappy.
 */
int vm_swappiness = 60;
long vm_total_pages;    /* The total number of pages which the VM controls */

static LIST_HEAD(shrinker_list);
static DECLARE_RWSEM(shrinker_rwsem);

#ifdef CONFIG_MEMCG
static bool global_reclaim(struct scan_control *sc)
{
    return !sc->target_mem_cgroup;
}
#else
static bool global_reclaim(struct scan_control *sc)
{
    return true;
}
#endif

static unsigned long get_lru_size(struct lruvec *lruvec, enum lru_list lru)
{
    if (!mem_cgroup_disabled())
        return mem_cgroup_get_lru_size(lruvec, lru);

    return zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru);
}

/*
 * Add a shrinker callback to be called from the vm
 */
void register_shrinker(struct shrinker *shrinker)
{
    atomic_long_set(&shrinker->nr_in_batch, 0);
    down_write(&shrinker_rwsem);
    list_add_tail(&shrinker->list, &shrinker_list);
    up_write(&shrinker_rwsem);
}
EXPORT_SYMBOL(register_shrinker);

/*
 * Remove one
 */
void unregister_shrinker(struct shrinker *shrinker)
{
    down_write(&shrinker_rwsem);
    list_del(&shrinker->list);
    up_write(&shrinker_rwsem);
}
EXPORT_SYMBOL(unregister_shrinker);

static inline int do_shrinker_shrink(struct shrinker *shrinker,
                     struct shrink_control *sc,
                     unsigned long nr_to_scan)
{
    sc->nr_to_scan = nr_to_scan;
    return (*shrinker->shrink)(shrinker, sc);
}

#define SHRINK_BATCH 128
/*
 * Call the shrink functions to age shrinkable caches
 *
 * Here we assume it costs one seek to replace a lru page and that it also
 * takes a seek to recreate a cache object.  With this in mind we age equal
 * percentages of the lru and ageable caches.  This should balance the seeks
 * generated by these structures.
 *
 * If the vm encountered mapped pages on the LRU it increase the pressure on
 * slab to avoid swapping.
 *
 * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
 *
 * `lru_pages' represents the number of on-LRU pages in all the zones which
 * are eligible for the caller's allocation attempt.  It is used for balancing
 * slab reclaim versus page reclaim.
 *
 * Returns the number of slab objects which we shrunk.
 */
unsigned long shrink_slab(struct shrink_control *shrink,
              unsigned long nr_pages_scanned,
              unsigned long lru_pages)
{
    struct shrinker *shrinker;
    unsigned long ret = 0;

    if (nr_pages_scanned == 0)
        nr_pages_scanned = SWAP_CLUSTER_MAX;

    if (!down_read_trylock(&shrinker_rwsem)) {
        /* Assume we'll be able to shrink next time */
        ret = 1;
        goto out;
    }

    list_for_each_entry(shrinker, &shrinker_list, list) {
        unsigned long long delta;
        long total_scan;
        long max_pass;
        int shrink_ret = 0;
        long nr;
        long new_nr;
        long batch_size = shrinker->batch ? shrinker->batch
                          : SHRINK_BATCH;

        max_pass = do_shrinker_shrink(shrinker, shrink, 0);
        if (max_pass <= 0)
            continue;

        /*
         * copy the current shrinker scan count into a local variable
         * and zero it so that other concurrent shrinker invocations
         * don't also do this scanning work.
         */
        nr = atomic_long_xchg(&shrinker->nr_in_batch, 0);

        total_scan = nr;
        delta = (4 * nr_pages_scanned) / shrinker->seeks;
        delta *= max_pass;
        do_div(delta, lru_pages + 1);
        total_scan += delta;
        if (total_scan < 0) {
            printk(KERN_ERR "shrink_slab: %pF negative objects to "
                   "delete nr=%ld\n",
                   shrinker->shrink, total_scan);
            total_scan = max_pass;
        }

        /*
         * We need to avoid excessive windup on filesystem shrinkers
         * due to large numbers of GFP_NOFS allocations causing the
         * shrinkers to return -1 all the time. This results in a large
         * nr being built up so when a shrink that can do some work
         * comes along it empties the entire cache due to nr >>>
         * max_pass.  This is bad for sustaining a working set in
         * memory.
         *
         * Hence only allow the shrinker to scan the entire cache when
         * a large delta change is calculated directly.
         */
        if (delta < max_pass / 4)
            total_scan = min(total_scan, max_pass / 2);

        /*
         * Avoid risking looping forever due to too large nr value:
         * never try to free more than twice the estimate number of
         * freeable entries.
         */
        if (total_scan > max_pass * 2)
            total_scan = max_pass * 2;

        trace_mm_shrink_slab_start(shrinker, shrink, nr,
                    nr_pages_scanned, lru_pages,
                    max_pass, delta, total_scan);

        while (total_scan >= batch_size) {
            int nr_before;

            nr_before = do_shrinker_shrink(shrinker, shrink, 0);
            shrink_ret = do_shrinker_shrink(shrinker, shrink,
                            batch_size);
            if (shrink_ret == -1)
                break;
            if (shrink_ret < nr_before)
                ret += nr_before - shrink_ret;
            count_vm_events(SLABS_SCANNED, batch_size);
            total_scan -= batch_size;

            cond_resched();
        }

        /*
         * move the unused scan count back into the shrinker in a
         * manner that handles concurrent updates. If we exhausted the
         * scan, there is no need to do an update.
         */
        if (total_scan > 0)
            new_nr = atomic_long_add_return(total_scan,
                    &shrinker->nr_in_batch);
        else
            new_nr = atomic_long_read(&shrinker->nr_in_batch);

        trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
    }
    up_read(&shrinker_rwsem);
out:
    cond_resched();
    return ret;
}

static inline int is_page_cache_freeable(struct page *page)
{
    /*
     * A freeable page cache page is referenced only by the caller
     * that isolated the page, the page cache radix tree and
     * optional buffer heads at page->private.
     */
    return page_count(page) - page_has_private(page) == 2;
}

static int may_write_to_queue(struct backing_dev_info *bdi,
                  struct scan_control *sc)
{
    if (current->flags & PF_SWAPWRITE)
        return 1;
    if (!bdi_write_congested(bdi))
        return 1;
    if (bdi == current->backing_dev_info)
        return 1;
    return 0;
}

/*
 * We detected a synchronous write error writing a page out.  Probably
 * -ENOSPC.  We need to propagate that into the address_space for a subsequent
 * fsync(), msync() or close().
 *
 * The tricky part is that after writepage we cannot touch the mapping: nothing
 * prevents it from being freed up.  But we have a ref on the page and once
 * that page is locked, the mapping is pinned.
 *
 * We're allowed to run sleeping lock_page() here because we know the caller has
 * __GFP_FS.
 */
static void handle_write_error(struct address_space *mapping,
                struct page *page, int error)
{
    lock_page(page);
    if (page_mapping(page) == mapping)
        mapping_set_error(mapping, error);
    unlock_page(page);
}

/* possible outcome of pageout() */
typedef enum {
    /* failed to write page out, page is locked */
    PAGE_KEEP,
    /* move page to the active list, page is locked */
    PAGE_ACTIVATE,
    /* page has been sent to the disk successfully, page is unlocked */
    PAGE_SUCCESS,
    /* page is clean and locked */
    PAGE_CLEAN,
} pageout_t;

/*
 * pageout is called by shrink_page_list() for each dirty page.
 * Calls ->writepage().
 */
static pageout_t pageout(struct page *page, struct address_space *mapping,
             struct scan_control *sc)
{
    /*
     * If the page is dirty, only perform writeback if that write
     * will be non-blocking.  To prevent this allocation from being
     * stalled by pagecache activity.  But note that there may be
     * stalls if we need to run get_block().  We could test
     * PagePrivate for that.
     *
     * If this process is currently in __generic_file_aio_write() against
     * this page's queue, we can perform writeback even if that
     * will block.
     *
     * If the page is swapcache, write it back even if that would
     * block, for some throttling. This happens by accident, because
     * swap_backing_dev_info is bust: it doesn't reflect the
     * congestion state of the swapdevs.  Easy to fix, if needed.
     */
    if (!is_page_cache_freeable(page))
        return PAGE_KEEP;
    if (!mapping) {
        /*
         * Some data journaling orphaned pages can have
         * page->mapping == NULL while being dirty with clean buffers.
         */
        if (page_has_private(page)) {
            if (try_to_free_buffers(page)) {
                ClearPageDirty(page);
                printk("%s: orphaned page\n", __func__);
                return PAGE_CLEAN;
            }
        }
        return PAGE_KEEP;
    }
    if (mapping->a_ops->writepage == NULL)
        return PAGE_ACTIVATE;
    if (!may_write_to_queue(mapping->backing_dev_info, sc))
        return PAGE_KEEP;

    if (clear_page_dirty_for_io(page)) {
        int res;
        struct writeback_control wbc = {
            .sync_mode = WB_SYNC_NONE,
            .nr_to_write = SWAP_CLUSTER_MAX,
            .range_start = 0,
            .range_end = LLONG_MAX,
            .for_reclaim = 1,
        };

        SetPageReclaim(page);
        res = mapping->a_ops->writepage(page, &wbc);
        if (res < 0)
            handle_write_error(mapping, page, res);
        if (res == AOP_WRITEPAGE_ACTIVATE) {
            ClearPageReclaim(page);
            return PAGE_ACTIVATE;
        }

        if (!PageWriteback(page)) {
            /* synchronous write or broken a_ops? */
            ClearPageReclaim(page);
        }
        trace_mm_vmscan_writepage(page, trace_reclaim_flags(page));
        inc_zone_page_state(page, NR_VMSCAN_WRITE);
        return PAGE_SUCCESS;
    }

    return PAGE_CLEAN;
}

/*
 * Same as remove_mapping, but if the page is removed from the mapping, it
 * gets returned with a refcount of 0.
 */
static int __remove_mapping(struct address_space *mapping, struct page *page)
{
    BUG_ON(!PageLocked(page));
    BUG_ON(mapping != page_mapping(page));

    spin_lock_irq(&mapping->tree_lock);
    /*
     * The non racy check for a busy page.
     *
     * Must be careful with the order of the tests. When someone has
     * a ref to the page, it may be possible that they dirty it then
     * drop the reference. So if PageDirty is tested before page_count
     * here, then the following race may occur:
     *
     * get_user_pages(&page);
     * [user mapping goes away]
     * write_to(page);
     *              !PageDirty(page)    [good]
     * SetPageDirty(page);
     * put_page(page);
     *              !page_count(page)   [good, discard it]
     *
     * [oops, our write_to data is lost]
     *
     * Reversing the order of the tests ensures such a situation cannot
     * escape unnoticed. The smp_rmb is needed to ensure the page->flags
     * load is not satisfied before that of page->_count.
     *
     * Note that if SetPageDirty is always performed via set_page_dirty,
     * and thus under tree_lock, then this ordering is not required.
     */
    if (!page_freeze_refs(page, 2))
        goto cannot_free;
    /* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
    if (unlikely(PageDirty(page))) {
        page_unfreeze_refs(page, 2);
        goto cannot_free;
    }

    if (PageSwapCache(page)) {
        swp_entry_t swap = { .val = page_private(page) };
        __delete_from_swap_cache(page);
        spin_unlock_irq(&mapping->tree_lock);
        swapcache_free(swap, page);
    } else {
        void (*freepage)(struct page *);

        freepage = mapping->a_ops->freepage;

        __delete_from_page_cache(page);
        spin_unlock_irq(&mapping->tree_lock);
        mem_cgroup_uncharge_cache_page(page);

        if (freepage != NULL)
            freepage(page);
    }

    return 1;

cannot_free:
    spin_unlock_irq(&mapping->tree_lock);
    return 0;
}

/*
 * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
 * someone else has a ref on the page, abort and return 0.  If it was
 * successfully detached, return 1.  Assumes the caller has a single ref on
 * this page.
 */
int remove_mapping(struct address_space *mapping, struct page *page)
{
    if (__remove_mapping(mapping, page)) {
        /*
         * Unfreezing the refcount with 1 rather than 2 effectively
         * drops the pagecache ref for us without requiring another
         * atomic operation.
         */
        page_unfreeze_refs(page, 1);
        return 1;
    }
    return 0;
}

void putback_lru_page(struct page *page)
{
    int lru;
    int active = !!TestClearPageActive(page);
    int was_unevictable = PageUnevictable(page);

    VM_BUG_ON(PageLRU(page));

redo:
    ClearPageUnevictable(page);

    if (page_evictable(page)) {
        /*
         * For evictable pages, we can use the cache.
         * In event of a race, worst case is we end up with an
         * unevictable page on [in]active list.
         * We know how to handle that.
         */
        lru = active + page_lru_base_type(page);
        lru_cache_add_lru(page, lru);
    } else {
        /*
         * Put unevictable pages directly on zone's unevictable
         * list.
         */
        lru = LRU_UNEVICTABLE;
        add_page_to_unevictable_list(page);
        /*
         * When racing with an mlock or AS_UNEVICTABLE clearing
         * (page is unlocked) make sure that if the other thread
         * does not observe our setting of PG_lru and fails
         * isolation/check_move_unevictable_pages,
         * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
         * the page back to the evictable list.
         *
         * The other side is TestClearPageMlocked() or shmem_lock().
         */
        smp_mb();
    }

    /*
     * page's status can change while we move it among lru. If an evictable
     * page is on unevictable list, it never be freed. To avoid that,
     * check after we added it to the list, again.
     */
    if (lru == LRU_UNEVICTABLE && page_evictable(page)) {
        if (!isolate_lru_page(page)) {
            put_page(page);
            goto redo;
        }
        /* This means someone else dropped this page from LRU
         * So, it will be freed or putback to LRU again. There is
         * nothing to do here.
         */
    }

    if (was_unevictable && lru != LRU_UNEVICTABLE)
        count_vm_event(UNEVICTABLE_PGRESCUED);
    else if (!was_unevictable && lru == LRU_UNEVICTABLE)
        count_vm_event(UNEVICTABLE_PGCULLED);

    put_page(page);     /* drop ref from isolate */
}

enum page_references {
    PAGEREF_RECLAIM,
    PAGEREF_RECLAIM_CLEAN,
    PAGEREF_KEEP,
    PAGEREF_ACTIVATE,
};

static enum page_references page_check_references(struct page *page,
                          struct scan_control *sc)
{
    int referenced_ptes, referenced_page;
    unsigned long vm_flags;

    referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
                      &vm_flags);
    referenced_page = TestClearPageReferenced(page);

    /*
     * Mlock lost the isolation race with us.  Let try_to_unmap()
     * move the page to the unevictable list.
     */
    if (vm_flags & VM_LOCKED)
        return PAGEREF_RECLAIM;

    if (referenced_ptes) {
        if (PageSwapBacked(page))
            return PAGEREF_ACTIVATE;
        /*
         * All mapped pages start out with page table
         * references from the instantiating fault, so we need
         * to look twice if a mapped file page is used more
         * than once.
         *
         * Mark it and spare it for another trip around the
         * inactive list.  Another page table reference will
         * lead to its activation.
         *
         * Note: the mark is set for activated pages as well
         * so that recently deactivated but used pages are
         * quickly recovered.
         */
        SetPageReferenced(page);

        if (referenced_page || referenced_ptes > 1)
            return PAGEREF_ACTIVATE;

        /*
         * Activate file-backed executable pages after first usage.
         */
        if (vm_flags & VM_EXEC)
            return PAGEREF_ACTIVATE;

        return PAGEREF_KEEP;
    }

    /* Reclaim if clean, defer dirty pages to writeback */
    if (referenced_page && !PageSwapBacked(page))
        return PAGEREF_RECLAIM_CLEAN;

    return PAGEREF_RECLAIM;
}

/*
 * shrink_page_list() returns the number of reclaimed pages
 */
static unsigned long shrink_page_list(struct list_head *page_list,
                      struct zone *zone,
                      struct scan_control *sc,
                      enum ttu_flags ttu_flags,
                      unsigned long *ret_nr_dirty,
                      unsigned long *ret_nr_writeback,
                      bool force_reclaim)
{
    LIST_HEAD(ret_pages);
    LIST_HEAD(free_pages);
    int pgactivate = 0;
    unsigned long nr_dirty = 0;
    unsigned long nr_congested = 0;
    unsigned long nr_reclaimed = 0;
    unsigned long nr_writeback = 0;

    cond_resched();

    mem_cgroup_uncharge_start();
    while (!list_empty(page_list)) {
        struct address_space *mapping;
        struct page *page;
        int may_enter_fs;
        enum page_references references = PAGEREF_RECLAIM_CLEAN;

        cond_resched();

        page = lru_to_page(page_list);
        list_del(&page->lru);

        if (!trylock_page(page))
            goto keep;

        VM_BUG_ON(PageActive(page));
        VM_BUG_ON(page_zone(page) != zone);

        sc->nr_scanned++;

        if (unlikely(!page_evictable(page)))
            goto cull_mlocked;

        if (!sc->may_unmap && page_mapped(page))
            goto keep_locked;

        /* Double the slab pressure for mapped and swapcache pages */
        if (page_mapped(page) || PageSwapCache(page))
            sc->nr_scanned++;

        may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
            (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));

        if (PageWriteback(page)) {
            /*
             * memcg doesn't have any dirty pages throttling so we
             * could easily OOM just because too many pages are in
             * writeback and there is nothing else to reclaim.
             *
             * Check __GFP_IO, certainly because a loop driver
             * thread might enter reclaim, and deadlock if it waits
             * on a page for which it is needed to do the write
             * (loop masks off __GFP_IO|__GFP_FS for this reason);
             * but more thought would probably show more reasons.
             *
             * Don't require __GFP_FS, since we're not going into
             * the FS, just waiting on its writeback completion.
             * Worryingly, ext4 gfs2 and xfs allocate pages with
             * grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so
             * testing may_enter_fs here is liable to OOM on them.
             */
            if (global_reclaim(sc) ||
                !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
                /*
                 * This is slightly racy - end_page_writeback()
                 * might have just cleared PageReclaim, then
                 * setting PageReclaim here end up interpreted
                 * as PageReadahead - but that does not matter
                 * enough to care.  What we do want is for this
                 * page to have PageReclaim set next time memcg
                 * reclaim reaches the tests above, so it will
                 * then wait_on_page_writeback() to avoid OOM;
                 * and it's also appropriate in global reclaim.
                 */
                SetPageReclaim(page);
                nr_writeback++;
                goto keep_locked;
            }
            wait_on_page_writeback(page);
        }

        if (!force_reclaim)
            references = page_check_references(page, sc);

        switch (references) {
        case PAGEREF_ACTIVATE:
            goto activate_locked;
        case PAGEREF_KEEP:
            goto keep_locked;
        case PAGEREF_RECLAIM:
        case PAGEREF_RECLAIM_CLEAN:
            ; /* try to reclaim the page below */
        }

        /*
         * Anonymous process memory has backing store?
         * Try to allocate it some swap space here.
         */
        if (PageAnon(page) && !PageSwapCache(page)) {
            if (!(sc->gfp_mask & __GFP_IO))
                goto keep_locked;
            if (!add_to_swap(page))
                goto activate_locked;
            may_enter_fs = 1;
        }

        mapping = page_mapping(page);

        /*
         * The page is mapped into the page tables of one or more
         * processes. Try to unmap it here.
         */
        if (page_mapped(page) && mapping) {
            switch (try_to_unmap(page, ttu_flags)) {
            case SWAP_FAIL:
                goto activate_locked;
            case SWAP_AGAIN:
                goto keep_locked;
            case SWAP_MLOCK:
                goto cull_mlocked;
            case SWAP_SUCCESS:
                ; /* try to free the page below */
            }
        }

        if (PageDirty(page)) {
            nr_dirty++;

            /*
             * Only kswapd can writeback filesystem pages to
             * avoid risk of stack overflow but do not writeback
             * unless under significant pressure.
             */
            if (page_is_file_cache(page) &&
                    (!current_is_kswapd() ||
                     sc->priority >= DEF_PRIORITY - 2)) {
                /*
                 * Immediately reclaim when written back.
                 * Similar in principal to deactivate_page()
                 * except we already have the page isolated
                 * and know it's dirty
                 */
                inc_zone_page_state(page, NR_VMSCAN_IMMEDIATE);
                SetPageReclaim(page);

                goto keep_locked;
            }

            if (references == PAGEREF_RECLAIM_CLEAN)
                goto keep_locked;
            if (!may_enter_fs)
                goto keep_locked;
            if (!sc->may_writepage)
                goto keep_locked;

            /* Page is dirty, try to write it out here */
            switch (pageout(page, mapping, sc)) {
            case PAGE_KEEP:
                nr_congested++;
                goto keep_locked;
            case PAGE_ACTIVATE:
                goto activate_locked;
            case PAGE_SUCCESS:
                if (PageWriteback(page))
                    goto keep;
                if (PageDirty(page))
                    goto keep;

                /*
                 * A synchronous write - probably a ramdisk.  Go
                 * ahead and try to reclaim the page.
                 */
                if (!trylock_page(page))
                    goto keep;
                if (PageDirty(page) || PageWriteback(page))
                    goto keep_locked;
                mapping = page_mapping(page);
            case PAGE_CLEAN:
                ; /* try to free the page below */
            }
        }

        /*
         * If the page has buffers, try to free the buffer mappings
         * associated with this page. If we succeed we try to free
         * the page as well.
         *
         * We do this even if the page is PageDirty().
         * try_to_release_page() does not perform I/O, but it is
         * possible for a page to have PageDirty set, but it is actually
         * clean (all its buffers are clean).  This happens if the
         * buffers were written out directly, with submit_bh(). ext3
         * will do this, as well as the blockdev mapping.
         * try_to_release_page() will discover that cleanness and will
         * drop the buffers and mark the page clean - it can be freed.
         *
         * Rarely, pages can have buffers and no ->mapping.  These are
         * the pages which were not successfully invalidated in
         * truncate_complete_page().  We try to drop those buffers here
         * and if that worked, and the page is no longer mapped into
         * process address space (page_count == 1) it can be freed.
         * Otherwise, leave the page on the LRU so it is swappable.
         */
        if (page_has_private(page)) {
            if (!try_to_release_page(page, sc->gfp_mask))
                goto activate_locked;
            if (!mapping && page_count(page) == 1) {
                unlock_page(page);
                if (put_page_testzero(page))
                    goto free_it;
                else {
                    /*
                     * rare race with speculative reference.
                     * the speculative reference will free
                     * this page shortly, so we may
                     * increment nr_reclaimed here (and
                     * leave it off the LRU).
                     */
                    nr_reclaimed++;
                    continue;
                }
            }
        }

        if (!mapping || !__remove_mapping(mapping, page))
            goto keep_locked;

        /*
         * At this point, we have no other references and there is
         * no way to pick any more up (removed from LRU, removed
         * from pagecache). Can use non-atomic bitops now (and
         * we obviously don't have to worry about waking up a process
         * waiting on the page lock, because there are no references.
         */
        __clear_page_locked(page);
free_it:
        nr_reclaimed++;

        /*
         * Is there need to periodically free_page_list? It would
         * appear not as the counts should be low
         */
        list_add(&page->lru, &free_pages);
        continue;

cull_mlocked:
        if (PageSwapCache(page))
            try_to_free_swap(page);
        unlock_page(page);
        putback_lru_page(page);
        continue;

activate_locked:
        /* Not a candidate for swapping, so reclaim swap space. */
        if (PageSwapCache(page) && vm_swap_full())
            try_to_free_swap(page);
        VM_BUG_ON(PageActive(page));
        SetPageActive(page);
        pgactivate++;
keep_locked:
        unlock_page(page);
keep:
        list_add(&page->lru, &ret_pages);
        VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
    }

    /*
     * Tag a zone as congested if all the dirty pages encountered were
     * backed by a congested BDI. In this case, reclaimers should just
     * back off and wait for congestion to clear because further reclaim
     * will encounter the same problem
     */
    if (nr_dirty && nr_dirty == nr_congested && global_reclaim(sc))
        zone_set_flag(zone, ZONE_CONGESTED);

    free_hot_cold_page_list(&free_pages, 1);

    list_splice(&ret_pages, page_list);
    count_vm_events(PGACTIVATE, pgactivate);
    mem_cgroup_uncharge_end();
    *ret_nr_dirty += nr_dirty;
    *ret_nr_writeback += nr_writeback;
    return nr_reclaimed;
}

unsigned long reclaim_clean_pages_from_list(struct zone *zone,
                        struct list_head *page_list)
{
    struct scan_control sc = {
        .gfp_mask = GFP_KERNEL,
        .priority = DEF_PRIORITY,
        .may_unmap = 1,
    };
    unsigned long ret, dummy1, dummy2;
    struct page *page, *next;
    LIST_HEAD(clean_pages);

    list_for_each_entry_safe(page, next, page_list, lru) {
        if (page_is_file_cache(page) && !PageDirty(page)) {
            ClearPageActive(page);
            list_move(&page->lru, &clean_pages);
        }
    }

    ret = shrink_page_list(&clean_pages, zone, &sc,
                TTU_UNMAP|TTU_IGNORE_ACCESS,
                &dummy1, &dummy2, true);
    list_splice(&clean_pages, page_list);
    __mod_zone_page_state(zone, NR_ISOLATED_FILE, -ret);
    return ret;
}

/*
 * Attempt to remove the specified page from its LRU.  Only take this page
 * if it is of the appropriate PageActive status.  Pages which are being
 * freed elsewhere are also ignored.
 *
 * page:    page to consider
 * mode:    one of the LRU isolation modes defined above
 *
 * returns 0 on success, -ve errno on failure.
 */
int __isolate_lru_page(struct page *page, isolate_mode_t mode)
{
    int ret = -EINVAL;

    /* Only take pages on the LRU. */
    if (!PageLRU(page))
        return ret;

    /* Compaction should not handle unevictable pages but CMA can do so */
    if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
        return ret;

    ret = -EBUSY;

    /*
     * To minimise LRU disruption, the caller can indicate that it only
     * wants to isolate pages it will be able to operate on without
     * blocking - clean pages for the most part.
     *
     * ISOLATE_CLEAN means that only clean pages should be isolated. This
     * is used by reclaim when it is cannot write to backing storage
     *
     * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
     * that it is possible to migrate without blocking
     */
    if (mode & (ISOLATE_CLEAN|ISOLATE_ASYNC_MIGRATE)) {
        /* All the caller can do on PageWriteback is block */
        if (PageWriteback(page))
            return ret;

        if (PageDirty(page)) {
            struct address_space *mapping;

            /* ISOLATE_CLEAN means only clean pages */
            if (mode & ISOLATE_CLEAN)
                return ret;

            /*
             * Only pages without mappings or that have a
             * ->migratepage callback are possible to migrate
             * without blocking
             */
            mapping = page_mapping(page);
            if (mapping && !mapping->a_ops->migratepage)
                return ret;
        }
    }

    if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
        return ret;

    if (likely(get_page_unless_zero(page))) {
        /*
         * 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.
         */
        ClearPageLRU(page);
        ret = 0;
    }

    return ret;
}

/*
 * zone->lru_lock is heavily contended.  Some of the functions that
 * shrink the lists perform better by taking out a batch of pages
 * and working on them outside the LRU lock.
 *
 * For pagecache intensive workloads, this function is the hottest
 * spot in the kernel (apart from copy_*_user functions).
 *
 * Appropriate locks must be held before calling this function.
 *
 * @nr_to_scan: The number of pages to look through on the list.
 * @lruvec: The LRU vector to pull pages from.
 * @dst:    The temp list to put pages on to.
 * @nr_scanned: The number of pages that were scanned.
 * @sc:     The scan_control struct for this reclaim session
 * @mode:   One of the LRU isolation modes
 * @lru:    LRU list id for isolating
 *
 * returns how many pages were moved onto *@dst.
 */
static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
        struct lruvec *lruvec, struct list_head *dst,
        unsigned long *nr_scanned, struct scan_control *sc,
        isolate_mode_t mode, enum lru_list lru)
{
    struct list_head *src = &lruvec->lists[lru];
    unsigned long nr_taken = 0;
    unsigned long scan;

    for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
        struct page *page;
        int nr_pages;

        page = lru_to_page(src);
        prefetchw_prev_lru_page(page, src, flags);

        VM_BUG_ON(!PageLRU(page));

        switch (__isolate_lru_page(page, mode)) {
        case 0:
            nr_pages = hpage_nr_pages(page);
            mem_cgroup_update_lru_size(lruvec, lru, -nr_pages);
            list_move(&page->lru, dst);
            nr_taken += nr_pages;
            break;

        case -EBUSY:
            /* else it is being freed elsewhere */
            list_move(&page->lru, src);
            continue;

        default:
            BUG();
        }
    }

    *nr_scanned = scan;
    trace_mm_vmscan_lru_isolate(sc->order, nr_to_scan, scan,
                    nr_taken, mode, is_file_lru(lru));
    return nr_taken;
}

int isolate_lru_page(struct page *page)
{
    int ret = -EBUSY;

    VM_BUG_ON(!page_count(page));

    if (PageLRU(page)) {
        struct zone *zone = page_zone(page);
        struct lruvec *lruvec;

        spin_lock_irq(&zone->lru_lock);
        lruvec = mem_cgroup_page_lruvec(page, zone);
        if (PageLRU(page)) {
            int lru = page_lru(page);
            get_page(page);
            ClearPageLRU(page);
            del_page_from_lru_list(page, lruvec, lru);
            ret = 0;
        }
        spin_unlock_irq(&zone->lru_lock);
    }
    return ret;
}

/*
 * Are there way too many processes in the direct reclaim path already?
 */
static int too_many_isolated(struct zone *zone, int file,
        struct scan_control *sc)
{
    unsigned long inactive, isolated;

    if (current_is_kswapd())
        return 0;

    if (!global_reclaim(sc))
        return 0;

    if (file) {
        inactive = zone_page_state(zone, NR_INACTIVE_FILE);
        isolated = zone_page_state(zone, NR_ISOLATED_FILE);
    } else {
        inactive = zone_page_state(zone, NR_INACTIVE_ANON);
        isolated = zone_page_state(zone, NR_ISOLATED_ANON);
    }

    return isolated > inactive;
}

static noinline_for_stack void
putback_inactive_pages(struct lruvec *lruvec, struct list_head *page_list)
{
    struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
    struct zone *zone = lruvec_zone(lruvec);
    LIST_HEAD(pages_to_free);

    /*
     * Put back any unfreeable pages.
     */
    while (!list_empty(page_list)) {
        struct page *page = lru_to_page(page_list);
        int lru;

        VM_BUG_ON(PageLRU(page));
        list_del(&page->lru);
        if (unlikely(!page_evictable(page))) {
            spin_unlock_irq(&zone->lru_lock);
            putback_lru_page(page);
            spin_lock_irq(&zone->lru_lock);
            continue;
        }

        lruvec = mem_cgroup_page_lruvec(page, zone);

        SetPageLRU(page);
        lru = page_lru(page);
        add_page_to_lru_list(page, lruvec, lru);

        if (is_active_lru(lru)) {
            int file = is_file_lru(lru);
            int numpages = hpage_nr_pages(page);
            reclaim_stat->recent_rotated[file] += numpages;
        }
        if (put_page_testzero(page)) {
            __ClearPageLRU(page);
            __ClearPageActive(page);
            del_page_from_lru_list(page, lruvec, lru);

            if (unlikely(PageCompound(page))) {
                spin_unlock_irq(&zone->lru_lock);
                (*get_compound_page_dtor(page))(page);
                spin_lock_irq(&zone->lru_lock);
            } else
                list_add(&page->lru, &pages_to_free);
        }
    }

    /*
     * To save our caller's stack, now use input list for pages to free.
     */
    list_splice(&pages_to_free, page_list);
}

/*
 * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
 * of reclaimed pages
 */
static noinline_for_stack unsigned long
shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
             struct scan_control *sc, enum lru_list lru)
{
    LIST_HEAD(page_list);
    unsigned long nr_scanned;
    unsigned long nr_reclaimed = 0;
    unsigned long nr_taken;
    unsigned long nr_dirty = 0;
    unsigned long nr_writeback = 0;
    isolate_mode_t isolate_mode = 0;
    int file = is_file_lru(lru);
    struct zone *zone = lruvec_zone(lruvec);
    struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;

    while (unlikely(too_many_isolated(zone, file, sc))) {
        congestion_wait(BLK_RW_ASYNC, HZ/10);

        /* We are about to die and free our memory. Return now. */
        if (fatal_signal_pending(current))
            return SWAP_CLUSTER_MAX;
    }

    lru_add_drain();

    if (!sc->may_unmap)
        isolate_mode |= ISOLATE_UNMAPPED;
    if (!sc->may_writepage)
        isolate_mode |= ISOLATE_CLEAN;

    spin_lock_irq(&zone->lru_lock);

    nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &page_list,
                     &nr_scanned, sc, isolate_mode, lru);

    __mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
    __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);

    if (global_reclaim(sc)) {
        zone->pages_scanned += nr_scanned;
        if (current_is_kswapd())
            __count_zone_vm_events(PGSCAN_KSWAPD, zone, nr_scanned);
        else
            __count_zone_vm_events(PGSCAN_DIRECT, zone, nr_scanned);
    }
    spin_unlock_irq(&zone->lru_lock);

    if (nr_taken == 0)
        return 0;

    nr_reclaimed = shrink_page_list(&page_list, zone, sc, TTU_UNMAP,
                    &nr_dirty, &nr_writeback, false);

    spin_lock_irq(&zone->lru_lock);

    reclaim_stat->recent_scanned[file] += nr_taken;

    if (global_reclaim(sc)) {
        if (current_is_kswapd())
            __count_zone_vm_events(PGSTEAL_KSWAPD, zone,
                           nr_reclaimed);
        else
            __count_zone_vm_events(PGSTEAL_DIRECT, zone,
                           nr_reclaimed);
    }

    putback_inactive_pages(lruvec, &page_list);

    __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);

    spin_unlock_irq(&zone->lru_lock);

    free_hot_cold_page_list(&page_list, 1);

    /*
     * If reclaim is isolating dirty pages under writeback, it implies
     * that the long-lived page allocation rate is exceeding the page
     * laundering rate. Either the global limits are not being effective
     * at throttling processes due to the page distribution throughout
     * zones or there is heavy usage of a slow backing device. The
     * only option is to throttle from reclaim context which is not ideal
     * as there is no guarantee the dirtying process is throttled in the
     * same way balance_dirty_pages() manages.
     *
     * This scales the number of dirty pages that must be under writeback
     * before throttling depending on priority. It is a simple backoff
     * function that has the most effect in the range DEF_PRIORITY to
     * DEF_PRIORITY-2 which is the priority reclaim is considered to be
     * in trouble and reclaim is considered to be in trouble.
     *
     * DEF_PRIORITY   100% isolated pages must be PageWriteback to throttle
     * DEF_PRIORITY-1  50% must be PageWriteback
     * DEF_PRIORITY-2  25% must be PageWriteback, kswapd in trouble
     * ...
     * DEF_PRIORITY-6 For SWAP_CLUSTER_MAX isolated pages, throttle if any
     *                     isolated page is PageWriteback
     */
    if (nr_writeback && nr_writeback >=
            (nr_taken >> (DEF_PRIORITY - sc->priority)))
        wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);

    trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
        zone_idx(zone),
        nr_scanned, nr_reclaimed,
        sc->priority,
        trace_shrink_flags(file));
    return nr_reclaimed;
}

/*
 * This moves pages from the active list to the inactive list.
 *
 * We move them the other way if the page is referenced by one or more
 * processes, from rmap.
 *
 * If the pages are mostly unmapped, the processing is fast and it is
 * appropriate to hold zone->lru_lock across the whole operation.  But if
 * the pages are mapped, the processing is slow (page_referenced()) so we
 * should drop zone->lru_lock around each page.  It's impossible to balance
 * this, so instead we remove the pages from the LRU while processing them.
 * It is safe to rely on PG_active against the non-LRU pages in here because
 * nobody will play with that bit on a non-LRU page.
 *
 * The downside is that we have to touch page->_count against each page.
 * But we had to alter page->flags anyway.
 */

static void move_active_pages_to_lru(struct lruvec *lruvec,
                     struct list_head *list,
                     struct list_head *pages_to_free,
                     enum lru_list lru)
{
    struct zone *zone = lruvec_zone(lruvec);
    unsigned long pgmoved = 0;
    struct page *page;
    int nr_pages;

    while (!list_empty(list)) {
        page = lru_to_page(list);
        lruvec = mem_cgroup_page_lruvec(page, zone);

        VM_BUG_ON(PageLRU(page));
        SetPageLRU(page);

        nr_pages = hpage_nr_pages(page);
        mem_cgroup_update_lru_size(lruvec, lru, nr_pages);
        list_move(&page->lru, &lruvec->lists[lru]);
        pgmoved += nr_pages;

        if (put_page_testzero(page)) {
            __ClearPageLRU(page);
            __ClearPageActive(page);
            del_page_from_lru_list(page, lruvec, lru);

            if (unlikely(PageCompound(page))) {
                spin_unlock_irq(&zone->lru_lock);
                (*get_compound_page_dtor(page))(page);
                spin_lock_irq(&zone->lru_lock);
            } else
                list_add(&page->lru, pages_to_free);
        }
    }
    __mod_zone_page_state(zone, NR_LRU_BASE + lru, pgmoved);
    if (!is_active_lru(lru))
        __count_vm_events(PGDEACTIVATE, pgmoved);
}

static void shrink_active_list(unsigned long nr_to_scan,
                   struct lruvec *lruvec,
                   struct scan_control *sc,
                   enum lru_list lru)
{
    unsigned long nr_taken;
    unsigned long nr_scanned;
    unsigned long vm_flags;
    LIST_HEAD(l_hold);  /* The pages which were snipped off */
    LIST_HEAD(l_active);
    LIST_HEAD(l_inactive);
    struct page *page;
    struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
    unsigned long nr_rotated = 0;
    isolate_mode_t isolate_mode = 0;
    int file = is_file_lru(lru);
    struct zone *zone = lruvec_zone(lruvec);

    lru_add_drain();

    if (!sc->may_unmap)
        isolate_mode |= ISOLATE_UNMAPPED;
    if (!sc->may_writepage)
        isolate_mode |= ISOLATE_CLEAN;

    spin_lock_irq(&zone->lru_lock);

    nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
                     &nr_scanned, sc, isolate_mode, lru);
    if (global_reclaim(sc))
        zone->pages_scanned += nr_scanned;

    reclaim_stat->recent_scanned[file] += nr_taken;

    __count_zone_vm_events(PGREFILL, zone, nr_scanned);
    __mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
    __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);
    spin_unlock_irq(&zone->lru_lock);

    while (!list_empty(&l_hold)) {
        cond_resched();
        page = lru_to_page(&l_hold);
        list_del(&page->lru);

        if (unlikely(!page_evictable(page))) {
            putback_lru_page(page);
            continue;
        }

        if (unlikely(buffer_heads_over_limit)) {
            if (page_has_private(page) && trylock_page(page)) {
                if (page_has_private(page))
                    try_to_release_page(page, 0);
                unlock_page(page);
            }
        }

        if (page_referenced(page, 0, sc->target_mem_cgroup,
                    &vm_flags)) {
            nr_rotated += hpage_nr_pages(page);
            /*
             * Identify referenced, file-backed active pages and
             * give them one more trip around the active list. So
             * that executable code get better chances to stay in
             * memory under moderate memory pressure.  Anon pages
             * are not likely to be evicted by use-once streaming
             * IO, plus JVM can create lots of anon VM_EXEC pages,
             * so we ignore them here.
             */
            if ((vm_flags & VM_EXEC) && page_is_file_cache(page)) {
                list_add(&page->lru, &l_active);
                continue;
            }
        }

        ClearPageActive(page);  /* we are de-activating */
        list_add(&page->lru, &l_inactive);
    }

    /*
     * Move pages back to the lru list.
     */
    spin_lock_irq(&zone->lru_lock);
    /*
     * Count referenced pages from currently used mappings as rotated,
     * even though only some of them are actually re-activated.  This
     * helps balance scan pressure between file and anonymous pages in
     * get_scan_ratio.
     */
    reclaim_stat->recent_rotated[file] += nr_rotated;

    move_active_pages_to_lru(lruvec, &l_active, &l_hold, lru);
    move_active_pages_to_lru(lruvec, &l_inactive, &l_hold, lru - LRU_ACTIVE);
    __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
    spin_unlock_irq(&zone->lru_lock);

    free_hot_cold_page_list(&l_hold, 1);
}

#ifdef CONFIG_SWAP
static int inactive_anon_is_low_global(struct zone *zone)
{
    unsigned long active, inactive;

    active = zone_page_state(zone, NR_ACTIVE_ANON);
    inactive = zone_page_state(zone, NR_INACTIVE_ANON);

    if (inactive * zone->inactive_ratio < active)
        return 1;

    return 0;
}

static int inactive_anon_is_low(struct lruvec *lruvec)
{
    /*
     * If we don't have swap space, anonymous page deactivation
     * is pointless.
     */
    if (!total_swap_pages)
        return 0;

    if (!mem_cgroup_disabled())
        return mem_cgroup_inactive_anon_is_low(lruvec);

    return inactive_anon_is_low_global(lruvec_zone(lruvec));
}
#else
static inline int inactive_anon_is_low(struct lruvec *lruvec)
{
    return 0;
}
#endif

static int inactive_file_is_low_global(struct zone *zone)
{
    unsigned long active, inactive;

    active = zone_page_state(zone, NR_ACTIVE_FILE);
    inactive = zone_page_state(zone, NR_INACTIVE_FILE);

    return (active > inactive);
}

static int inactive_file_is_low(struct lruvec *lruvec)
{
    if (!mem_cgroup_disabled())
        return mem_cgroup_inactive_file_is_low(lruvec);

    return inactive_file_is_low_global(lruvec_zone(lruvec));
}

static int inactive_list_is_low(struct lruvec *lruvec, enum lru_list lru)
{
    if (is_file_lru(lru))
        return inactive_file_is_low(lruvec);
    else
        return inactive_anon_is_low(lruvec);
}

static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
                 struct lruvec *lruvec, struct scan_control *sc)
{
    if (is_active_lru(lru)) {
        if (inactive_list_is_low(lruvec, lru))
            shrink_active_list(nr_to_scan, lruvec, sc, lru);
        return 0;
    }

    return shrink_inactive_list(nr_to_scan, lruvec, sc, lru);
}

static int vmscan_swappiness(struct scan_control *sc)
{
    if (global_reclaim(sc))
        return vm_swappiness;
    return mem_cgroup_swappiness(sc->target_mem_cgroup);
}

/*
 * Determine how aggressively the anon and file LRU lists should be
 * scanned.  The relative value of each set of LRU lists is determined
 * by looking at the fraction of the pages scanned we did rotate back
 * onto the active list instead of evict.
 *
 * nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
 * nr[2] = file inactive pages to scan; nr[3] = file active pages to scan
 */
static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
               unsigned long *nr)
{
    unsigned long anon, file, free;
    unsigned long anon_prio, file_prio;
    unsigned long ap, fp;
    struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
    u64 fraction[2], denominator;
    enum lru_list lru;
    int noswap = 0;
    bool force_scan = false;
    struct zone *zone = lruvec_zone(lruvec);

    /*
     * If the zone or memcg is small, nr[l] can be 0.  This
     * results in no scanning on this priority and a potential
     * priority drop.  Global direct reclaim can go to the next
     * zone and tends to have no problems. Global kswapd is for
     * zone balancing and it needs to scan a minimum amount. When
     * reclaiming for a memcg, a priority drop can cause high
     * latencies, so it's better to scan a minimum amount there as
     * well.
     */
    if (current_is_kswapd() && zone->all_unreclaimable)
        force_scan = true;
    if (!global_reclaim(sc))
        force_scan = true;

    /* If we have no swap space, do not bother scanning anon pages. */
    if (!sc->may_swap || (nr_swap_pages <= 0)) {
        noswap = 1;
        fraction[0] = 0;
        fraction[1] = 1;
        denominator = 1;
        goto out;
    }

    anon  = get_lru_size(lruvec, LRU_ACTIVE_ANON) +
        get_lru_size(lruvec, LRU_INACTIVE_ANON);
    file  = get_lru_size(lruvec, LRU_ACTIVE_FILE) +
        get_lru_size(lruvec, LRU_INACTIVE_FILE);

    if (global_reclaim(sc)) {
        free  = zone_page_state(zone, NR_FREE_PAGES);
        /* If we have very few page cache pages,
           force-scan anon pages. */
        if (unlikely(file + free <= high_wmark_pages(zone))) {
            fraction[0] = 1;
            fraction[1] = 0;
            denominator = 1;
            goto out;
        }
    }

    /*
     * With swappiness at 100, anonymous and file have the same priority.
     * This scanning priority is essentially the inverse of IO cost.
     */
    anon_prio = vmscan_swappiness(sc);
    file_prio = 200 - anon_prio;

    /*
     * OK, so we have swap space and a fair amount of page cache
     * pages.  We use the recently rotated / recently scanned
     * ratios to determine how valuable each cache is.
     *
     * Because workloads change over time (and to avoid overflow)
     * we keep these statistics as a floating average, which ends
     * up weighing recent references more than old ones.
     *
     * anon in [0], file in [1]
     */
    spin_lock_irq(&zone->lru_lock);
    if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {
        reclaim_stat->recent_scanned[0] /= 2;
        reclaim_stat->recent_rotated[0] /= 2;
    }

    if (unlikely(reclaim_stat->recent_scanned[1] > file / 4)) {
        reclaim_stat->recent_scanned[1] /= 2;
        reclaim_stat->recent_rotated[1] /= 2;
    }

    /*
     * The amount of pressure on anon vs file pages is inversely
     * proportional to the fraction of recently scanned pages on
     * each list that were recently referenced and in active use.
     */
    ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
    ap /= reclaim_stat->recent_rotated[0] + 1;

    fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
    fp /= reclaim_stat->recent_rotated[1] + 1;
    spin_unlock_irq(&zone->lru_lock);

    fraction[0] = ap;
    fraction[1] = fp;
    denominator = ap + fp + 1;
out:
    for_each_evictable_lru(lru) {
        int file = is_file_lru(lru);
        unsigned long scan;

        scan = get_lru_size(lruvec, lru);
        if (sc->priority || noswap || !vmscan_swappiness(sc)) {
            scan >>= sc->priority;
            if (!scan && force_scan)
                scan = SWAP_CLUSTER_MAX;
            scan = div64_u64(scan * fraction[file], denominator);
        }
        nr[lru] = scan;
    }
}

/* Use reclaim/compaction for costly allocs or under memory pressure */
static bool in_reclaim_compaction(struct scan_control *sc)
{
    if (COMPACTION_BUILD && sc->order &&
            (sc->order > PAGE_ALLOC_COSTLY_ORDER ||
             sc->priority < DEF_PRIORITY - 2))
        return true;

    return false;
}

/*
 * Reclaim/compaction is used for high-order allocation requests. It reclaims
 * order-0 pages before compacting the zone. should_continue_reclaim() returns
 * true if more pages should be reclaimed such that when the page allocator
 * calls try_to_compact_zone() that it will have enough free pages to succeed.
 * It will give up earlier than that if there is difficulty reclaiming pages.
 */
static inline bool should_continue_reclaim(struct lruvec *lruvec,
                    unsigned long nr_reclaimed,
                    unsigned long nr_scanned,
                    struct scan_control *sc)
{
    unsigned long pages_for_compaction;
    unsigned long inactive_lru_pages;

    /* If not in reclaim/compaction mode, stop */
    if (!in_reclaim_compaction(sc))
        return false;

    /* Consider stopping depending on scan and reclaim activity */
    if (sc->gfp_mask & __GFP_REPEAT) {
        /*
         * For __GFP_REPEAT allocations, stop reclaiming if the
         * full LRU list has been scanned and we are still failing
         * to reclaim pages. This full LRU scan is potentially
         * expensive but a __GFP_REPEAT caller really wants to succeed
         */
        if (!nr_reclaimed && !nr_scanned)
            return false;
    } else {
        /*
         * For non-__GFP_REPEAT allocations which can presumably
         * fail without consequence, stop if we failed to reclaim
         * any pages from the last SWAP_CLUSTER_MAX number of
         * pages that were scanned. This will return to the
         * caller faster at the risk reclaim/compaction and
         * the resulting allocation attempt fails
         */
        if (!nr_reclaimed)
            return false;
    }

    /*
     * If we have not reclaimed enough pages for compaction and the
     * inactive lists are large enough, continue reclaiming
     */
    pages_for_compaction = (2UL << sc->order);
    inactive_lru_pages = get_lru_size(lruvec, LRU_INACTIVE_FILE);
    if (nr_swap_pages > 0)
        inactive_lru_pages += get_lru_size(lruvec, LRU_INACTIVE_ANON);
    if (sc->nr_reclaimed < pages_for_compaction &&
            inactive_lru_pages > pages_for_compaction)
        return true;

    /* If compaction would go ahead or the allocation would succeed, stop */
    switch (compaction_suitable(lruvec_zone(lruvec), sc->order)) {
    case COMPACT_PARTIAL:
    case COMPACT_CONTINUE:
        return false;
    default:
        return true;
    }
}

/*
 * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
 */
static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
{
    unsigned long nr[NR_LRU_LISTS];
    unsigned long nr_to_scan;
    enum lru_list lru;
    unsigned long nr_reclaimed, nr_scanned;
    unsigned long nr_to_reclaim = sc->nr_to_reclaim;
    struct blk_plug plug;

restart:
    nr_reclaimed = 0;
    nr_scanned = sc->nr_scanned;
    get_scan_count(lruvec, sc, nr);

    blk_start_plug(&plug);
    while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
                    nr[LRU_INACTIVE_FILE]) {
        for_each_evictable_lru(lru) {
            if (nr[lru]) {
                nr_to_scan = min_t(unsigned long,
                           nr[lru], SWAP_CLUSTER_MAX);
                nr[lru] -= nr_to_scan;

                nr_reclaimed += shrink_list(lru, nr_to_scan,
                                lruvec, sc);
            }
        }
        /*
         * On large memory systems, scan >> priority can become
         * really large. This is fine for the starting priority;
         * we want to put equal scanning pressure on each zone.
         * However, if the VM has a harder time of freeing pages,
         * with multiple processes reclaiming pages, the total
         * freeing target can get unreasonably large.
         */
        if (nr_reclaimed >= nr_to_reclaim &&
            sc->priority < DEF_PRIORITY)
            break;
    }
    blk_finish_plug(&plug);
    sc->nr_reclaimed += nr_reclaimed;

    /*
     * Even if we did not try to evict anon pages at all, we want to
     * rebalance the anon lru active/inactive ratio.
     */
    if (inactive_anon_is_low(lruvec))
        shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                   sc, LRU_ACTIVE_ANON);

    /* reclaim/compaction might need reclaim to continue */
    if (should_continue_reclaim(lruvec, nr_reclaimed,
                    sc->nr_scanned - nr_scanned, sc))
        goto restart;

    throttle_vm_writeout(sc->gfp_mask);
}

static void shrink_zone(struct zone *zone, struct scan_control *sc)
{
    struct mem_cgroup *root = sc->target_mem_cgroup;
    struct mem_cgroup_reclaim_cookie reclaim = {
        .zone = zone,
        .priority = sc->priority,
    };
    struct mem_cgroup *memcg;

    memcg = mem_cgroup_iter(root, NULL, &reclaim);
    do {
        struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);

        shrink_lruvec(lruvec, sc);

        /*
         * Limit reclaim has historically picked one memcg and
         * scanned it with decreasing priority levels until
         * nr_to_reclaim had been reclaimed.  This priority
         * cycle is thus over after a single memcg.
         *
         * Direct reclaim and kswapd, on the other hand, have
         * to scan all memory cgroups to fulfill the overall
         * scan target for the zone.
         */
        if (!global_reclaim(sc)) {
            mem_cgroup_iter_break(root, memcg);
            break;
        }
        memcg = mem_cgroup_iter(root, memcg, &reclaim);
    } while (memcg);
}

/* Returns true if compaction should go ahead for a high-order request */
static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
{
    unsigned long balance_gap, watermark;
    bool watermark_ok;

    /* Do not consider compaction for orders reclaim is meant to satisfy */
    if (sc->order <= PAGE_ALLOC_COSTLY_ORDER)
        return false;

    /*
     * Compaction takes time to run and there are potentially other
     * callers using the pages just freed. Continue reclaiming until
     * there is a buffer of free pages available to give compaction
     * a reasonable chance of completing and allocating the page
     */
    balance_gap = min(low_wmark_pages(zone),
        (zone->present_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
            KSWAPD_ZONE_BALANCE_GAP_RATIO);
    watermark = high_wmark_pages(zone) + balance_gap + (2UL << sc->order);
    watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, 0, 0);

    /*
     * If compaction is deferred, reclaim up to a point where
     * compaction will have a chance of success when re-enabled
     */
    if (compaction_deferred(zone, sc->order))
        return watermark_ok;

    /* If compaction is not ready to start, keep reclaiming */
    if (!compaction_suitable(zone, sc->order))
        return false;

    return watermark_ok;
}

/*
 * This is the direct reclaim path, for page-allocating processes.  We only
 * try to reclaim pages from zones which will satisfy the caller's allocation
 * request.
 *
 * We reclaim from a zone even if that zone is over high_wmark_pages(zone).
 * Because:
 * a) The caller may be trying to free *extra* pages to satisfy a higher-order
 *    allocation or
 * b) The target zone may be at high_wmark_pages(zone) but the lower zones
 *    must go *over* high_wmark_pages(zone) to satisfy the `incremental min'
 *    zone defense algorithm.
 *
 * If a zone is deemed to be full of pinned pages then just give it a light
 * scan then give up on it.
 *
 * This function returns true if a zone is being reclaimed for a costly
 * high-order allocation and compaction is ready to begin. This indicates to
 * the caller that it should consider retrying the allocation instead of
 * further reclaim.
 */
static bool shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
{
    struct zoneref *z;
    struct zone *zone;
    unsigned long nr_soft_reclaimed;
    unsigned long nr_soft_scanned;
    bool aborted_reclaim = false;

    /*
     * If the number of buffer_heads in the machine exceeds the maximum
     * allowed level, force direct reclaim to scan the highmem zone as
     * highmem pages could be pinning lowmem pages storing buffer_heads
     */
    if (buffer_heads_over_limit)
        sc->gfp_mask |= __GFP_HIGHMEM;

    for_each_zone_zonelist_nodemask(zone, z, zonelist,
                    gfp_zone(sc->gfp_mask), sc->nodemask) {
        if (!populated_zone(zone))
            continue;
        /*
         * Take care memory controller reclaiming has small influence
         * to global LRU.
         */
        if (global_reclaim(sc)) {
            if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                continue;
            if (zone->all_unreclaimable &&
                    sc->priority != DEF_PRIORITY)
                continue;   /* Let kswapd poll it */
            if (COMPACTION_BUILD) {
                /*
                 * If we already have plenty of memory free for
                 * compaction in this zone, don't free any more.
                 * Even though compaction is invoked for any
                 * non-zero order, only frequent costly order
                 * reclamation is disruptive enough to become a
                 * noticeable problem, like transparent huge
                 * page allocations.
                 */
                if (compaction_ready(zone, sc)) {
                    aborted_reclaim = true;
                    continue;
                }
            }
            /*
             * This steals pages from memory cgroups over softlimit
             * and returns the number of reclaimed pages and
             * scanned pages. This works for global memory pressure
             * and balancing, not for a memcg's limit.
             */
            nr_soft_scanned = 0;
            nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
                        sc->order, sc->gfp_mask,
                        &nr_soft_scanned);
            sc->nr_reclaimed += nr_soft_reclaimed;
            sc->nr_scanned += nr_soft_scanned;
            /* need some check for avoid more shrink_zone() */
        }

        shrink_zone(zone, sc);
    }

    return aborted_reclaim;
}

static bool zone_reclaimable(struct zone *zone)
{
    return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
}

/* All zones in zonelist are unreclaimable? */
static bool all_unreclaimable(struct zonelist *zonelist,
        struct scan_control *sc)
{
    struct zoneref *z;
    struct zone *zone;

    for_each_zone_zonelist_nodemask(zone, z, zonelist,
            gfp_zone(sc->gfp_mask), sc->nodemask) {
        if (!populated_zone(zone))
            continue;
        if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
            continue;
        if (!zone->all_unreclaimable)
            return false;
    }

    return true;
}

/*
 * This is the main entry point to direct page reclaim.
 *
 * If a full scan of the inactive list fails to free enough memory then we
 * are "out of memory" and something needs to be killed.
 *
 * If the caller is !__GFP_FS then the probability of a failure is reasonably
 * high - the zone may be full of dirty or under-writeback pages, which this
 * caller can't do much about.  We kick the writeback threads and take explicit
 * naps in the hope that some of these pages can be written.  But if the
 * allocating task holds filesystem locks which prevent writeout this might not
 * work, and the allocation attempt will fail.
 *
 * returns: 0, if no pages reclaimed
 *      else, the number of pages reclaimed
 */
static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
                    struct scan_control *sc,
                    struct shrink_control *shrink)
{
    unsigned long total_scanned = 0;
    struct reclaim_state *reclaim_state = current->reclaim_state;
    struct zoneref *z;
    struct zone *zone;
    unsigned long writeback_threshold;
    bool aborted_reclaim;

    delayacct_freepages_start();

    if (global_reclaim(sc))
        count_vm_event(ALLOCSTALL);

    do {
        sc->nr_scanned = 0;
        aborted_reclaim = shrink_zones(zonelist, sc);

        /*
         * Don't shrink slabs when reclaiming memory from
         * over limit cgroups
         */
        if (global_reclaim(sc)) {
            unsigned long lru_pages = 0;
            for_each_zone_zonelist(zone, z, zonelist,
                    gfp_zone(sc->gfp_mask)) {
                if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                    continue;

                lru_pages += zone_reclaimable_pages(zone);
            }

            shrink_slab(shrink, sc->nr_scanned, lru_pages);
            if (reclaim_state) {
                sc->nr_reclaimed += reclaim_state->reclaimed_slab;
                reclaim_state->reclaimed_slab = 0;
            }
        }
        total_scanned += sc->nr_scanned;
        if (sc->nr_reclaimed >= sc->nr_to_reclaim)
            goto out;

        /*
         * Try to write back as many pages as we just scanned.  This
         * tends to cause slow streaming writers to write data to the
         * disk smoothly, at the dirtying rate, which is nice.   But
         * that's undesirable in laptop mode, where we *want* lumpy
         * writeout.  So in laptop mode, write out the whole world.
         */
        writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
        if (total_scanned > writeback_threshold) {
            wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
                        WB_REASON_TRY_TO_FREE_PAGES);
            sc->may_writepage = 1;
        }

        /* Take a nap, wait for some writeback to complete */
        if (!sc->hibernation_mode && sc->nr_scanned &&
            sc->priority < DEF_PRIORITY - 2) {
            struct zone *preferred_zone;

            first_zones_zonelist(zonelist, gfp_zone(sc->gfp_mask),
                        &cpuset_current_mems_allowed,
                        &preferred_zone);
            wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/10);
        }
    } while (--sc->priority >= 0);

out:
    delayacct_freepages_end();

    if (sc->nr_reclaimed)
        return sc->nr_reclaimed;

    /*
     * As hibernation is going on, kswapd is freezed so that it can't mark
     * the zone into all_unreclaimable. Thus bypassing all_unreclaimable
     * check.
     */
    if (oom_killer_disabled)
        return 0;

    /* Aborted reclaim to try compaction? don't OOM, then */
    if (aborted_reclaim)
        return 1;

    /* top priority shrink_zones still had more to do? don't OOM, then */
    if (global_reclaim(sc) && !all_unreclaimable(zonelist, sc))
        return 1;

    return 0;
}

static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
{
    struct zone *zone;
    unsigned long pfmemalloc_reserve = 0;
    unsigned long free_pages = 0;
    int i;
    bool wmark_ok;

    for (i = 0; i <= ZONE_NORMAL; i++) {
        zone = &pgdat->node_zones[i];
        pfmemalloc_reserve += min_wmark_pages(zone);
        free_pages += zone_page_state(zone, NR_FREE_PAGES);
    }

    wmark_ok = free_pages > pfmemalloc_reserve / 2;

    /* kswapd must be awake if processes are being throttled */
    if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
        pgdat->classzone_idx = min(pgdat->classzone_idx,
                        (enum zone_type)ZONE_NORMAL);
        wake_up_interruptible(&pgdat->kswapd_wait);
    }

    return wmark_ok;
}

/*
 * Throttle direct reclaimers if backing storage is backed by the network
 * and the PFMEMALLOC reserve for the preferred node is getting dangerously
 * depleted. kswapd will continue to make progress and wake the processes
 * when the low watermark is reached.
 *
 * Returns true if a fatal signal was delivered during throttling. If this
 * happens, the page allocator should not consider triggering the OOM killer.
 */
static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
                    nodemask_t *nodemask)
{
    struct zone *zone;
    int high_zoneidx = gfp_zone(gfp_mask);
    pg_data_t *pgdat;

    /*
     * Kernel threads should not be throttled as they may be indirectly
     * responsible for cleaning pages necessary for reclaim to make forward
     * progress. kjournald for example may enter direct reclaim while
     * committing a transaction where throttling it could forcing other
     * processes to block on log_wait_commit().
     */
    if (current->flags & PF_KTHREAD)
        goto out;

    /*
     * If a fatal signal is pending, this process should not throttle.
     * It should return quickly so it can exit and free its memory
     */
    if (fatal_signal_pending(current))
        goto out;

    /* Check if the pfmemalloc reserves are ok */
    first_zones_zonelist(zonelist, high_zoneidx, NULL, &zone);
    pgdat = zone->zone_pgdat;
    if (pfmemalloc_watermark_ok(pgdat))
        goto out;

    /* Account for the throttling */
    count_vm_event(PGSCAN_DIRECT_THROTTLE);

    /*
     * If the caller cannot enter the filesystem, it's possible that it
     * is due to the caller holding an FS lock or performing a journal
     * transaction in the case of a filesystem like ext[3|4]. In this case,
     * it is not safe to block on pfmemalloc_wait as kswapd could be
     * blocked waiting on the same lock. Instead, throttle for up to a
     * second before continuing.
     */
    if (!(gfp_mask & __GFP_FS)) {
        wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
            pfmemalloc_watermark_ok(pgdat), HZ);

        goto check_pending;
    }

    /* Throttle until kswapd wakes the process */
    wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
        pfmemalloc_watermark_ok(pgdat));

check_pending:
    if (fatal_signal_pending(current))
        return true;

out:
    return false;
}

unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
                gfp_t gfp_mask, nodemask_t *nodemask)
{
    unsigned long nr_reclaimed;
    struct scan_control sc = {
        .gfp_mask = gfp_mask,
        .may_writepage = !laptop_mode,
        .nr_to_reclaim = SWAP_CLUSTER_MAX,
        .may_unmap = 1,
        .may_swap = 1,
        .order = order,
        .priority = DEF_PRIORITY,
        .target_mem_cgroup = NULL,
        .nodemask = nodemask,
    };
    struct shrink_control shrink = {
        .gfp_mask = sc.gfp_mask,
    };

    /*
     * Do not enter reclaim if fatal signal was delivered while throttled.
     * 1 is returned so that the page allocator does not OOM kill at this
     * point.
     */
    if (throttle_direct_reclaim(gfp_mask, zonelist, nodemask))
        return 1;

    trace_mm_vmscan_direct_reclaim_begin(order,
                sc.may_writepage,
                gfp_mask);

    nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);

    trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);

    return nr_reclaimed;
}

#ifdef CONFIG_MEMCG

unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *memcg,
                        gfp_t gfp_mask, bool noswap,
                        struct zone *zone,
                        unsigned long *nr_scanned)
{
    struct scan_control sc = {
        .nr_scanned = 0,
        .nr_to_reclaim = SWAP_CLUSTER_MAX,
        .may_writepage = !laptop_mode,
        .may_unmap = 1,
        .may_swap = !noswap,
        .order = 0,
        .priority = 0,
        .target_mem_cgroup = memcg,
    };
    struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);

    sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
            (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);

    trace_mm_vmscan_memcg_softlimit_reclaim_begin(sc.order,
                              sc.may_writepage,
                              sc.gfp_mask);

    /*
     * NOTE: Although we can get the priority field, using it
     * here is not a good idea, since it limits the pages we can scan.
     * if we don't reclaim here, the shrink_zone from balance_pgdat
     * will pick up pages from other mem cgroup's as well. We hack
     * the priority and make it zero.
     */
    shrink_lruvec(lruvec, &sc);

    trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);

    *nr_scanned = sc.nr_scanned;
    return sc.nr_reclaimed;
}

unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
                       gfp_t gfp_mask,
                       bool noswap)
{
    struct zonelist *zonelist;
    unsigned long nr_reclaimed;
    int nid;
    struct scan_control sc = {
        .may_writepage = !laptop_mode,
        .may_unmap = 1,
        .may_swap = !noswap,
        .nr_to_reclaim = SWAP_CLUSTER_MAX,
        .order = 0,
        .priority = DEF_PRIORITY,
        .target_mem_cgroup = memcg,
        .nodemask = NULL, /* we don't care the placement */
        .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
                (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
    };
    struct shrink_control shrink = {
        .gfp_mask = sc.gfp_mask,
    };

    /*
     * Unlike direct reclaim via alloc_pages(), memcg's reclaim doesn't
     * take care of from where we get pages. So the node where we start the
     * scan does not need to be the current node.
     */
    nid = mem_cgroup_select_victim_node(memcg);

    zonelist = NODE_DATA(nid)->node_zonelists;

    trace_mm_vmscan_memcg_reclaim_begin(0,
                        sc.may_writepage,
                        sc.gfp_mask);

    nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);

    trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);

    return nr_reclaimed;
}
#endif

static void age_active_anon(struct zone *zone, struct scan_control *sc)
{
    struct mem_cgroup *memcg;

    if (!total_swap_pages)
        return;

    memcg = mem_cgroup_iter(NULL, NULL, NULL);
    do {
        struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);

        if (inactive_anon_is_low(lruvec))
            shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                       sc, LRU_ACTIVE_ANON);

        memcg = mem_cgroup_iter(NULL, memcg, NULL);
    } while (memcg);
}

static bool zone_balanced(struct zone *zone, int order,
              unsigned long balance_gap, int classzone_idx)
{
    if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone) +
                    balance_gap, classzone_idx, 0))
        return false;

    if (COMPACTION_BUILD && order && !compaction_suitable(zone, order))
        return false;

    return true;
}

/*
 * pgdat_balanced is used when checking if a node is balanced for high-order
 * allocations. Only zones that meet watermarks and are in a zone allowed
 * by the callers classzone_idx are added to balanced_pages. The total of
 * balanced pages must be at least 25% of the zones allowed by classzone_idx
 * for the node to be considered balanced. Forcing all zones to be balanced
 * for high orders can cause excessive reclaim when there are imbalanced zones.
 * The choice of 25% is due to
 *   o a 16M DMA zone that is balanced will not balance a zone on any
 *     reasonable sized machine
 *   o On all other machines, the top zone must be at least a reasonable
 *     percentage of the middle zones. For example, on 32-bit x86, highmem
 *     would need to be at least 256M for it to be balance a whole node.
 *     Similarly, on x86-64 the Normal zone would need to be at least 1G
 *     to balance a node on its own. These seemed like reasonable ratios.
 */
static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
                        int classzone_idx)
{
    unsigned long present_pages = 0;
    int i;

    for (i = 0; i <= classzone_idx; i++)
        present_pages += pgdat->node_zones[i].present_pages;

    /* A special case here: if zone has no page, we think it's balanced */
    return balanced_pages >= (present_pages >> 2);
}

/*
 * Prepare kswapd for sleeping. This verifies that there are no processes
 * waiting in throttle_direct_reclaim() and that watermarks have been met.
 *
 * Returns true if kswapd is ready to sleep
 */
static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
                    int classzone_idx)
{
    int i;
    unsigned long balanced = 0;
    bool all_zones_ok = true;

    /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
    if (remaining)
        return false;

    /*
     * There is a potential race between when kswapd checks its watermarks
     * and a process gets throttled. There is also a potential race if
     * processes get throttled, kswapd wakes, a large process exits therby
     * balancing the zones that causes kswapd to miss a wakeup. If kswapd
     * is going to sleep, no process should be sleeping on pfmemalloc_wait
     * so wake them now if necessary. If necessary, processes will wake
     * kswapd and get throttled again
     */
    if (waitqueue_active(&pgdat->pfmemalloc_wait)) {
        wake_up(&pgdat->pfmemalloc_wait);
        return false;
    }

    /* Check the watermark levels */
    for (i = 0; i <= classzone_idx; i++) {
        struct zone *zone = pgdat->node_zones + i;

        if (!populated_zone(zone))
            continue;

        /*
         * balance_pgdat() skips over all_unreclaimable after
         * DEF_PRIORITY. Effectively, it considers them balanced so
         * they must be considered balanced here as well if kswapd
         * is to sleep
         */
        if (zone->all_unreclaimable) {
            balanced += zone->present_pages;
            continue;
        }

        if (!zone_balanced(zone, order, 0, i))
            all_zones_ok = false;
        else
            balanced += zone->present_pages;
    }

    /*
     * For high-order requests, the balanced zones must contain at least
     * 25% of the nodes pages for kswapd to sleep. For order-0, all zones
     * must be balanced
     */
    if (order)
        return pgdat_balanced(pgdat, balanced, classzone_idx);
    else
        return all_zones_ok;
}

/*
 * For kswapd, balance_pgdat() will work across all this node's zones until
 * they are all at high_wmark_pages(zone).
 *
 * Returns the final order kswapd was reclaiming at
 *
 * There is special handling here for zones which are full of pinned pages.
 * This can happen if the pages are all mlocked, or if they are all used by
 * device drivers (say, ZONE_DMA).  Or if they are all in use by hugetlb.
 * What we do is to detect the case where all pages in the zone have been
 * scanned twice and there has been zero successful reclaim.  Mark the zone as
 * dead and from now on, only perform a short scan.  Basically we're polling
 * the zone for when the problem goes away.
 *
 * kswapd scans the zones in the highmem->normal->dma direction.  It skips
 * zones which have free_pages > high_wmark_pages(zone), but once a zone is
 * found to have free_pages <= high_wmark_pages(zone), we scan that zone and the
 * lower zones regardless of the number of free pages in the lower zones. This
 * interoperates with the page allocator fallback scheme to ensure that aging
 * of pages is balanced across the zones.
 */
static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
                            int *classzone_idx)
{
    int all_zones_ok;
    unsigned long balanced;
    int i;
    int end_zone = 0;   /* Inclusive.  0 = ZONE_DMA */
    unsigned long total_scanned;
    struct reclaim_state *reclaim_state = current->reclaim_state;
    unsigned long nr_soft_reclaimed;
    unsigned long nr_soft_scanned;
    struct scan_control sc = {
        .gfp_mask = GFP_KERNEL,
        .may_unmap = 1,
        .may_swap = 1,
        /*
         * kswapd doesn't want to be bailed out while reclaim. because
         * we want to put equal scanning pressure on each zone.
         */
        .nr_to_reclaim = ULONG_MAX,
        .order = order,
        .target_mem_cgroup = NULL,
    };
    struct shrink_control shrink = {
        .gfp_mask = sc.gfp_mask,
    };
loop_again:
    total_scanned = 0;
    sc.priority = DEF_PRIORITY;
    sc.nr_reclaimed = 0;
    sc.may_writepage = !laptop_mode;
    count_vm_event(PAGEOUTRUN);

    do {
        unsigned long lru_pages = 0;
        int has_under_min_watermark_zone = 0;

        all_zones_ok = 1;
        balanced = 0;

        /*
         * Scan in the highmem->dma direction for the highest
         * zone which needs scanning
         */
        for (i = pgdat->nr_zones - 1; i >= 0; i--) {
            struct zone *zone = pgdat->node_zones + i;

            if (!populated_zone(zone))
                continue;

            if (zone->all_unreclaimable &&
                sc.priority != DEF_PRIORITY)
                continue;

            /*
             * Do some background aging of the anon list, to give
             * pages a chance to be referenced before reclaiming.
             */
            age_active_anon(zone, &sc);

            /*
             * If the number of buffer_heads in the machine
             * exceeds the maximum allowed level and this node
             * has a highmem zone, force kswapd to reclaim from
             * it to relieve lowmem pressure.
             */
            if (buffer_heads_over_limit && is_highmem_idx(i)) {
                end_zone = i;
                break;
            }

            if (!zone_balanced(zone, order, 0, 0)) {
                end_zone = i;
                break;
            } else {
                /* If balanced, clear the congested flag */
                zone_clear_flag(zone, ZONE_CONGESTED);
            }
        }
        if (i < 0)
            goto out;

        for (i = 0; i <= end_zone; i++) {
            struct zone *zone = pgdat->node_zones + i;

            lru_pages += zone_reclaimable_pages(zone);
        }

        /*
         * Now scan the zone in the dma->highmem direction, stopping
         * at the last zone which needs scanning.
         *
         * We do this because the page allocator works in the opposite
         * direction.  This prevents the page allocator from allocating
         * pages behind kswapd's direction of progress, which would
         * cause too much scanning of the lower zones.
         */
        for (i = 0; i <= end_zone; i++) {
            struct zone *zone = pgdat->node_zones + i;
            int nr_slab, testorder;
            unsigned long balance_gap;

            if (!populated_zone(zone))
                continue;

            if (zone->all_unreclaimable &&
                sc.priority != DEF_PRIORITY)
                continue;

            sc.nr_scanned = 0;

            nr_soft_scanned = 0;
            /*
             * Call soft limit reclaim before calling shrink_zone.
             */
            nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
                            order, sc.gfp_mask,
                            &nr_soft_scanned);
            sc.nr_reclaimed += nr_soft_reclaimed;
            total_scanned += nr_soft_scanned;

            /*
             * We put equal pressure on every zone, unless
             * one zone has way too many pages free
             * already. The "too many pages" is defined
             * as the high wmark plus a "gap" where the
             * gap is either the low watermark or 1%
             * of the zone, whichever is smaller.
             */
            balance_gap = min(low_wmark_pages(zone),
                (zone->present_pages +
                    KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
                KSWAPD_ZONE_BALANCE_GAP_RATIO);
            /*
             * Kswapd reclaims only single pages with compaction
             * enabled. Trying too hard to reclaim until contiguous
             * free pages have become available can hurt performance
             * by evicting too much useful data from memory.
             * Do not reclaim more than needed for compaction.
             */
            testorder = order;
            if (COMPACTION_BUILD && order &&
                    compaction_suitable(zone, order) !=
                        COMPACT_SKIPPED)
                testorder = 0;

            if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
                !zone_balanced(zone, testorder,
                       balance_gap, end_zone)) {
                shrink_zone(zone, &sc);

                reclaim_state->reclaimed_slab = 0;
                nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
                sc.nr_reclaimed += reclaim_state->reclaimed_slab;
                total_scanned += sc.nr_scanned;

                if (nr_slab == 0 && !zone_reclaimable(zone))
                    zone->all_unreclaimable = 1;
            }

            /*
             * If we've done a decent amount of scanning and
             * the reclaim ratio is low, start doing writepage
             * even in laptop mode
             */
            if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
                total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
                sc.may_writepage = 1;

            if (zone->all_unreclaimable) {
                if (end_zone && end_zone == i)
                    end_zone--;
                continue;
            }

            if (!zone_balanced(zone, testorder, 0, end_zone)) {
                all_zones_ok = 0;
                /*
                 * We are still under min water mark.  This
                 * means that we have a GFP_ATOMIC allocation
                 * failure risk. Hurry up!
                 */
                if (!zone_watermark_ok_safe(zone, order,
                        min_wmark_pages(zone), end_zone, 0))
                    has_under_min_watermark_zone = 1;
            } else {
                /*
                 * If a zone reaches its high watermark,
                 * consider it to be no longer congested. It's
                 * possible there are dirty pages backed by
                 * congested BDIs but as pressure is relieved,
                 * speculatively avoid congestion waits
                 */
                zone_clear_flag(zone, ZONE_CONGESTED);
                if (i <= *classzone_idx)
                    balanced += zone->present_pages;
            }

        }

        /*
         * If the low watermark is met there is no need for processes
         * to be throttled on pfmemalloc_wait as they should not be
         * able to safely make forward progress. Wake them
         */
        if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
                pfmemalloc_watermark_ok(pgdat))
            wake_up(&pgdat->pfmemalloc_wait);

        if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
            break;      /* kswapd: all done */
        /*
         * OK, kswapd is getting into trouble.  Take a nap, then take
         * another pass across the zones.
         */
        if (total_scanned && (sc.priority < DEF_PRIORITY - 2)) {
            if (has_under_min_watermark_zone)
                count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
            else
                congestion_wait(BLK_RW_ASYNC, HZ/10);
        }

        /*
         * We do this so kswapd doesn't build up large priorities for
         * example when it is freeing in parallel with allocators. It
         * matches the direct reclaim path behaviour in terms of impact
         * on zone->*_priority.
         */
        if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
            break;
    } while (--sc.priority >= 0);
out:

    /*
     * order-0: All zones must meet high watermark for a balanced node
     * high-order: Balanced zones must make up at least 25% of the node
     *             for the node to be balanced
     */
    if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))) {
        cond_resched();

        try_to_freeze();

        /*
         * Fragmentation may mean that the system cannot be
         * rebalanced for high-order allocations in all zones.
         * At this point, if nr_reclaimed < SWAP_CLUSTER_MAX,
         * it means the zones have been fully scanned and are still
         * not balanced. For high-order allocations, there is
         * little point trying all over again as kswapd may
         * infinite loop.
         *
         * Instead, recheck all watermarks at order-0 as they
         * are the most important. If watermarks are ok, kswapd will go
         * back to sleep. High-order users can still perform direct
         * reclaim if they wish.
         */
        if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
            order = sc.order = 0;

        goto loop_again;
    }

    /*
     * If kswapd was reclaiming at a higher order, it has the option of
     * sleeping without all zones being balanced. Before it does, it must
     * ensure that the watermarks for order-0 on *all* zones are met and
     * that the congestion flags are cleared. The congestion flag must
     * be cleared as kswapd is the only mechanism that clears the flag
     * and it is potentially going to sleep here.
     */
    if (order) {
        int zones_need_compaction = 1;

        for (i = 0; i <= end_zone; i++) {
            struct zone *zone = pgdat->node_zones + i;

            if (!populated_zone(zone))
                continue;

            /* Check if the memory needs to be defragmented. */
            if (zone_watermark_ok(zone, order,
                    low_wmark_pages(zone), *classzone_idx, 0))
                zones_need_compaction = 0;
        }

        if (zones_need_compaction)
            compact_pgdat(pgdat, order);
    }

    /*
     * Return the order we were reclaiming at so prepare_kswapd_sleep()
     * makes a decision on the order we were last reclaiming at. However,
     * if another caller entered the allocator slow path while kswapd
     * was awake, order will remain at the higher level
     */
    *classzone_idx = end_zone;
    return order;
}

static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
{
    long remaining = 0;
    DEFINE_WAIT(wait);

    if (freezing(current) || kthread_should_stop())
        return;

    prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);

    /* Try to sleep for a short interval */
    if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
        remaining = schedule_timeout(HZ/10);
        finish_wait(&pgdat->kswapd_wait, &wait);
        prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
    }

    /*
     * After a short sleep, check if it was a premature sleep. If not, then
     * go fully to sleep until explicitly woken up.
     */
    if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
        trace_mm_vmscan_kswapd_sleep(pgdat->node_id);

        /*
         * vmstat counters are not perfectly accurate and the estimated
         * value for counters such as NR_FREE_PAGES can deviate from the
         * true value by nr_online_cpus * threshold. To avoid the zone
         * watermarks being breached while under pressure, we reduce the
         * per-cpu vmstat threshold while kswapd is awake and restore
         * them before going back to sleep.
         */
        set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);

        /*
         * Compaction records what page blocks it recently failed to
         * isolate pages from and skips them in the future scanning.
         * When kswapd is going to sleep, it is reasonable to assume
         * that pages and compaction may succeed so reset the cache.
         */
        reset_isolation_suitable(pgdat);

        if (!kthread_should_stop())
            schedule();

        set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
    } else {
        if (remaining)
            count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
        else
            count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
    }
    finish_wait(&pgdat->kswapd_wait, &wait);
}

/*
 * The background pageout daemon, started as a kernel thread
 * from the init process.
 *
 * This basically trickles out pages so that we have _some_
 * free memory available even if there is no other activity
 * that frees anything up. This is needed for things like routing
 * etc, where we otherwise might have all activity going on in
 * asynchronous contexts that cannot page things out.
 *
 * If there are applications that are active memory-allocators
 * (most normal use), this basically shouldn't matter.
 */
static int kswapd(void *p)
{
    unsigned long order, new_order;
    unsigned balanced_order;
    int classzone_idx, new_classzone_idx;
    int balanced_classzone_idx;
    pg_data_t *pgdat = (pg_data_t*)p;
    struct task_struct *tsk = current;

    struct reclaim_state reclaim_state = {
        .reclaimed_slab = 0,
    };
    const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);

    lockdep_set_current_reclaim_state(GFP_KERNEL);

    if (!cpumask_empty(cpumask))
        set_cpus_allowed_ptr(tsk, cpumask);
    current->reclaim_state = &reclaim_state;

    /*
     * Tell the memory management that we're a "memory allocator",
     * and that if we need more memory we should get access to it
     * regardless (see "__alloc_pages()"). "kswapd" should
     * never get caught in the normal page freeing logic.
     *
     * (Kswapd normally doesn't need memory anyway, but sometimes
     * you need a small amount of memory in order to be able to
     * page out something else, and this flag essentially protects
     * us from recursively trying to free more memory as we're
     * trying to free the first piece of memory in the first place).
     */
    tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
    set_freezable();

    order = new_order = 0;
    balanced_order = 0;
    classzone_idx = new_classzone_idx = pgdat->nr_zones - 1;
    balanced_classzone_idx = classzone_idx;
    for ( ; ; ) {
        int ret;

        /*
         * If the last balance_pgdat was unsuccessful it's unlikely a
         * new request of a similar or harder type will succeed soon
         * so consider going to sleep on the basis we reclaimed at
         */
        if (balanced_classzone_idx >= new_classzone_idx &&
                    balanced_order == new_order) {
            new_order = pgdat->kswapd_max_order;
            new_classzone_idx = pgdat->classzone_idx;
            pgdat->kswapd_max_order =  0;
            pgdat->classzone_idx = pgdat->nr_zones - 1;
        }

        if (order < new_order || classzone_idx > new_classzone_idx) {
            /*
             * Don't sleep if someone wants a larger 'order'
             * allocation or has tigher zone constraints
             */
            order = new_order;
            classzone_idx = new_classzone_idx;
        } else {
            kswapd_try_to_sleep(pgdat, balanced_order,
                        balanced_classzone_idx);
            order = pgdat->kswapd_max_order;
            classzone_idx = pgdat->classzone_idx;
            new_order = order;
            new_classzone_idx = classzone_idx;
            pgdat->kswapd_max_order = 0;
            pgdat->classzone_idx = pgdat->nr_zones - 1;
        }

        ret = try_to_freeze();
        if (kthread_should_stop())
            break;

        /*
         * We can speed up thawing tasks if we don't call balance_pgdat
         * after returning from the refrigerator
         */
        if (!ret) {
            trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
            balanced_classzone_idx = classzone_idx;
            balanced_order = balance_pgdat(pgdat, order,
                        &balanced_classzone_idx);
        }
    }

    current->reclaim_state = NULL;
    return 0;
}

/*
 * A zone is low on free memory, so wake its kswapd task to service it.
 */
void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
{
    pg_data_t *pgdat;

    if (!populated_zone(zone))
        return;

    if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
        return;
    pgdat = zone->zone_pgdat;
    if (pgdat->kswapd_max_order < order) {
        pgdat->kswapd_max_order = order;
        pgdat->classzone_idx = min(pgdat->classzone_idx, classzone_idx);
    }
    if (!waitqueue_active(&pgdat->kswapd_wait))
        return;
    if (zone_watermark_ok_safe(zone, order, low_wmark_pages(zone), 0, 0))
        return;

    trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
    wake_up_interruptible(&pgdat->kswapd_wait);
}

/*
 * The reclaimable count would be mostly accurate.
 * The less reclaimable pages may be
 * - mlocked pages, which will be moved to unevictable list when encountered
 * - mapped pages, which may require several travels to be reclaimed
 * - dirty pages, which is not "instantly" reclaimable
 */
unsigned long global_reclaimable_pages(void)
{
    int nr;

    nr = global_page_state(NR_ACTIVE_FILE) +
         global_page_state(NR_INACTIVE_FILE);

    if (nr_swap_pages > 0)
        nr += global_page_state(NR_ACTIVE_ANON) +
              global_page_state(NR_INACTIVE_ANON);

    return nr;
}

unsigned long zone_reclaimable_pages(struct zone *zone)
{
    int nr;

    nr = zone_page_state(zone, NR_ACTIVE_FILE) +
         zone_page_state(zone, NR_INACTIVE_FILE);

    if (nr_swap_pages > 0)
        nr += zone_page_state(zone, NR_ACTIVE_ANON) +
              zone_page_state(zone, NR_INACTIVE_ANON);

    return nr;
}

#ifdef CONFIG_HIBERNATION
/*
 * Try to free `nr_to_reclaim' of memory, system-wide, and return the number of
 * freed pages.
 *
 * Rather than trying to age LRUs the aim is to preserve the overall
 * LRU order by reclaiming preferentially
 * inactive > active > active referenced > active mapped
 */
unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
{
    struct reclaim_state reclaim_state;
    struct scan_control sc = {
        .gfp_mask = GFP_HIGHUSER_MOVABLE,
        .may_swap = 1,
        .may_unmap = 1,
        .may_writepage = 1,
        .nr_to_reclaim = nr_to_reclaim,
        .hibernation_mode = 1,
        .order = 0,
        .priority = DEF_PRIORITY,
    };
    struct shrink_control shrink = {
        .gfp_mask = sc.gfp_mask,
    };
    struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
    struct task_struct *p = current;
    unsigned long nr_reclaimed;

    p->flags |= PF_MEMALLOC;
    lockdep_set_current_reclaim_state(sc.gfp_mask);
    reclaim_state.reclaimed_slab = 0;
    p->reclaim_state = &reclaim_state;

    nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);

    p->reclaim_state = NULL;
    lockdep_clear_current_reclaim_state();
    p->flags &= ~PF_MEMALLOC;

    return nr_reclaimed;
}
#endif /* CONFIG_HIBERNATION */

/* It's optimal to keep kswapds 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 __devinit 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_HIGH_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->kswapd, mask);
        }
    }
    return NOTIFY_OK;
}

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

    if (pgdat->kswapd)
        return 0;

    pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
    if (IS_ERR(pgdat->kswapd)) {
        /* failure at boot is fatal */
        BUG_ON(system_state == SYSTEM_BOOTING);
        pgdat->kswapd = NULL;
        pr_err("Failed to start kswapd on node %d\n", nid);
        ret = PTR_ERR(pgdat->kswapd);
    }
    return ret;
}

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

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

static int __init kswapd_init(void)
{
    int nid;

    swap_setup();
    for_each_node_state(nid, N_HIGH_MEMORY)
        kswapd_run(nid);
    hotcpu_notifier(cpu_callback, 0);
    return 0;
}

module_init(kswapd_init)

#ifdef CONFIG_NUMA
/*
 * Zone reclaim mode
 *
 * If non-zero call zone_reclaim when the number of free pages falls below
 * the watermarks.
 */
int zone_reclaim_mode __read_mostly;

#define RECLAIM_OFF 0
#define RECLAIM_ZONE (1<<0) /* Run shrink_inactive_list on the zone */
#define RECLAIM_WRITE (1<<1)    /* Writeout pages during reclaim */
#define RECLAIM_SWAP (1<<2) /* Swap pages out during reclaim */

/*
 * Priority for ZONE_RECLAIM. This determines the fraction of pages
 * of a node considered for each zone_reclaim. 4 scans 1/16th of
 * a zone.
 */
#define ZONE_RECLAIM_PRIORITY 4

/*
 * Percentage of pages in a zone that must be unmapped for zone_reclaim to
 * occur.
 */
int sysctl_min_unmapped_ratio = 1;

/*
 * If the number of slab pages in a zone grows beyond this percentage then
 * slab reclaim needs to occur.
 */
int sysctl_min_slab_ratio = 5;

static inline unsigned long zone_unmapped_file_pages(struct zone *zone)
{
    unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);
    unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE) +
        zone_page_state(zone, NR_ACTIVE_FILE);

    /*
     * It's possible for there to be more file mapped pages than
     * accounted for by the pages on the file LRU lists because
     * tmpfs pages accounted for as ANON can also be FILE_MAPPED
     */
    return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
}

/* Work out how many page cache pages we can reclaim in this reclaim_mode */
static long zone_pagecache_reclaimable(struct zone *zone)
{
    long nr_pagecache_reclaimable;
    long delta = 0;

    /*
     * If RECLAIM_SWAP is set, then all file pages are considered
     * potentially reclaimable. Otherwise, we have to worry about
     * pages like swapcache and zone_unmapped_file_pages() provides
     * a better estimate
     */
    if (zone_reclaim_mode & RECLAIM_SWAP)
        nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
    else
        nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);

    /* If we can't clean pages, remove dirty pages from consideration */
    if (!(zone_reclaim_mode & RECLAIM_WRITE))
        delta += zone_page_state(zone, NR_FILE_DIRTY);

    /* Watch for any possible underflows due to delta */
    if (unlikely(delta > nr_pagecache_reclaimable))
        delta = nr_pagecache_reclaimable;

    return nr_pagecache_reclaimable - delta;
}

/*
 * Try to free up some pages from this zone through reclaim.
 */
static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
{
    /* Minimum pages needed in order to stay on node */
    const unsigned long nr_pages = 1 << order;
    struct task_struct *p = current;
    struct reclaim_state reclaim_state;
    struct scan_control sc = {
        .may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
        .may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
        .may_swap = 1,
        .nr_to_reclaim = max_t(unsigned long, nr_pages,
                       SWAP_CLUSTER_MAX),
        .gfp_mask = gfp_mask,
        .order = order,
        .priority = ZONE_RECLAIM_PRIORITY,
    };
    struct shrink_control shrink = {
        .gfp_mask = sc.gfp_mask,
    };
    unsigned long nr_slab_pages0, nr_slab_pages1;

    cond_resched();
    /*
     * We need to be able to allocate from the reserves for RECLAIM_SWAP
     * and we also need to be able to write out pages for RECLAIM_WRITE
     * and RECLAIM_SWAP.
     */
    p->flags |= PF_MEMALLOC | PF_SWAPWRITE;
    lockdep_set_current_reclaim_state(gfp_mask);
    reclaim_state.reclaimed_slab = 0;
    p->reclaim_state = &reclaim_state;

    if (zone_pagecache_reclaimable(zone) > zone->min_unmapped_pages) {
        /*
         * Free memory by calling shrink zone with increasing
         * priorities until we have enough memory freed.
         */
        do {
            shrink_zone(zone, &sc);
        } while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
    }

    nr_slab_pages0 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
    if (nr_slab_pages0 > zone->min_slab_pages) {
        /*
         * shrink_slab() does not currently allow us to determine how
         * many pages were freed in this zone. So we take the current
         * number of slab pages and shake the slab until it is reduced
         * by the same nr_pages that we used for reclaiming unmapped
         * pages.
         *
         * Note that shrink_slab will free memory on all zones and may
         * take a long time.
         */
        for (;;) {
            unsigned long lru_pages = zone_reclaimable_pages(zone);

            /* No reclaimable slab or very low memory pressure */
            if (!shrink_slab(&shrink, sc.nr_scanned, lru_pages))
                break;

            /* Freed enough memory */
            nr_slab_pages1 = zone_page_state(zone,
                            NR_SLAB_RECLAIMABLE);
            if (nr_slab_pages1 + nr_pages <= nr_slab_pages0)
                break;
        }

        /*
         * Update nr_reclaimed by the number of slab pages we
         * reclaimed from this zone.
         */
        nr_slab_pages1 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
        if (nr_slab_pages1 < nr_slab_pages0)
            sc.nr_reclaimed += nr_slab_pages0 - nr_slab_pages1;
    }

    p->reclaim_state = NULL;
    current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);
    lockdep_clear_current_reclaim_state();
    return sc.nr_reclaimed >= nr_pages;
}

int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
{
    int node_id;
    int ret;

    /*
     * Zone reclaim reclaims unmapped file backed pages and
     * slab pages if we are over the defined limits.
     *
     * A small portion of unmapped file backed pages is needed for
     * file I/O otherwise pages read by file I/O will be immediately
     * thrown out if the zone is overallocated. So we do not reclaim
     * if less than a specified percentage of the zone is used by
     * unmapped file backed pages.
     */
    if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
        zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
        return ZONE_RECLAIM_FULL;

    if (zone->all_unreclaimable)
        return ZONE_RECLAIM_FULL;

    /*
     * Do not scan if the allocation should not be delayed.
     */
    if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
        return ZONE_RECLAIM_NOSCAN;

    /*
     * Only run zone reclaim on the local zone or on zones that do not
     * have associated processors. This will favor the local processor
     * over remote processors and spread off node memory allocations
     * as wide as possible.
     */
    node_id = zone_to_nid(zone);
    if (node_state(node_id, N_CPU) && node_id != numa_node_id())
        return ZONE_RECLAIM_NOSCAN;

    if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
        return ZONE_RECLAIM_NOSCAN;

    ret = __zone_reclaim(zone, gfp_mask, order);
    zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);

    if (!ret)
        count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);

    return ret;
}
#endif

/*
 * page_evictable - test whether a page is evictable
 * @page: the page to test
 *
 * Test whether page is evictable--i.e., should be placed on active/inactive
 * lists vs unevictable list.
 *
 * Reasons page might not be evictable:
 * (1) page's mapping marked unevictable
 * (2) page is part of an mlocked VMA
 *
 */
int page_evictable(struct page *page)
{
    return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
}

#ifdef CONFIG_SHMEM

void check_move_unevictable_pages(struct page **pages, int nr_pages)
{
    struct lruvec *lruvec;
    struct zone *zone = NULL;
    int pgscanned = 0;
    int pgrescued = 0;
    int i;

    for (i = 0; i < nr_pages; i++) {
        struct page *page = pages[i];
        struct zone *pagezone;

        pgscanned++;
        pagezone = page_zone(page);
        if (pagezone != zone) {
            if (zone)
                spin_unlock_irq(&zone->lru_lock);
            zone = pagezone;
            spin_lock_irq(&zone->lru_lock);
        }
        lruvec = mem_cgroup_page_lruvec(page, zone);

        if (!PageLRU(page) || !PageUnevictable(page))
            continue;

        if (page_evictable(page)) {
            enum lru_list lru = page_lru_base_type(page);

            VM_BUG_ON(PageActive(page));
            ClearPageUnevictable(page);
            del_page_from_lru_list(page, lruvec, LRU_UNEVICTABLE);
            add_page_to_lru_list(page, lruvec, lru);
            pgrescued++;
        }
    }

    if (zone) {
        __count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
        __count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
        spin_unlock_irq(&zone->lru_lock);
    }
}
#endif /* CONFIG_SHMEM */

static void warn_scan_unevictable_pages(void)
{
    printk_once(KERN_WARNING
            "%s: The scan_unevictable_pages sysctl/node-interface has been "
            "disabled for lack of a legitimate use case.  If you have "
            "one, please send an email to [email protected].\n",
            current->comm);
}

/*
 * scan_unevictable_pages [vm] sysctl handler.  On demand re-scan of
 * all nodes' unevictable lists for evictable pages
 */
unsigned long scan_unevictable_pages;

int scan_unevictable_handler(struct ctl_table *table, int write,
               void __user *buffer,
               size_t *length, loff_t *ppos)
{
    warn_scan_unevictable_pages();
    proc_doulongvec_minmax(table, write, buffer, length, ppos);
    scan_unevictable_pages = 0;
    return 0;
}

#ifdef CONFIG_NUMA
/*
 * per node 'scan_unevictable_pages' attribute.  On demand re-scan of
 * a specified node's per zone unevictable lists for evictable pages.
 */

static ssize_t read_scan_unevictable_node(struct device *dev,
                      struct device_attribute *attr,
                      char *buf)
{
    warn_scan_unevictable_pages();
    return sprintf(buf, "0\n"); /* always zero; should fit... */
}

static ssize_t write_scan_unevictable_node(struct device *dev,
                       struct device_attribute *attr,
                    const char *buf, size_t count)
{
    warn_scan_unevictable_pages();
    return 1;
}


static DEVICE_ATTR(scan_unevictable_pages, S_IRUGO | S_IWUSR,
            read_scan_unevictable_node,
            write_scan_unevictable_node);

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

void scan_unevictable_unregister_node(struct node *node)
{
    device_remove_file(&node->dev, &dev_attr_scan_unevictable_pages);
}
#endif