[pandora-kernel.git] / mm / vmscan.c

/*
 *  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 (kanoj@sgi.com).
 *  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/pagevec.h>
#include <linux/backing-dev.h>
#include <linux/rmap.h>
#include <linux/topology.h>
#include <linux/cpu.h>
#include <linux/cpuset.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 <asm/tlbflush.h>
#include <asm/div64.h>

#include <linux/swapops.h>

#include "internal.h"

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

/*
 * lumpy_mode determines how the inactive list is shrunk
 * LUMPY_MODE_SINGLE: Reclaim only order-0 pages
 * LUMPY_MODE_ASYNC:  Do not block
 * LUMPY_MODE_SYNC:   Allow blocking e.g. call wait_on_page_writeback
 * LUMPY_MODE_CONTIGRECLAIM: For high-order allocations, take a reference
 *                      page from the LRU and reclaim all pages within a
 *                      naturally aligned range
 */
typedef unsigned __bitwise__ lumpy_mode;
#define LUMPY_MODE_SINGLE               ((__force lumpy_mode)0x01u)
#define LUMPY_MODE_ASYNC                ((__force lumpy_mode)0x02u)
#define LUMPY_MODE_SYNC                 ((__force lumpy_mode)0x04u)
#define LUMPY_MODE_CONTIGRECLAIM        ((__force lumpy_mode)0x08u)

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 swappiness;

        int order;

        /*
         * Intend to reclaim enough continuous memory rather than reclaim
         * enough amount of memory. i.e, mode for high order allocation.
         */
        lumpy_mode lumpy_reclaim_mode;

        /* Which cgroup do we reclaim from */
        struct mem_cgroup *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_CGROUP_MEM_RES_CTLR
#define scanning_global_lru(sc) (!(sc)->mem_cgroup)
#else
#define scanning_global_lru(sc) (1)
#endif

static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
                                                  struct scan_control *sc)
{
        if (!scanning_global_lru(sc))
                return mem_cgroup_get_reclaim_stat(sc->mem_cgroup, zone);

        return &zone->reclaim_stat;
}

static unsigned long zone_nr_lru_pages(struct zone *zone,
                                struct scan_control *sc, enum lru_list lru)
{
        if (!scanning_global_lru(sc))
                return mem_cgroup_zone_nr_pages(sc->mem_cgroup, zone, lru);

        return zone_page_state(zone, NR_LRU_BASE + lru);
}


/*
 * Add a shrinker callback to be called from the vm
 */
void register_shrinker(struct shrinker *shrinker)
{
        shrinker->nr = 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);

#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(unsigned long scanned, gfp_t gfp_mask,
                        unsigned long lru_pages)
{
        struct shrinker *shrinker;
        unsigned long ret = 0;

        if (scanned == 0)
                scanned = SWAP_CLUSTER_MAX;

        if (!down_read_trylock(&shrinker_rwsem))
                return 1;       /* Assume we'll be able to shrink next time */

        list_for_each_entry(shrinker, &shrinker_list, list) {
                unsigned long long delta;
                unsigned long total_scan;
                unsigned long max_pass;

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

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

                total_scan = shrinker->nr;
                shrinker->nr = 0;

                while (total_scan >= SHRINK_BATCH) {
                        long this_scan = SHRINK_BATCH;
                        int shrink_ret;
                        int nr_before;

                        nr_before = (*shrinker->shrink)(shrinker, 0, gfp_mask);
                        shrink_ret = (*shrinker->shrink)(shrinker, this_scan,
                                                                gfp_mask);
                        if (shrink_ret == -1)
                                break;
                        if (shrink_ret < nr_before)
                                ret += nr_before - shrink_ret;
                        count_vm_events(SLABS_SCANNED, this_scan);
                        total_scan -= this_scan;

                        cond_resched();
                }

                shrinker->nr += total_scan;
        }
        up_read(&shrinker_rwsem);
        return ret;
}

static void set_lumpy_reclaim_mode(int priority, struct scan_control *sc,
                                   bool sync)
{
        lumpy_mode syncmode = sync ? LUMPY_MODE_SYNC : LUMPY_MODE_ASYNC;

        /*
         * Some reclaim have alredy been failed. No worth to try synchronous
         * lumpy reclaim.
         */
        if (sync && sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE)
                return;

        /*
         * If we need a large contiguous chunk of memory, or have
         * trouble getting a small set of contiguous pages, we
         * will reclaim both active and inactive pages.
         */
        sc->lumpy_reclaim_mode = LUMPY_MODE_CONTIGRECLAIM;
        if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
                sc->lumpy_reclaim_mode |= syncmode;
        else if (sc->order && priority < DEF_PRIORITY - 2)
                sc->lumpy_reclaim_mode |= syncmode;
        else
                sc->lumpy_reclaim_mode = LUMPY_MODE_SINGLE | LUMPY_MODE_ASYNC;
}

static void disable_lumpy_reclaim_mode(struct scan_control *sc)
{
        sc->lumpy_reclaim_mode = LUMPY_MODE_SINGLE | LUMPY_MODE_ASYNC;
}

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;

        /* lumpy reclaim for hugepage often need a lot of write */
        if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
                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_nosync(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;
                }

                /*
                 * Wait on writeback if requested to. This happens when
                 * direct reclaiming a large contiguous area and the
                 * first attempt to free a range of pages fails.
                 */
                if (PageWriteback(page) &&
                    (sc->lumpy_reclaim_mode & LUMPY_MODE_SYNC))
                        wait_on_page_writeback(page);

                if (!PageWriteback(page)) {
                        /* synchronous write or broken a_ops? */
                        ClearPageReclaim(page);
                }
                trace_mm_vmscan_writepage(page,
                        trace_reclaim_flags(page, sc->lumpy_reclaim_mode));
                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;

                __remove_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;
}

/**
 * putback_lru_page - put previously isolated page onto appropriate LRU list
 * @page: page to be put back to appropriate lru list
 *
 * Add previously isolated @page to appropriate LRU list.
 * Page may still be unevictable for other reasons.
 *
 * lru_lock must not be held, interrupts must be enabled.
 */
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, NULL)) {
                /*
                 * 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 clearing (page is
                 * unlocked), make sure that if the other thread does
                 * not observe our setting of PG_lru and fails
                 * isolation, we see PG_mlocked cleared below and move
                 * the page back to the evictable list.
                 *
                 * The other side is TestClearPageMlocked().
                 */
                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, NULL)) {
                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->mem_cgroup, &vm_flags);
        referenced_page = TestClearPageReferenced(page);

        /* Lumpy reclaim - ignore references */
        if (sc->lumpy_reclaim_mode & LUMPY_MODE_CONTIGRECLAIM)
                return PAGEREF_RECLAIM;

        /*
         * 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 (PageAnon(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)
                        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;
}

static noinline_for_stack void free_page_list(struct list_head *free_pages)
{
        struct pagevec freed_pvec;
        struct page *page, *tmp;

        pagevec_init(&freed_pvec, 1);

        list_for_each_entry_safe(page, tmp, free_pages, lru) {
                list_del(&page->lru);
                if (!pagevec_add(&freed_pvec, page)) {
                        __pagevec_free(&freed_pvec);
                        pagevec_reinit(&freed_pvec);
                }
        }

        pagevec_free(&freed_pvec);
}

/*
 * 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)
{
        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;

        cond_resched();

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

                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, NULL)))
                        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)) {
                        /*
                         * Synchronous reclaim is performed in two passes,
                         * first an asynchronous pass over the list to
                         * start parallel writeback, and a second synchronous
                         * pass to wait for the IO to complete.  Wait here
                         * for any page for which writeback has already
                         * started.
                         */
                        if ((sc->lumpy_reclaim_mode & LUMPY_MODE_SYNC) &&
                            may_enter_fs)
                                wait_on_page_writeback(page);
                        else {
                                unlock_page(page);
                                goto keep_lumpy;
                        }
                }

                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_UNMAP)) {
                        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++;

                        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_lumpy;
                                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);
                disable_lumpy_reclaim_mode(sc);
                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:
                disable_lumpy_reclaim_mode(sc);
keep_lumpy:
                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_congested && nr_dirty != 0)
                zone_set_flag(zone, ZONE_CONGESTED);

        free_page_list(&free_pages);

        list_splice(&ret_pages, page_list);
        count_vm_events(PGACTIVATE, pgactivate);
        return nr_reclaimed;
}

/*
 * 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, int mode, int file)
{
        int ret = -EINVAL;

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

        /*
         * When checking the active state, we need to be sure we are
         * dealing with comparible boolean values.  Take the logical not
         * of each.
         */
        if (mode != ISOLATE_BOTH && (!PageActive(page) != !mode))
                return ret;

        if (mode != ISOLATE_BOTH && page_is_file_cache(page) != file)
                return ret;

        /*
         * When this function is being called for lumpy reclaim, we
         * initially look into all LRU pages, active, inactive and
         * unevictable; only give shrink_page_list evictable pages.
         */
        if (PageUnevictable(page))
                return ret;

        ret = -EBUSY;

        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.
 * @src:        The LRU list to pull pages off.
 * @dst:        The temp list to put pages on to.
 * @scanned:    The number of pages that were scanned.
 * @order:      The caller's attempted allocation order
 * @mode:       One of the LRU isolation modes
 * @file:       True [1] if isolating file [!anon] pages
 *
 * returns how many pages were moved onto *@dst.
 */
static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
                struct list_head *src, struct list_head *dst,
                unsigned long *scanned, int order, int mode, int file)
{
        unsigned long nr_taken = 0;
        unsigned long nr_lumpy_taken = 0;
        unsigned long nr_lumpy_dirty = 0;
        unsigned long nr_lumpy_failed = 0;
        unsigned long scan;

        for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
                struct page *page;
                unsigned long pfn;
                unsigned long end_pfn;
                unsigned long page_pfn;
                int zone_id;

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

                VM_BUG_ON(!PageLRU(page));

                switch (__isolate_lru_page(page, mode, file)) {
                case 0:
                        list_move(&page->lru, dst);
                        mem_cgroup_del_lru(page);
                        nr_taken++;
                        break;

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

                default:
                        BUG();
                }

                if (!order)
                        continue;

                /*
                 * Attempt to take all pages in the order aligned region
                 * surrounding the tag page.  Only take those pages of
                 * the same active state as that tag page.  We may safely
                 * round the target page pfn down to the requested order
                 * as the mem_map is guarenteed valid out to MAX_ORDER,
                 * where that page is in a different zone we will detect
                 * it from its zone id and abort this block scan.
                 */
                zone_id = page_zone_id(page);
                page_pfn = page_to_pfn(page);
                pfn = page_pfn & ~((1 << order) - 1);
                end_pfn = pfn + (1 << order);
                for (; pfn < end_pfn; pfn++) {
                        struct page *cursor_page;

                        /* The target page is in the block, ignore it. */
                        if (unlikely(pfn == page_pfn))
                                continue;

                        /* Avoid holes within the zone. */
                        if (unlikely(!pfn_valid_within(pfn)))
                                break;

                        cursor_page = pfn_to_page(pfn);

                        /* Check that we have not crossed a zone boundary. */
                        if (unlikely(page_zone_id(cursor_page) != zone_id))
                                break;

                        /*
                         * If we don't have enough swap space, reclaiming of
                         * anon page which don't already have a swap slot is
                         * pointless.
                         */
                        if (nr_swap_pages <= 0 && PageAnon(cursor_page) &&
                            !PageSwapCache(cursor_page))
                                break;

                        if (__isolate_lru_page(cursor_page, mode, file) == 0) {
                                list_move(&cursor_page->lru, dst);
                                mem_cgroup_del_lru(cursor_page);
                                nr_taken++;
                                nr_lumpy_taken++;
                                if (PageDirty(cursor_page))
                                        nr_lumpy_dirty++;
                                scan++;
                        } else {
                                /* the page is freed already. */
                                if (!page_count(cursor_page))
                                        continue;
                                break;
                        }
                }

                /* If we break out of the loop above, lumpy reclaim failed */
                if (pfn < end_pfn)
                        nr_lumpy_failed++;
        }

        *scanned = scan;

        trace_mm_vmscan_lru_isolate(order,
                        nr_to_scan, scan,
                        nr_taken,
                        nr_lumpy_taken, nr_lumpy_dirty, nr_lumpy_failed,
                        mode);
        return nr_taken;
}

static unsigned long isolate_pages_global(unsigned long nr,
                                        struct list_head *dst,
                                        unsigned long *scanned, int order,
                                        int mode, struct zone *z,
                                        int active, int file)
{
        int lru = LRU_BASE;
        if (active)
                lru += LRU_ACTIVE;
        if (file)
                lru += LRU_FILE;
        return isolate_lru_pages(nr, &z->lru[lru].list, dst, scanned, order,
                                                                mode, file);
}

/*
 * clear_active_flags() is a helper for shrink_active_list(), clearing
 * any active bits from the pages in the list.
 */
static unsigned long clear_active_flags(struct list_head *page_list,
                                        unsigned int *count)
{
        int nr_active = 0;
        int lru;
        struct page *page;

        list_for_each_entry(page, page_list, lru) {
                lru = page_lru_base_type(page);
                if (PageActive(page)) {
                        lru += LRU_ACTIVE;
                        ClearPageActive(page);
                        nr_active++;
                }
                if (count)
                        count[lru]++;
        }

        return nr_active;
}

/**
 * isolate_lru_page - tries to isolate a page from its LRU list
 * @page: page to isolate from its LRU list
 *
 * Isolates a @page from an LRU list, clears PageLRU and adjusts the
 * vmstat statistic corresponding to whatever LRU list the page was on.
 *
 * Returns 0 if the page was removed from an LRU list.
 * Returns -EBUSY if the page was not on an LRU list.
 *
 * The returned page will have PageLRU() cleared.  If it was found on
 * the active list, it will have PageActive set.  If it was found on
 * the unevictable list, it will have the PageUnevictable bit set. That flag
 * may need to be cleared by the caller before letting the page go.
 *
 * The vmstat statistic corresponding to the list on which the page was
 * found will be decremented.
 *
 * Restrictions:
 * (1) Must be called with an elevated refcount on the page. This is a
 *     fundamentnal difference from isolate_lru_pages (which is called
 *     without a stable reference).
 * (2) the lru_lock must not be held.
 * (3) interrupts must be enabled.
 */
int isolate_lru_page(struct page *page)
{
        int ret = -EBUSY;

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

                spin_lock_irq(&zone->lru_lock);
                if (PageLRU(page) && get_page_unless_zero(page)) {
                        int lru = page_lru(page);
                        ret = 0;
                        ClearPageLRU(page);

                        del_page_from_lru_list(zone, page, lru);
                }
                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 (!scanning_global_lru(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;
}

/*
 * TODO: Try merging with migrations version of putback_lru_pages
 */
static noinline_for_stack void
putback_lru_pages(struct zone *zone, struct scan_control *sc,
                                unsigned long nr_anon, unsigned long nr_file,
                                struct list_head *page_list)
{
        struct page *page;
        struct pagevec pvec;
        struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);

        pagevec_init(&pvec, 1);

        /*
         * Put back any unfreeable pages.
         */
        spin_lock(&zone->lru_lock);
        while (!list_empty(page_list)) {
                int lru;
                page = lru_to_page(page_list);
                VM_BUG_ON(PageLRU(page));
                list_del(&page->lru);
                if (unlikely(!page_evictable(page, NULL))) {
                        spin_unlock_irq(&zone->lru_lock);
                        putback_lru_page(page);
                        spin_lock_irq(&zone->lru_lock);
                        continue;
                }
                SetPageLRU(page);
                lru = page_lru(page);
                add_page_to_lru_list(zone, page, lru);
                if (is_active_lru(lru)) {
                        int file = is_file_lru(lru);
                        reclaim_stat->recent_rotated[file]++;
                }
                if (!pagevec_add(&pvec, page)) {
                        spin_unlock_irq(&zone->lru_lock);
                        __pagevec_release(&pvec);
                        spin_lock_irq(&zone->lru_lock);
                }
        }
        __mod_zone_page_state(zone, NR_ISOLATED_ANON, -nr_anon);
        __mod_zone_page_state(zone, NR_ISOLATED_FILE, -nr_file);

        spin_unlock_irq(&zone->lru_lock);
        pagevec_release(&pvec);
}

static noinline_for_stack void update_isolated_counts(struct zone *zone,
                                        struct scan_control *sc,
                                        unsigned long *nr_anon,
                                        unsigned long *nr_file,
                                        struct list_head *isolated_list)
{
        unsigned long nr_active;
        unsigned int count[NR_LRU_LISTS] = { 0, };
        struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);

        nr_active = clear_active_flags(isolated_list, count);
        __count_vm_events(PGDEACTIVATE, nr_active);

        __mod_zone_page_state(zone, NR_ACTIVE_FILE,
                              -count[LRU_ACTIVE_FILE]);
        __mod_zone_page_state(zone, NR_INACTIVE_FILE,
                              -count[LRU_INACTIVE_FILE]);
        __mod_zone_page_state(zone, NR_ACTIVE_ANON,
                              -count[LRU_ACTIVE_ANON]);
        __mod_zone_page_state(zone, NR_INACTIVE_ANON,
                              -count[LRU_INACTIVE_ANON]);

        *nr_anon = count[LRU_ACTIVE_ANON] + count[LRU_INACTIVE_ANON];
        *nr_file = count[LRU_ACTIVE_FILE] + count[LRU_INACTIVE_FILE];
        __mod_zone_page_state(zone, NR_ISOLATED_ANON, *nr_anon);
        __mod_zone_page_state(zone, NR_ISOLATED_FILE, *nr_file);

        reclaim_stat->recent_scanned[0] += *nr_anon;
        reclaim_stat->recent_scanned[1] += *nr_file;
}

/*
 * Returns true if the caller should wait to clean dirty/writeback pages.
 *
 * If we are direct reclaiming for contiguous pages and we do not reclaim
 * everything in the list, try again and wait for writeback IO to complete.
 * This will stall high-order allocations noticeably. Only do that when really
 * need to free the pages under high memory pressure.
 */
static inline bool should_reclaim_stall(unsigned long nr_taken,
                                        unsigned long nr_freed,
                                        int priority,
                                        struct scan_control *sc)
{
        int lumpy_stall_priority;

        /* kswapd should not stall on sync IO */
        if (current_is_kswapd())
                return false;

        /* Only stall on lumpy reclaim */
        if (sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE)
                return false;

        /* If we have relaimed everything on the isolated list, no stall */
        if (nr_freed == nr_taken)
                return false;

        /*
         * For high-order allocations, there are two stall thresholds.
         * High-cost allocations stall immediately where as lower
         * order allocations such as stacks require the scanning
         * priority to be much higher before stalling.
         */
        if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
                lumpy_stall_priority = DEF_PRIORITY;
        else
                lumpy_stall_priority = DEF_PRIORITY / 3;

        return priority <= lumpy_stall_priority;
}

/*
 * 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 zone *zone,
                        struct scan_control *sc, int priority, int file)
{
        LIST_HEAD(page_list);
        unsigned long nr_scanned;
        unsigned long nr_reclaimed = 0;
        unsigned long nr_taken;
        unsigned long nr_anon;
        unsigned long nr_file;

        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;
        }

        set_lumpy_reclaim_mode(priority, sc, false);
        lru_add_drain();
        spin_lock_irq(&zone->lru_lock);

        if (scanning_global_lru(sc)) {
                nr_taken = isolate_pages_global(nr_to_scan,
                        &page_list, &nr_scanned, sc->order,
                        sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE ?
                                        ISOLATE_INACTIVE : ISOLATE_BOTH,
                        zone, 0, file);
                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);
        } else {
                nr_taken = mem_cgroup_isolate_pages(nr_to_scan,
                        &page_list, &nr_scanned, sc->order,
                        sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE ?
                                        ISOLATE_INACTIVE : ISOLATE_BOTH,
                        zone, sc->mem_cgroup,
                        0, file);
                /*
                 * mem_cgroup_isolate_pages() keeps track of
                 * scanned pages on its own.
                 */
        }

        if (nr_taken == 0) {
                spin_unlock_irq(&zone->lru_lock);
                return 0;
        }

        update_isolated_counts(zone, sc, &nr_anon, &nr_file, &page_list);

        spin_unlock_irq(&zone->lru_lock);

        nr_reclaimed = shrink_page_list(&page_list, zone, sc);

        /* Check if we should syncronously wait for writeback */
        if (should_reclaim_stall(nr_taken, nr_reclaimed, priority, sc)) {
                set_lumpy_reclaim_mode(priority, sc, true);
                nr_reclaimed += shrink_page_list(&page_list, zone, sc);
        }

        local_irq_disable();
        if (current_is_kswapd())
                __count_vm_events(KSWAPD_STEAL, nr_reclaimed);
        __count_zone_vm_events(PGSTEAL, zone, nr_reclaimed);

        putback_lru_pages(zone, sc, nr_anon, nr_file, &page_list);

        trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
                zone_idx(zone),
                nr_scanned, nr_reclaimed,
                priority,
                trace_shrink_flags(file, sc->lumpy_reclaim_mode));
        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 zone *zone,
                                     struct list_head *list,
                                     enum lru_list lru)
{
        unsigned long pgmoved = 0;
        struct pagevec pvec;
        struct page *page;

        pagevec_init(&pvec, 1);

        while (!list_empty(list)) {
                page = lru_to_page(list);

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

                list_move(&page->lru, &zone->lru[lru].list);
                mem_cgroup_add_lru_list(page, lru);
                pgmoved++;

                if (!pagevec_add(&pvec, page) || list_empty(list)) {
                        spin_unlock_irq(&zone->lru_lock);
                        if (buffer_heads_over_limit)
                                pagevec_strip(&pvec);
                        __pagevec_release(&pvec);
                        spin_lock_irq(&zone->lru_lock);
                }
        }
        __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_pages, struct zone *zone,
                        struct scan_control *sc, int priority, int file)
{
        unsigned long nr_taken;
        unsigned long pgscanned;
        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 = get_reclaim_stat(zone, sc);
        unsigned long nr_rotated = 0;

        lru_add_drain();
        spin_lock_irq(&zone->lru_lock);
        if (scanning_global_lru(sc)) {
                nr_taken = isolate_pages_global(nr_pages, &l_hold,
                                                &pgscanned, sc->order,
                                                ISOLATE_ACTIVE, zone,
                                                1, file);
                zone->pages_scanned += pgscanned;
        } else {
                nr_taken = mem_cgroup_isolate_pages(nr_pages, &l_hold,
                                                &pgscanned, sc->order,
                                                ISOLATE_ACTIVE, zone,
                                                sc->mem_cgroup, 1, file);
                /*
                 * mem_cgroup_isolate_pages() keeps track of
                 * scanned pages on its own.
                 */
        }

        reclaim_stat->recent_scanned[file] += nr_taken;

        __count_zone_vm_events(PGREFILL, zone, pgscanned);
        if (file)
                __mod_zone_page_state(zone, NR_ACTIVE_FILE, -nr_taken);
        else
                __mod_zone_page_state(zone, NR_ACTIVE_ANON, -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, NULL))) {
                        putback_lru_page(page);
                        continue;
                }

                if (page_referenced(page, 0, sc->mem_cgroup, &vm_flags)) {
                        nr_rotated++;
                        /*
                         * 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(zone, &l_active,
                                                LRU_ACTIVE + file * LRU_FILE);
        move_active_pages_to_lru(zone, &l_inactive,
                                                LRU_BASE   + file * LRU_FILE);
        __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
        spin_unlock_irq(&zone->lru_lock);
}

#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;
}

/**
 * inactive_anon_is_low - check if anonymous pages need to be deactivated
 * @zone: zone to check
 * @sc:   scan control of this context
 *
 * Returns true if the zone does not have enough inactive anon pages,
 * meaning some active anon pages need to be deactivated.
 */
static int inactive_anon_is_low(struct zone *zone, struct scan_control *sc)
{
        int low;

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

        if (scanning_global_lru(sc))
                low = inactive_anon_is_low_global(zone);
        else
                low = mem_cgroup_inactive_anon_is_low(sc->mem_cgroup);
        return low;
}
#else
static inline int inactive_anon_is_low(struct zone *zone,
                                        struct scan_control *sc)
{
        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);
}

/**
 * inactive_file_is_low - check if file pages need to be deactivated
 * @zone: zone to check
 * @sc:   scan control of this context
 *
 * When the system is doing streaming IO, memory pressure here
 * ensures that active file pages get deactivated, until more
 * than half of the file pages are on the inactive list.
 *
 * Once we get to that situation, protect the system's working
 * set from being evicted by disabling active file page aging.
 *
 * This uses a different ratio than the anonymous pages, because
 * the page cache uses a use-once replacement algorithm.
 */
static int inactive_file_is_low(struct zone *zone, struct scan_control *sc)
{
        int low;

        if (scanning_global_lru(sc))
                low = inactive_file_is_low_global(zone);
        else
                low = mem_cgroup_inactive_file_is_low(sc->mem_cgroup);
        return low;
}

static int inactive_list_is_low(struct zone *zone, struct scan_control *sc,
                                int file)
{
        if (file)
                return inactive_file_is_low(zone, sc);
        else
                return inactive_anon_is_low(zone, sc);
}

static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
        struct zone *zone, struct scan_control *sc, int priority)
{
        int file = is_file_lru(lru);

        if (is_active_lru(lru)) {
                if (inactive_list_is_low(zone, sc, file))
                    shrink_active_list(nr_to_scan, zone, sc, priority, file);
                return 0;
        }

        return shrink_inactive_list(nr_to_scan, zone, sc, priority, file);
}

/*
 * Smallish @nr_to_scan's are deposited in @nr_saved_scan,
 * until we collected @swap_cluster_max pages to scan.
 */
static unsigned long nr_scan_try_batch(unsigned long nr_to_scan,
                                       unsigned long *nr_saved_scan)
{
        unsigned long nr;

        *nr_saved_scan += nr_to_scan;
        nr = *nr_saved_scan;

        if (nr >= SWAP_CLUSTER_MAX)
                *nr_saved_scan = 0;
        else
                nr = 0;

        return nr;
}

/*
 * 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 pages to scan; nr[1] = file pages to scan
 */
static void get_scan_count(struct zone *zone, struct scan_control *sc,
                                        unsigned long *nr, int priority)
{
        unsigned long anon, file, free;
        unsigned long anon_prio, file_prio;
        unsigned long ap, fp;
        struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
        u64 fraction[2], denominator;
        enum lru_list l;
        int noswap = 0;

        /* 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  = zone_nr_lru_pages(zone, sc, LRU_ACTIVE_ANON) +
                zone_nr_lru_pages(zone, sc, LRU_INACTIVE_ANON);
        file  = zone_nr_lru_pages(zone, sc, LRU_ACTIVE_FILE) +
                zone_nr_lru_pages(zone, sc, LRU_INACTIVE_FILE);

        if (scanning_global_lru(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 = sc->swappiness;
        file_prio = 200 - sc->swappiness;

        /*
         * 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 + 1) * (reclaim_stat->recent_scanned[0] + 1);
        ap /= reclaim_stat->recent_rotated[0] + 1;

        fp = (file_prio + 1) * (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(l) {
                int file = is_file_lru(l);
                unsigned long scan;

                scan = zone_nr_lru_pages(zone, sc, l);
                if (priority || noswap) {
                        scan >>= priority;
                        scan = div64_u64(scan * fraction[file], denominator);
                }
                nr[l] = nr_scan_try_batch(scan,
                                          &reclaim_stat->nr_saved_scan[l]);
        }
}

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

        get_scan_count(zone, sc, nr, priority);

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

                                nr_reclaimed += shrink_list(l, nr_to_scan,
                                                            zone, sc, priority);
                        }
                }
                /*
                 * 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 && priority < DEF_PRIORITY)
                        break;
        }

        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(zone, sc))
                shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);

        throttle_vm_writeout(sc->gfp_mask);
}

/*
 * 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.
 */
static void shrink_zones(int priority, 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;
                /*
                 * Take care memory controller reclaiming has small influence
                 * to global LRU.
                 */
                if (scanning_global_lru(sc)) {
                        if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                                continue;
                        if (zone->all_unreclaimable && priority != DEF_PRIORITY)
                                continue;       /* Let kswapd poll it */
                }

                shrink_zone(priority, zone, sc);
        }
}

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

/*
 * As hibernation is going on, kswapd is freezed so that it can't mark
 * the zone into all_unreclaimable. It can't handle OOM during hibernation.
 * So let's check zone's unreclaimable in direct reclaim as well as kswapd.
 */
static bool all_unreclaimable(struct zonelist *zonelist,
                struct scan_control *sc)
{
        struct zoneref *z;
        struct zone *zone;
        bool all_unreclaimable = true;

        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_reclaimable(zone)) {
                        all_unreclaimable = false;
                        break;
                }
        }

        return all_unreclaimable;
}

/*
 * 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)
{
        int priority;
        unsigned long total_scanned = 0;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        struct zoneref *z;
        struct zone *zone;
        unsigned long writeback_threshold;

        get_mems_allowed();
        delayacct_freepages_start();

        if (scanning_global_lru(sc))
                count_vm_event(ALLOCSTALL);

        for (priority = DEF_PRIORITY; priority >= 0; priority--) {
                sc->nr_scanned = 0;
                if (!priority)
                        disable_swap_token();
                shrink_zones(priority, zonelist, sc);
                /*
                 * Don't shrink slabs when reclaiming memory from
                 * over limit cgroups
                 */
                if (scanning_global_lru(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(sc->nr_scanned, sc->gfp_mask, 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);
                        sc->may_writepage = 1;
                }

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

                        first_zones_zonelist(zonelist, gfp_zone(sc->gfp_mask),
                                                        NULL, &preferred_zone);
                        wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/10);
                }
        }

out:
        delayacct_freepages_end();
        put_mems_allowed();

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

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

        return 0;
}

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,
                .swappiness = vm_swappiness,
                .order = order,
                .mem_cgroup = NULL,
                .nodemask = nodemask,
        };

        trace_mm_vmscan_direct_reclaim_begin(order,
                                sc.may_writepage,
                                gfp_mask);

        nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

        trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);

        return nr_reclaimed;
}

#ifdef CONFIG_CGROUP_MEM_RES_CTLR

unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *mem,
                                                gfp_t gfp_mask, bool noswap,
                                                unsigned int swappiness,
                                                struct zone *zone)
{
        struct scan_control sc = {
                .nr_to_reclaim = SWAP_CLUSTER_MAX,
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .may_swap = !noswap,
                .swappiness = swappiness,
                .order = 0,
                .mem_cgroup = mem,
        };
        sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
                        (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);

        trace_mm_vmscan_memcg_softlimit_reclaim_begin(0,
                                                      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_zone(0, zone, &sc);

        trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);

        return sc.nr_reclaimed;
}

unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
                                           gfp_t gfp_mask,
                                           bool noswap,
                                           unsigned int swappiness)
{
        struct zonelist *zonelist;
        unsigned long nr_reclaimed;
        struct scan_control sc = {
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .may_swap = !noswap,
                .nr_to_reclaim = SWAP_CLUSTER_MAX,
                .swappiness = swappiness,
                .order = 0,
                .mem_cgroup = mem_cont,
                .nodemask = NULL, /* we don't care the placement */
        };

        sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
                        (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
        zonelist = NODE_DATA(numa_node_id())->node_zonelists;

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

        nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

        trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);

        return nr_reclaimed;
}
#endif

/* is kswapd sleeping prematurely? */
static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
{
        int i;

        /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
        if (remaining)
                return 1;

        /* If after HZ/10, a zone is below the high mark, it's premature */
        for (i = 0; i < pgdat->nr_zones; i++) {
                struct zone *zone = pgdat->node_zones + i;

                if (!populated_zone(zone))
                        continue;

                if (zone->all_unreclaimable)
                        continue;

                if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone),
                                                                0, 0))
                        return 1;
        }

        return 0;
}

/*
 * For kswapd, balance_pgdat() will work across all this node's zones until
 * they are all at high_wmark_pages(zone).
 *
 * Returns the number of pages which were actually freed.
 *
 * 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 all_zones_ok;
        int priority;
        int i;
        unsigned long total_scanned;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        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,
                .swappiness = vm_swappiness,
                .order = order,
                .mem_cgroup = NULL,
        };
loop_again:
        total_scanned = 0;
        sc.nr_reclaimed = 0;
        sc.may_writepage = !laptop_mode;
        count_vm_event(PAGEOUTRUN);

        for (priority = DEF_PRIORITY; priority >= 0; priority--) {
                int end_zone = 0;       /* Inclusive.  0 = ZONE_DMA */
                unsigned long lru_pages = 0;
                int has_under_min_watermark_zone = 0;

                /* The swap token gets in the way of swapout... */
                if (!priority)
                        disable_swap_token();

                all_zones_ok = 1;

                /*
                 * 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 && priority != DEF_PRIORITY)
                                continue;

                        /*
                         * Do some background aging of the anon list, to give
                         * pages a chance to be referenced before reclaiming.
                         */
                        if (inactive_anon_is_low(zone, &sc))
                                shrink_active_list(SWAP_CLUSTER_MAX, zone,
                                                        &sc, priority, 0);

                        if (!zone_watermark_ok_safe(zone, order,
                                        high_wmark_pages(zone), 0, 0)) {
                                end_zone = i;
                                break;
                        }
                }
                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;

                        if (!populated_zone(zone))
                                continue;

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

                        sc.nr_scanned = 0;

                        /*
                         * Call soft limit reclaim before calling shrink_zone.
                         * For now we ignore the return value
                         */
                        mem_cgroup_soft_limit_reclaim(zone, order, sc.gfp_mask);

                        /*
                         * We put equal pressure on every zone, unless one
                         * zone has way too many pages free already.
                         */
                        if (!zone_watermark_ok_safe(zone, order,
                                        8*high_wmark_pages(zone), end_zone, 0))
                                shrink_zone(priority, zone, &sc);
                        reclaim_state->reclaimed_slab = 0;
                        nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,
                                                lru_pages);
                        sc.nr_reclaimed += reclaim_state->reclaimed_slab;
                        total_scanned += sc.nr_scanned;
                        if (zone->all_unreclaimable)
                                continue;
                        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_watermark_ok_safe(zone, order,
                                        high_wmark_pages(zone), end_zone, 0)) {
                                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,
                                 * spectulatively avoid congestion waits
                                 */
                                zone_clear_flag(zone, ZONE_CONGESTED);
                        }

                }
                if (all_zones_ok)
                        break;          /* kswapd: all done */
                /*
                 * OK, kswapd is getting into trouble.  Take a nap, then take
                 * another pass across the zones.
                 */
                if (total_scanned && (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;
        }
out:
        if (!all_zones_ok) {
                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;
        }

        return sc.nr_reclaimed;
}

static void kswapd_try_to_sleep(pg_data_t *pgdat, int order)
{
        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 (!sleeping_prematurely(pgdat, order, remaining)) {
                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 (!sleeping_prematurely(pgdat, order, remaining)) {
                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);
                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;
        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 = 0;
        for ( ; ; ) {
                unsigned long new_order;
                int ret;

                new_order = pgdat->kswapd_max_order;
                pgdat->kswapd_max_order = 0;
                if (order < new_order) {
                        /*
                         * Don't sleep if someone wants a larger 'order'
                         * allocation
                         */
                        order = new_order;
                } else {
                        kswapd_try_to_sleep(pgdat, order);
                        order = pgdat->kswapd_max_order;
                }

                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);
                        balance_pgdat(pgdat, order);
                }
        }
        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)
{
        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;
        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,
                .swappiness = vm_swappiness,
                .order = 0,
        };
        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);

        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);
                printk("Failed to start kswapd on node %d\n",nid);
                ret = -1;
        }
        return ret;
}

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

        if (kswapd)
                kthread_stop(kswapd);
}

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;
        int priority;
        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,
                .swappiness = vm_swappiness,
                .order = order,
        };
        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.
                 */
                priority = ZONE_RECLAIM_PRIORITY;
                do {
                        shrink_zone(priority, zone, &sc);
                        priority--;
                } while (priority >= 0 && sc.nr_reclaimed < nr_pages);
        }

        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(sc.nr_scanned, gfp_mask, 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
 * @vma: the VMA in which the page is or will be mapped, may be NULL
 *
 * Test whether page is evictable--i.e., should be placed on active/inactive
 * lists vs unevictable list.  The vma argument is !NULL when called from the
 * fault path to determine how to instantate a new page.
 *
 * 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, struct vm_area_struct *vma)
{

        if (mapping_unevictable(page_mapping(page)))
                return 0;

        if (PageMlocked(page) || (vma && is_mlocked_vma(vma, page)))
                return 0;

        return 1;
}

/**
 * check_move_unevictable_page - check page for evictability and move to appropriate zone lru list
 * @page: page to check evictability and move to appropriate lru list
 * @zone: zone page is in
 *
 * Checks a page for evictability and moves the page to the appropriate
 * zone lru list.
 *
 * Restrictions: zone->lru_lock must be held, page must be on LRU and must
 * have PageUnevictable set.
 */
static void check_move_unevictable_page(struct page *page, struct zone *zone)
{
        VM_BUG_ON(PageActive(page));

retry:
        ClearPageUnevictable(page);
        if (page_evictable(page, NULL)) {
                enum lru_list l = page_lru_base_type(page);

                __dec_zone_state(zone, NR_UNEVICTABLE);
                list_move(&page->lru, &zone->lru[l].list);
                mem_cgroup_move_lists(page, LRU_UNEVICTABLE, l);
                __inc_zone_state(zone, NR_INACTIVE_ANON + l);
                __count_vm_event(UNEVICTABLE_PGRESCUED);
        } else {
                /*
                 * rotate unevictable list
                 */
                SetPageUnevictable(page);
                list_move(&page->lru, &zone->lru[LRU_UNEVICTABLE].list);
                mem_cgroup_rotate_lru_list(page, LRU_UNEVICTABLE);
                if (page_evictable(page, NULL))
                        goto retry;
        }
}

/**
 * scan_mapping_unevictable_pages - scan an address space for evictable pages
 * @mapping: struct address_space to scan for evictable pages
 *
 * Scan all pages in mapping.  Check unevictable pages for
 * evictability and move them to the appropriate zone lru list.
 */
void scan_mapping_unevictable_pages(struct address_space *mapping)
{
        pgoff_t next = 0;
        pgoff_t end   = (i_size_read(mapping->host) + PAGE_CACHE_SIZE - 1) >>
                         PAGE_CACHE_SHIFT;
        struct zone *zone;
        struct pagevec pvec;

        if (mapping->nrpages == 0)
                return;

        pagevec_init(&pvec, 0);
        while (next < end &&
                pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
                int i;
                int pg_scanned = 0;

                zone = NULL;

                for (i = 0; i < pagevec_count(&pvec); i++) {
                        struct page *page = pvec.pages[i];
                        pgoff_t page_index = page->index;
                        struct zone *pagezone = page_zone(page);

                        pg_scanned++;
                        if (page_index > next)
                                next = page_index;
                        next++;

                        if (pagezone != zone) {
                                if (zone)
                                        spin_unlock_irq(&zone->lru_lock);
                                zone = pagezone;
                                spin_lock_irq(&zone->lru_lock);
                        }

                        if (PageLRU(page) && PageUnevictable(page))
                                check_move_unevictable_page(page, zone);
                }
                if (zone)
                        spin_unlock_irq(&zone->lru_lock);
                pagevec_release(&pvec);

                count_vm_events(UNEVICTABLE_PGSCANNED, pg_scanned);
        }

}

/**
 * scan_zone_unevictable_pages - check unevictable list for evictable pages
 * @zone - zone of which to scan the unevictable list
 *
 * Scan @zone's unevictable LRU lists to check for pages that have become
 * evictable.  Move those that have to @zone's inactive list where they
 * become candidates for reclaim, unless shrink_inactive_zone() decides
 * to reactivate them.  Pages that are still unevictable are rotated
 * back onto @zone's unevictable list.
 */
#define SCAN_UNEVICTABLE_BATCH_SIZE 16UL /* arbitrary lock hold batch size */
static void scan_zone_unevictable_pages(struct zone *zone)
{
        struct list_head *l_unevictable = &zone->lru[LRU_UNEVICTABLE].list;
        unsigned long scan;
        unsigned long nr_to_scan = zone_page_state(zone, NR_UNEVICTABLE);

        while (nr_to_scan > 0) {
                unsigned long batch_size = min(nr_to_scan,
                                                SCAN_UNEVICTABLE_BATCH_SIZE);

                spin_lock_irq(&zone->lru_lock);
                for (scan = 0;  scan < batch_size; scan++) {
                        struct page *page = lru_to_page(l_unevictable);

                        if (!trylock_page(page))
                                continue;

                        prefetchw_prev_lru_page(page, l_unevictable, flags);

                        if (likely(PageLRU(page) && PageUnevictable(page)))
                                check_move_unevictable_page(page, zone);

                        unlock_page(page);
                }
                spin_unlock_irq(&zone->lru_lock);

                nr_to_scan -= batch_size;
        }
}


/**
 * scan_all_zones_unevictable_pages - scan all unevictable lists for evictable pages
 *
 * A really big hammer:  scan all zones' unevictable LRU lists to check for
 * pages that have become evictable.  Move those back to the zones'
 * inactive list where they become candidates for reclaim.
 * This occurs when, e.g., we have unswappable pages on the unevictable lists,
 * and we add swap to the system.  As such, it runs in the context of a task
 * that has possibly/probably made some previously unevictable pages
 * evictable.
 */
static void scan_all_zones_unevictable_pages(void)
{
        struct zone *zone;

        for_each_zone(zone) {
                scan_zone_unevictable_pages(zone);
        }
}

/*
 * 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)
{
        proc_doulongvec_minmax(table, write, buffer, length, ppos);

        if (write && *(unsigned long *)table->data)
                scan_all_zones_unevictable_pages();

        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 sys_device *dev,
                                          struct sysdev_attribute *attr,
                                          char *buf)
{
        return sprintf(buf, "0\n");     /* always zero; should fit... */
}

static ssize_t write_scan_unevictable_node(struct sys_device *dev,
                                           struct sysdev_attribute *attr,
                                        const char *buf, size_t count)
{
        struct zone *node_zones = NODE_DATA(dev->id)->node_zones;
        struct zone *zone;
        unsigned long res;
        unsigned long req = strict_strtoul(buf, 10, &res);

        if (!req)
                return 1;       /* zero is no-op */

        for (zone = node_zones; zone - node_zones < MAX_NR_ZONES; ++zone) {
                if (!populated_zone(zone))
                        continue;
                scan_zone_unevictable_pages(zone);
        }
        return 1;
}


static SYSDEV_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 sysdev_create_file(&node->sysdev, &attr_scan_unevictable_pages);
}

void scan_unevictable_unregister_node(struct node *node)
{
        sysdev_remove_file(&node->sysdev, &attr_scan_unevictable_pages);
}
#endif