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.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#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/vmpressure.h>
#include <linux/vmstat.h>
#include <linux/file.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h> /* for try_to_release_page(),
     buffer_heads_over_limit */
#include <linux/mm_inline.h>
#include <linux/backing-dev.h>
#include <linux/rmap.h>
#include <linux/topology.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/compaction.h>
#include <linux/notifier.h>
#include <linux/rwsem.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/memcontrol.h>
#include <linux/delayacct.h>
#include <linux/sysctl.h>
#include <linux/oom.h>
#include <linux/prefetch.h>
#include <linux/printk.h>
#include <linux/dax.h>

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

#include <linux/swapops.h>
#include <linux/balloon_compaction.h>

#include "internal.h"

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

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

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

 /* Allocation order */
 int order;

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

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

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

 /* The highest zone to isolate pages for reclaim from */
 enum zone_type reclaim_idx;

 unsigned int may_writepage:1;

 /* Can mapped pages be reclaimed? */
 unsigned int may_unmap:1;

 /* Can pages be swapped as part of reclaim? */
 unsigned int may_swap:1;

 /* Can cgroups be reclaimed below their normal consumption range? */
 unsigned int may_thrash:1;

 unsigned int hibernation_mode:1;

 /* One of the zones is ready for compaction */
 unsigned int compaction_ready:1;

 /* 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;
};

#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;
/*
 * The total number of pages which are beyond the high watermark within all
 * zones.
 */
unsigned long vm_total_pages;

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

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

/**
 * sane_reclaim - is the usual dirty throttling mechanism operational?
 * @sc: scan_control in question
 *
 * The normal page dirty throttling mechanism in balance_dirty_pages() is
 * completely broken with the legacy memcg and direct stalling in
 * shrink_page_list() is used for throttling instead, which lacks all the
 * niceties such as fairness, adaptive pausing, bandwidth proportional
 * allocation and configurability.
 *
 * This function tests whether the vmscan currently in progress can assume
 * that the normal dirty throttling mechanism is operational.
 */
static bool sane_reclaim(struct scan_control *sc)
{
 struct mem_cgroup *memcg = sc->target_mem_cgroup;

 if (!memcg)
  return true;
#ifdef CONFIG_CGROUP_WRITEBACK
 if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
  return true;
#endif
 return false;
}
#else
static bool global_reclaim(struct scan_control *sc)
{
 return true;
}

static bool sane_reclaim(struct scan_control *sc)
{
 return true;
}
#endif

/*
 * This misses isolated pages which are not accounted for to save counters.
 * As the data only determines if reclaim or compaction continues, it is
 * not expected that isolated pages will be a dominating factor.
 */
unsigned long zone_reclaimable_pages(struct zone *zone)
{
 unsigned long nr;

 nr = zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_FILE) +
  zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_FILE);
 if (get_nr_swap_pages() > 0)
  nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
   zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);

 return nr;
}

unsigned long pgdat_reclaimable_pages(struct pglist_data *pgdat)
{
 unsigned long nr;

 nr = node_page_state_snapshot(pgdat, NR_ACTIVE_FILE) +
      node_page_state_snapshot(pgdat, NR_INACTIVE_FILE) +
      node_page_state_snapshot(pgdat, NR_ISOLATED_FILE);

 if (get_nr_swap_pages() > 0)
  nr += node_page_state_snapshot(pgdat, NR_ACTIVE_ANON) +
        node_page_state_snapshot(pgdat, NR_INACTIVE_ANON) +
        node_page_state_snapshot(pgdat, NR_ISOLATED_ANON);

 return nr;
}

bool pgdat_reclaimable(struct pglist_data *pgdat)
{
 return node_page_state_snapshot(pgdat, NR_PAGES_SCANNED) <
  pgdat_reclaimable_pages(pgdat) * 6;
}

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

 return node_page_state(lruvec_pgdat(lruvec), NR_LRU_BASE + lru);
}

/*
 * Add a shrinker callback to be called from the vm.
 */
int register_shrinker(struct shrinker *shrinker)
{
 size_t size = sizeof(*shrinker->nr_deferred);

 if (shrinker->flags & SHRINKER_NUMA_AWARE)
  size *= nr_node_ids;

 shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
 if (!shrinker->nr_deferred)
  return -ENOMEM;

 down_write(&shrinker_rwsem);
 list_add_tail(&shrinker->list, &shrinker_list);
 up_write(&shrinker_rwsem);
 return 0;
}
EXPORT_SYMBOL(register_shrinker);

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

#define SHRINK_BATCH 128

static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
        struct shrinker *shrinker,
        unsigned long nr_scanned,
        unsigned long nr_eligible)
{
 unsigned long freed = 0;
 unsigned long long delta;
 long total_scan;
 long freeable;
 long nr;
 long new_nr;
 int nid = shrinkctl->nid;
 long batch_size = shrinker->batch ? shrinker->batch
       : SHRINK_BATCH;

 freeable = shrinker->count_objects(shrinker, shrinkctl);
 if (freeable == 0)
  return 0;

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

 total_scan = nr;
 delta = (4 * nr_scanned) / shrinker->seeks;
 delta *= freeable;
 do_div(delta, nr_eligible + 1);
 total_scan += delta;
 if (total_scan < 0) {
  pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",
         shrinker->scan_objects, total_scan);
  total_scan = freeable;
 }

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

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

 trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
       nr_scanned, nr_eligible,
       freeable, delta, total_scan);

 /*
  * Normally, we should not scan less than batch_size objects in one
  * pass to avoid too frequent shrinker calls, but if the slab has less
  * than batch_size objects in total and we are really tight on memory,
  * we will try to reclaim all available objects, otherwise we can end
  * up failing allocations although there are plenty of reclaimable
  * objects spread over several slabs with usage less than the
  * batch_size.
  *
  * We detect the "tight on memory" situations by looking at the total
  * number of objects we want to scan (total_scan). If it is greater
  * than the total number of objects on slab (freeable), we must be
  * scanning at high prio and therefore should try to reclaim as much as
  * possible.
  */
 while (total_scan >= batch_size ||
        total_scan >= freeable) {
  unsigned long ret;
  unsigned long nr_to_scan = min(batch_size, total_scan);

  shrinkctl->nr_to_scan = nr_to_scan;
  ret = shrinker->scan_objects(shrinker, shrinkctl);
  if (ret == SHRINK_STOP)
   break;
  freed += ret;

  count_vm_events(SLABS_SCANNED, nr_to_scan);
  total_scan -= nr_to_scan;

  cond_resched();
 }

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

 trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan);
 return freed;
}

/**
 * shrink_slab - shrink slab caches
 * @gfp_mask: allocation context
 * @nid: node whose slab caches to target
 * @memcg: memory cgroup whose slab caches to target
 * @nr_scanned: pressure numerator
 * @nr_eligible: pressure denominator
 *
 * Call the shrink functions to age shrinkable caches.
 *
 * @nid is passed along to shrinkers with SHRINKER_NUMA_AWARE set,
 * unaware shrinkers will receive a node id of 0 instead.
 *
 * @memcg specifies the memory cgroup to target. If it is not NULL,
 * only shrinkers with SHRINKER_MEMCG_AWARE set will be called to scan
 * objects from the memory cgroup specified. Otherwise, only unaware
 * shrinkers are called.
 *
 * @nr_scanned and @nr_eligible form a ratio that indicate how much of
 * the available objects should be scanned.  Page reclaim for example
 * passes the number of pages scanned and the number of pages on the
 * LRU lists that it considered on @nid, plus a bias in @nr_scanned
 * when it encountered mapped pages.  The ratio is further biased by
 * the ->seeks setting of the shrink function, which indicates the
 * cost to recreate an object relative to that of an LRU page.
 *
 * Returns the number of reclaimed slab objects.
 */
static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
     struct mem_cgroup *memcg,
     unsigned long nr_scanned,
     unsigned long nr_eligible)
{
 struct shrinker *shrinker;
 unsigned long freed = 0;

 if (memcg && (!memcg_kmem_enabled() || !mem_cgroup_online(memcg)))
  return 0;

 if (nr_scanned == 0)
  nr_scanned = SWAP_CLUSTER_MAX;

 if (!down_read_trylock(&shrinker_rwsem)) {
  /*
   * If we would return 0, our callers would understand that we
   * have nothing else to shrink and give up trying. By returning
   * 1 we keep it going and assume we'll be able to shrink next
   * time.
   */
  freed = 1;
  goto out;
 }

 list_for_each_entry(shrinker, &shrinker_list, list) {
  struct shrink_control sc = {
   .gfp_mask = gfp_mask,
   .nid = nid,
   .memcg = memcg,
  };

  /*
   * If kernel memory accounting is disabled, we ignore
   * SHRINKER_MEMCG_AWARE flag and call all shrinkers
   * passing NULL for memcg.
   */
  if (memcg_kmem_enabled() &&
      !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
   continue;

  if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
   sc.nid = 0;

  freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
 }

 up_read(&shrinker_rwsem);
out:
 cond_resched();
 return freed;
}

void drop_slab_node(int nid)
{
 unsigned long freed;

 do {
  struct mem_cgroup *memcg = NULL;

  freed = 0;
  do {
   freed += shrink_slab(GFP_KERNEL, nid, memcg,
          1000, 1000);
  } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
 } while (freed > 10);
}

void drop_slab(void)
{
 int nid;

 for_each_online_node(nid)
  drop_slab_node(nid);
}

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_inode(struct inode *inode, struct scan_control *sc)
{
 if (current->flags & PF_SWAPWRITE)
  return 1;
 if (!inode_write_congested(inode))
  return 1;
 if (inode_to_bdi(inode) == current->backing_dev_info)
  return 1;
 return 0;
}

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

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

/*
 * pageout is called by shrink_page_list() for each dirty page.
 * Calls ->writepage().
 */
static pageout_t pageout(struct page *page, struct address_space *mapping,
    struct scan_control *sc)
{
 /*
  * If the page is dirty, only perform writeback if that write
  * will be non-blocking.  To prevent this allocation from being
  * stalled by pagecache activity.  But note that there may be
  * stalls if we need to run get_block().  We could test
  * PagePrivate for that.
  *
  * If this process is currently in __generic_file_write_iter() 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);
    pr_info("%s: orphaned page\n", __func__);
    return PAGE_CLEAN;
   }
  }
  return PAGE_KEEP;
 }
 if (mapping->a_ops->writepage == NULL)
  return PAGE_ACTIVATE;
 if (!may_write_to_inode(mapping->host, sc))
  return PAGE_KEEP;

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

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

  if (!PageWriteback(page)) {
   /* synchronous write or broken a_ops? */
   ClearPageReclaim(page);
  }
  trace_mm_vmscan_writepage(page);
  inc_node_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,
       bool reclaimed)
{
 unsigned long flags;

 BUG_ON(!PageLocked(page));
 BUG_ON(mapping != page_mapping(page));

 spin_lock_irqsave(&mapping->tree_lock, flags);
 /*
  * 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->_refcount.
  *
  * Note that if SetPageDirty is always performed via set_page_dirty,
  * and thus under tree_lock, then this ordering is not required.
  */
 if (!page_ref_freeze(page, 2))
  goto cannot_free;
 /* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
 if (unlikely(PageDirty(page))) {
  page_ref_unfreeze(page, 2);
  goto cannot_free;
 }

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

  freepage = mapping->a_ops->freepage;
  /*
   * Remember a shadow entry for reclaimed file cache in
   * order to detect refaults, thus thrashing, later on.
   *
   * But don't store shadows in an address space that is
   * already exiting.  This is not just an optizimation,
   * inode reclaim needs to empty out the radix tree or
   * the nodes are lost.  Don't plant shadows behind its
   * back.
   *
   * We also don't store shadows for DAX mappings because the
   * only page cache pages found in these are zero pages
   * covering holes, and because we don't want to mix DAX
   * exceptional entries and shadow exceptional entries in the
   * same page_tree.
   */
  if (reclaimed && page_is_file_cache(page) &&
      !mapping_exiting(mapping) && !dax_mapping(mapping))
   shadow = workingset_eviction(mapping, page);
  __delete_from_page_cache(page, shadow);
  spin_unlock_irqrestore(&mapping->tree_lock, flags);

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

 return 1;

cannot_free:
 spin_unlock_irqrestore(&mapping->tree_lock, flags);
 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, false)) {
  /*
   * Unfreezing the refcount with 1 rather than 2 effectively
   * drops the pagecache ref for us without requiring another
   * atomic operation.
   */
  page_ref_unfreeze(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)
{
 bool is_unevictable;
 int was_unevictable = PageUnevictable(page);

 VM_BUG_ON_PAGE(PageLRU(page), page);

redo:
 ClearPageUnevictable(page);

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

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

 if (was_unevictable && !is_unevictable)
  count_vm_event(UNEVICTABLE_PGRESCUED);
 else if (!was_unevictable && is_unevictable)
  count_vm_event(UNEVICTABLE_PGCULLED);

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

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

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

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

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

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

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

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

  return PAGEREF_KEEP;
 }

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

 return PAGEREF_RECLAIM;
}

/* Check if a page is dirty or under writeback */
static void page_check_dirty_writeback(struct page *page,
           bool *dirty, bool *writeback)
{
 struct address_space *mapping;

 /*
  * Anonymous pages are not handled by flushers and must be written
  * from reclaim context. Do not stall reclaim based on them
  */
 if (!page_is_file_cache(page)) {
  *dirty = false;
  *writeback = false;
  return;
 }

 /* By default assume that the page flags are accurate */
 *dirty = PageDirty(page);
 *writeback = PageWriteback(page);

 /* Verify dirty/writeback state if the filesystem supports it */
 if (!page_has_private(page))
  return;

 mapping = page_mapping(page);
 if (mapping && mapping->a_ops->is_dirty_writeback)
  mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
}

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

 cond_resched();

 while (!list_empty(page_list)) {
  struct address_space *mapping;
  struct page *page;
  int may_enter_fs;
  enum page_references references = PAGEREF_RECLAIM_CLEAN;
  bool dirty, writeback;
  bool lazyfree = false;
  int ret = SWAP_SUCCESS;

  cond_resched();

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

  if (!trylock_page(page))
   goto keep;

  VM_BUG_ON_PAGE(PageActive(page), page);

  sc->nr_scanned++;

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

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

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

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

  /*
   * The number of dirty pages determines if a zone is marked
   * reclaim_congested which affects wait_iff_congested. kswapd
   * will stall and start writing pages if the tail of the LRU
   * is all dirty unqueued pages.
   */
  page_check_dirty_writeback(page, &dirty, &writeback);
  if (dirty || writeback)
   nr_dirty++;

  if (dirty && !writeback)
   nr_unqueued_dirty++;

  /*
   * Treat this page as congested if the underlying BDI is or if
   * pages are cycling through the LRU so quickly that the
   * pages marked for immediate reclaim are making it to the
   * end of the LRU a second time.
   */
  mapping = page_mapping(page);
  if (((dirty || writeback) && mapping &&
       inode_write_congested(mapping->host)) ||
      (writeback && PageReclaim(page)))
   nr_congested++;

  /*
   * If a page at the tail of the LRU is under writeback, there
   * are three cases to consider.
   *
   * 1) If reclaim is encountering an excessive number of pages
   *    under writeback and this page is both under writeback and
   *    PageReclaim then it indicates that pages are being queued
   *    for IO but are being recycled through the LRU before the
   *    IO can complete. Waiting on the page itself risks an
   *    indefinite stall if it is impossible to writeback the
   *    page due to IO error or disconnected storage so instead
   *    note that the LRU is being scanned too quickly and the
   *    caller can stall after page list has been processed.
   *
   * 2) Global or new memcg reclaim encounters a page that is
   *    not marked for immediate reclaim, or the caller does not
   *    have __GFP_FS (or __GFP_IO if it's simply going to swap,
   *    not to fs). In this case mark the page for immediate
   *    reclaim and continue scanning.
   *
   *    Require may_enter_fs because we would wait on fs, which
   *    may not have submitted IO yet. And the loop driver might
   *    enter reclaim, and deadlock if it waits on a page for
   *    which it is needed to do the write (loop masks off
   *    __GFP_IO|__GFP_FS for this reason); but more thought
   *    would probably show more reasons.
   *
   * 3) Legacy memcg encounters a page that is already marked
   *    PageReclaim. memcg does not have any dirty pages
   *    throttling so we could easily OOM just because too many
   *    pages are in writeback and there is nothing else to
   *    reclaim. Wait for the writeback to complete.
   */
  if (PageWriteback(page)) {
   /* Case 1 above */
   if (current_is_kswapd() &&
       PageReclaim(page) &&
       test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
    nr_immediate++;
    goto keep_locked;

   /* Case 2 above */
   } else if (sane_reclaim(sc) ||
       !PageReclaim(page) || !may_enter_fs) {
    /*
     * This is slightly racy - end_page_writeback()
     * might have just cleared PageReclaim, then
     * setting PageReclaim here end up interpreted
     * as PageReadahead - but that does not matter
     * enough to care.  What we do want is for this
     * page to have PageReclaim set next time memcg
     * reclaim reaches the tests above, so it will
     * then wait_on_page_writeback() to avoid OOM;
     * and it's also appropriate in global reclaim.
     */
    SetPageReclaim(page);
    nr_writeback++;
    goto keep_locked;

   /* Case 3 above */
   } else {
    unlock_page(page);
    wait_on_page_writeback(page);
    /* then go back and try same page again */
    list_add_tail(&page->lru, page_list);
    continue;
   }
  }

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

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

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

   /* Adding to swap updated mapping */
   mapping = page_mapping(page);
  } else if (unlikely(PageTransHuge(page))) {
   /* Split file THP */
   if (split_huge_page_to_list(page, page_list))
    goto keep_locked;
  }

  VM_BUG_ON_PAGE(PageTransHuge(page), 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 (ret = try_to_unmap(page, lazyfree ?
    (ttu_flags | TTU_BATCH_FLUSH | TTU_LZFREE) :
    (ttu_flags | TTU_BATCH_FLUSH))) {
   case SWAP_FAIL:
    goto activate_locked;
   case SWAP_AGAIN:
    goto keep_locked;
   case SWAP_MLOCK:
    goto cull_mlocked;
   case SWAP_LZFREE:
    goto lazyfree;
   case SWAP_SUCCESS:
    ; /* try to free the page below */
   }
  }

  if (PageDirty(page)) {
   /*
    * Only kswapd can writeback filesystem pages to
    * avoid risk of stack overflow but only writeback
    * if many dirty pages have been encountered.
    */
   if (page_is_file_cache(page) &&
     (!current_is_kswapd() ||
      !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
    /*
     * Immediately reclaim when written back.
     * Similar in principal to deactivate_page()
     * except we already have the page isolated
     * and know it's dirty
     */
    inc_node_page_state(page, NR_VMSCAN_IMMEDIATE);
    SetPageReclaim(page);

    goto keep_locked;
   }

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

   /*
    * Page is dirty. Flush the TLB if a writable entry
    * potentially exists to avoid CPU writes after IO
    * starts and then write it out here.
    */
   try_to_unmap_flush_dirty();
   switch (pageout(page, mapping, sc)) {
   case PAGE_KEEP:
    goto keep_locked;
   case PAGE_ACTIVATE:
    goto activate_locked;
   case PAGE_SUCCESS:
    if (PageWriteback(page))
     goto keep;
    if (PageDirty(page))
     goto keep;

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

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

lazyfree:
  if (!mapping || !__remove_mapping(mapping, page, true))
   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.
   */
  __ClearPageLocked(page);
free_it:
  if (ret == SWAP_LZFREE)
   count_vm_event(PGLAZYFREED);

  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);
  list_add(&page->lru, &ret_pages);
  continue;

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

 mem_cgroup_uncharge_list(&free_pages);
 try_to_unmap_flush();
 free_hot_cold_page_list(&free_pages, true);

 list_splice(&ret_pages, page_list);
 count_vm_events(PGACTIVATE, pgactivate);

 *ret_nr_dirty += nr_dirty;
 *ret_nr_congested += nr_congested;
 *ret_nr_unqueued_dirty += nr_unqueued_dirty;
 *ret_nr_writeback += nr_writeback;
 *ret_nr_immediate += nr_immediate;
 return nr_reclaimed;
}

unsigned long reclaim_clean_pages_from_list(struct zone *zone,
         struct list_head *page_list)
{
 struct scan_control sc = {
  .gfp_mask = GFP_KERNEL,
  .priority = DEF_PRIORITY,
  .may_unmap = 1,
 };
 unsigned long ret, dummy1, dummy2, dummy3, dummy4, dummy5;
 struct page *page, *next;
 LIST_HEAD(clean_pages);

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

 ret = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
   TTU_UNMAP|TTU_IGNORE_ACCESS,
   &dummy1, &dummy2, &dummy3, &dummy4, &dummy5, true);
 list_splice(&clean_pages, page_list);
 mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE, -ret);
 return ret;
}

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

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

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

 ret = -EBUSY;

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

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

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

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

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

 if (likely(get_page_unless_zero(page))) {
  /*
   * Be careful not to clear PageLRU until after we're
   * sure the page is not being freed elsewhere -- the
   * page release code relies on it.
   */
  ClearPageLRU(page);
  ret = 0;
 }

 return ret;
}

/*
 * Update LRU sizes after isolating pages. The LRU size updates must
 * be complete before mem_cgroup_update_lru_size due to a santity check.
 */
static __always_inline void update_lru_sizes(struct lruvec *lruvec,
   enum lru_list lru, unsigned long *nr_zone_taken,
   unsigned long nr_taken)
{
 int zid;

 for (zid = 0; zid < MAX_NR_ZONES; zid++) {
  if (!nr_zone_taken[zid])
   continue;

  __update_lru_size(lruvec, lru, zid, -nr_zone_taken[zid]);
 }

#ifdef CONFIG_MEMCG
 mem_cgroup_update_lru_size(lruvec, lru, -nr_taken);
#endif
}

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

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

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

  VM_BUG_ON_PAGE(!PageLRU(page), page);

  if (page_zonenum(page) > sc->reclaim_idx) {
   list_move(&page->lru, &pages_skipped);
   nr_skipped[page_zonenum(page)]++;
   continue;
  }

  /*
   * Account for scanned and skipped separetly to avoid the pgdat
   * being prematurely marked unreclaimable by pgdat_reclaimable.
   */
  scan++;

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

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

  default:
   BUG();
  }
 }

 /*
  * Splice any skipped pages to the start of the LRU list. Note that
  * this disrupts the LRU order when reclaiming for lower zones but
  * we cannot splice to the tail. If we did then the SWAP_CLUSTER_MAX
  * scanning would soon rescan the same pages to skip and put the
  * system at risk of premature OOM.
  */
 if (!list_empty(&pages_skipped)) {
  int zid;
  unsigned long total_skipped = 0;

  for (zid = 0; zid < MAX_NR_ZONES; zid++) {
   if (!nr_skipped[zid])
    continue;

   __count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
   total_skipped += nr_skipped[zid];
  }

  /*
   * Account skipped pages as a partial scan as the pgdat may be
   * close to unreclaimable. If the LRU list is empty, account
   * skipped pages as a full scan.
   */
  scan += list_empty(src) ? total_skipped : total_skipped >> 2;

  list_splice(&pages_skipped, src);
 }
 *nr_scanned = scan;
 trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan, scan,
        nr_taken, mode, is_file_lru(lru));
 update_lru_sizes(lruvec, lru, nr_zone_taken, nr_taken);
 return nr_taken;
}

/**
 * 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;

 VM_BUG_ON_PAGE(!page_count(page), page);
 WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");

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

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

/*
 * A direct reclaimer may isolate SWAP_CLUSTER_MAX pages from the LRU list and
 * then get resheduled. When there are massive number of tasks doing page
 * allocation, such sleeping direct reclaimers may keep piling up on each CPU,
 * the LRU list will go small and be scanned faster than necessary, leading to
 * unnecessary swapping, thrashing and OOM.
 */
static int too_many_isolated(struct pglist_data *pgdat, int file,
  struct scan_control *sc)
{
 unsigned long inactive, isolated;

 if (current_is_kswapd())
  return 0;

 if (!sane_reclaim(sc))
  return 0;

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

 /*
  * GFP_NOIO/GFP_NOFS callers are allowed to isolate more pages, so they
  * won't get blocked by normal direct-reclaimers, forming a circular
  * deadlock.
  */
 if ((sc->gfp_mask & (__GFP_IO | __GFP_FS)) == (__GFP_IO | __GFP_FS))
  inactive >>= 3;

 return isolated > inactive;
}

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

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

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

  lruvec = mem_cgroup_page_lruvec(page, pgdat);

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

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

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

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

/*
 * If a kernel thread (such as nfsd for loop-back mounts) services
 * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
 * In that case we should only throttle if the backing device it is
 * writing to is congested.  In other cases it is safe to throttle.
 */
static int current_may_throttle(void)
{
 return !(current->flags & PF_LESS_THROTTLE) ||
  current->backing_dev_info == NULL ||
  bdi_write_congested(current->backing_dev_info);
}

static bool inactive_reclaimable_pages(struct lruvec *lruvec,
    struct scan_control *sc, enum lru_list lru)
{
 int zid;
 struct zone *zone;
 int file = is_file_lru(lru);
 struct pglist_data *pgdat = lruvec_pgdat(lruvec);

 if (!global_reclaim(sc))
  return true;

 for (zid = sc->reclaim_idx; zid >= 0; zid--) {
  zone = &pgdat->node_zones[zid];
  if (!managed_zone(zone))
   continue;

  if (zone_page_state_snapshot(zone, NR_ZONE_LRU_BASE +
    LRU_FILE * file) >= SWAP_CLUSTER_MAX)
   return true;
 }

 return false;
}

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

 if (!inactive_reclaimable_pages(lruvec, sc, lru))
  return 0;

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

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

 lru_add_drain();

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

 spin_lock_irq(&pgdat->lru_lock);

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

 __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
 reclaim_stat->recent_scanned[file] += nr_taken;

 if (global_reclaim(sc)) {
  __mod_node_page_state(pgdat, NR_PAGES_SCANNED, nr_scanned);
  if (current_is_kswapd())
   __count_vm_events(PGSCAN_KSWAPD, nr_scanned);
  else
   __count_vm_events(PGSCAN_DIRECT, nr_scanned);
 }
 spin_unlock_irq(&pgdat->lru_lock);

 if (nr_taken == 0)
  return 0;

 nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, TTU_UNMAP,
    &nr_dirty, &nr_unqueued_dirty, &nr_congested,
    &nr_writeback, &nr_immediate,
    false);

 spin_lock_irq(&pgdat->lru_lock);

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

 putback_inactive_pages(lruvec, &page_list);

 __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);

 spin_unlock_irq(&pgdat->lru_lock);

 mem_cgroup_uncharge_list(&page_list);
 free_hot_cold_page_list(&page_list, true);

 /*
  * If reclaim is isolating dirty pages under writeback, it implies
  * that the long-lived page allocation rate is exceeding the page
  * laundering rate. Either the global limits are not being effective
  * at throttling processes due to the page distribution throughout
  * zones or there is heavy usage of a slow backing device. The
  * only option is to throttle from reclaim context which is not ideal
  * as there is no guarantee the dirtying process is throttled in the
  * same way balance_dirty_pages() manages.
  *
  * Once a zone is flagged ZONE_WRITEBACK, kswapd will count the number
  * of pages under pages flagged for immediate reclaim and stall if any
  * are encountered in the nr_immediate check below.
  */
 if (nr_writeback && nr_writeback == nr_taken)
  set_bit(PGDAT_WRITEBACK, &pgdat->flags);

 /*
  * Legacy memcg will stall in page writeback so avoid forcibly
  * stalling here.
  */
 if (sane_reclaim(sc)) {
  /*
   * Tag a zone as congested if all the dirty pages scanned were
   * backed by a congested BDI and wait_iff_congested will stall.
   */
  if (nr_dirty && nr_dirty == nr_congested)
   set_bit(PGDAT_CONGESTED, &pgdat->flags);

  /*
   * If dirty pages are scanned that are not queued for IO, it
   * implies that flushers are not keeping up. In this case, flag
   * the pgdat PGDAT_DIRTY and kswapd will start writing pages from
   * reclaim context.
   */
  if (nr_unqueued_dirty == nr_taken)
   set_bit(PGDAT_DIRTY, &pgdat->flags);

  /*
   * If kswapd scans pages marked marked for immediate
   * reclaim and under writeback (nr_immediate), it implies
   * that pages are cycling through the LRU faster than
   * they are written so also forcibly stall.
   */
  if (nr_immediate && current_may_throttle())
   congestion_wait(BLK_RW_ASYNC, HZ/10);
 }

 /*
  * Stall direct reclaim for IO completions if underlying BDIs or zone
  * is congested. Allow kswapd to continue until it starts encountering
  * unqueued dirty pages or cycling through the LRU too quickly.
  */
 if (!sc->hibernation_mode && !current_is_kswapd() &&
     current_may_throttle())
  wait_iff_congested(pgdat, BLK_RW_ASYNC, HZ/10);

 trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
   nr_scanned, nr_reclaimed,
   sc->priority, file);
 return nr_reclaimed;
}

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

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

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

  VM_BUG_ON_PAGE(PageLRU(page), page);
  SetPageLRU(page);

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

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

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

 if (!is_active_lru(lru))
  __count_vm_events(PGDEACTIVATE, pgmoved);
}

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

 lru_add_drain();

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

 spin_lock_irq(&pgdat->lru_lock);

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

 __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
 reclaim_stat->recent_scanned[file] += nr_taken;

 if (global_reclaim(sc))
  __mod_node_page_state(pgdat, NR_PAGES_SCANNED, nr_scanned);
 __count_vm_events(PGREFILL, nr_scanned);

 spin_unlock_irq(&pgdat->lru_lock);

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

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

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

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

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

 /*
  * Move pages back to the lru list.
  */
 spin_lock_irq(&pgdat->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_count.
  */
 reclaim_stat->recent_rotated[file] += nr_rotated;

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

 mem_cgroup_uncharge_list(&l_hold);
 free_hot_cold_page_list(&l_hold, true);
}

/*
 * The inactive anon list should be small enough that the VM never has
 * to do too much work.
 *
 * The inactive file list should be small enough to leave most memory
 * to the established workingset on the scan-resistant active list,
 * but large enough to avoid thrashing the aggregate readahead window.
 *
 * Both inactive lists should also be large enough that each inactive
 * page has a chance to be referenced again before it is reclaimed.
 *
 * The inactive_ratio is the target ratio of ACTIVE to INACTIVE pages
 * on this LRU, maintained by the pageout code. A zone->inactive_ratio
 * of 3 means 3:1 or 25% of the pages are kept on the inactive list.
 *
 * total     target    max
 * memory    ratio     inactive
 * -------------------------------------
 *   10MB       1         5MB
 *  100MB       1        50MB
 *    1GB       3       250MB
 *   10GB      10       0.9GB
 *  100GB      31         3GB
 *    1TB     101        10GB
 *   10TB     320        32GB
 */
static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
      struct scan_control *sc)
{
 unsigned long inactive_ratio;
 unsigned long inactive;
 unsigned long active;
 unsigned long gb;
 struct pglist_data *pgdat = lruvec_pgdat(lruvec);
 int zid;

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

 inactive = lruvec_lru_size(lruvec, file * LRU_FILE);
 active = lruvec_lru_size(lruvec, file * LRU_FILE + LRU_ACTIVE);

 /*
  * For zone-constrained allocations, it is necessary to check if
  * deactivations are required for lowmem to be reclaimed. This
  * calculates the inactive/active pages available in eligible zones.
  */
 for (zid = sc->reclaim_idx + 1; zid < MAX_NR_ZONES; zid++) {
  struct zone *zone = &pgdat->node_zones[zid];
  unsigned long inactive_zone, active_zone;

  if (!managed_zone(zone))
   continue;

  inactive_zone = zone_page_state(zone,
    NR_ZONE_LRU_BASE + (file * LRU_FILE));
  active_zone = zone_page_state(zone,
    NR_ZONE_LRU_BASE + (file * LRU_FILE) + LRU_ACTIVE);

  inactive -= min(inactive, inactive_zone);
  active -= min(active, active_zone);
 }

 gb = (inactive + active) >> (30 - PAGE_SHIFT);
 if (gb)
  inactive_ratio = int_sqrt(10 * gb);
 else
  inactive_ratio = 1;

 return inactive * inactive_ratio < active;
}

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

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

enum scan_balance {
 SCAN_EQUAL,
 SCAN_FRACT,
 SCAN_ANON,
 SCAN_FILE,
};

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

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

 /* If we have no swap space, do not bother scanning anon pages. */
 if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
  scan_balance = SCAN_FILE;
  goto out;
 }

 /*
  * Global reclaim will swap to prevent OOM even with no
  * swappiness, but memcg users want to use this knob to
  * disable swapping for individual groups completely when
  * using the memory controller's swap limit feature would be
  * too expensive.
  */
 if (!global_reclaim(sc) && !swappiness) {
  scan_balance = SCAN_FILE;
  goto out;
 }

 /*
  * Do not apply any pressure balancing cleverness when the
  * system is close to OOM, scan both anon and file equally
  * (unless the swappiness setting disagrees with swapping).
  */
 if (!sc->priority && swappiness) {
  scan_balance = SCAN_EQUAL;
  goto out;
 }

 /*
  * Prevent the reclaimer from falling into the cache trap: as
  * cache pages start out inactive, every cache fault will tip
  * the scan balance towards the file LRU.  And as the file LRU
  * shrinks, so does the window for rotation from references.
  * This means we have a runaway feedback loop where a tiny
  * thrashing file LRU becomes infinitely more attractive than
  * anon pages.  Try to detect this based on file LRU size.
  */
 if (global_reclaim(sc)) {
  unsigned long pgdatfile;
  unsigned long pgdatfree;
  int z;
  unsigned long total_high_wmark = 0;

  pgdatfree = sum_zone_node_page_state(pgdat->node_id, NR_FREE_PAGES);
  pgdatfile = node_page_state(pgdat, NR_ACTIVE_FILE) +
      node_page_state(pgdat, NR_INACTIVE_FILE);

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

   total_high_wmark += high_wmark_pages(zone);
  }

  if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
   scan_balance = SCAN_ANON;
   goto out;
  }
 }

 /*
  * If there is enough inactive page cache, i.e. if the size of the
  * inactive list is greater than that of the active list *and* the
  * inactive list actually has some pages to scan on this priority, we
  * do not reclaim anything from the anonymous working set right now.
  * Without the second condition we could end up never scanning an
  * lruvec even if it has plenty of old anonymous pages unless the
  * system is under heavy pressure.
  */
 if (!inactive_list_is_low(lruvec, true, sc) &&
     lruvec_lru_size(lruvec, LRU_INACTIVE_FILE) >> sc->priority) {
  scan_balance = SCAN_FILE;
  goto out;
 }

 scan_balance = SCAN_FRACT;

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

 /*
  * OK, so we have swap space and a fair amount of page cache
  * pages.  We use the recently rotated / recently scanned
  * ratios to determine how valuable each cache is.
  *
  * Because workloads change over time (and to avoid overflow)
  * we keep these statistics as a floating average, which ends
  * up weighing recent references more than old ones.
  *
  * anon in [0], file in [1]
  */

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

 spin_lock_irq(&pgdat->lru_lock);
 if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {
  reclaim_stat->recent_scanned[0] /= 2;
  reclaim_stat->recent_rotated[0] /= 2;
 }

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

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

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

 fraction[0] = ap;
 fraction[1] = fp;
 denominator = ap + fp + 1;
out:
 some_scanned = false;
 /* Only use force_scan on second pass. */
 for (pass = 0; !some_scanned && pass < 2; pass++) {
  *lru_pages = 0;
  for_each_evictable_lru(lru) {
   int file = is_file_lru(lru);
   unsigned long size;
   unsigned long scan;

   size = lruvec_lru_size(lruvec, lru);
   scan = size >> sc->priority;

   if (!scan && pass && force_scan)
    scan = min(size, SWAP_CLUSTER_MAX);

   switch (scan_balance) {
   case SCAN_EQUAL:
    /* Scan lists relative to size */
    break;
   case SCAN_FRACT:
    /*
     * Scan types proportional to swappiness and
     * their relative recent reclaim efficiency.
     */
    scan = div64_u64(scan * fraction[file],
       denominator);
    break;
   case SCAN_FILE:
   case SCAN_ANON:
    /* Scan one type exclusively */
    if ((scan_balance == SCAN_FILE) != file) {
     size = 0;
     scan = 0;
    }
    break;
   default:
    /* Look ma, no brain */
    BUG();
   }

   *lru_pages += size;
   nr[lru] = scan;

   /*
    * Skip the second pass and don't force_scan,
    * if we found something to scan.
    */
   some_scanned |= !!scan;
  }
 }
}

/*
 * This is a basic per-node page freer.  Used by both kswapd and direct reclaim.
 */
static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memcg,
         struct scan_control *sc, unsigned long *lru_pages)
{
 struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);
 unsigned long nr[NR_LRU_LISTS];
 unsigned long targets[NR_LRU_LISTS];
 unsigned long nr_to_scan;
 enum lru_list lru;
 unsigned long nr_reclaimed = 0;
 unsigned long nr_to_reclaim = sc->nr_to_reclaim;
 struct blk_plug plug;
 bool scan_adjusted;

 get_scan_count(lruvec, memcg, sc, nr, lru_pages);

 /* Record the original scan target for proportional adjustments later */
 memcpy(targets, nr, sizeof(nr));

 /*
  * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
  * event that can occur when there is little memory pressure e.g.
  * multiple streaming readers/writers. Hence, we do not abort scanning
  * when the requested number of pages are reclaimed when scanning at
  * DEF_PRIORITY on the assumption that the fact we are direct
  * reclaiming implies that kswapd is not keeping up and it is best to
  * do a batch of work at once. For memcg reclaim one check is made to
  * abort proportional reclaim if either the file or anon lru has already
  * dropped to zero at the first pass.
  */
 scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() &&
    sc->priority == DEF_PRIORITY);

 blk_start_plug(&plug);
 while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
     nr[LRU_INACTIVE_FILE]) {
  unsigned long nr_anon, nr_file, percentage;
  unsigned long nr_scanned;

  for_each_evictable_lru(lru) {
   if (nr[lru]) {
    nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
    nr[lru] -= nr_to_scan;

    nr_reclaimed += shrink_list(lru, nr_to_scan,
           lruvec, sc);
   }
  }

  if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
   continue;

  /*
   * For kswapd and memcg, reclaim at least the number of pages
   * requested. Ensure that the anon and file LRUs are scanned
   * proportionally what was requested by get_scan_count(). We
   * stop reclaiming one LRU and reduce the amount scanning
   * proportional to the original scan target.
   */
  nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
  nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];

  /*
   * It's just vindictive to attack the larger once the smaller
   * has gone to zero.  And given the way we stop scanning the
   * smaller below, this makes sure that we only make one nudge
   * towards proportionality once we've got nr_to_reclaim.
   */
  if (!nr_file || !nr_anon)
   break;

  if (nr_file > nr_anon) {
   unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
      targets[LRU_ACTIVE_ANON] + 1;
   lru = LRU_BASE;
   percentage = nr_anon * 100 / scan_target;
  } else {
   unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
      targets[LRU_ACTIVE_FILE] + 1;
   lru = LRU_FILE;
   percentage = nr_file * 100 / scan_target;
  }

  /* Stop scanning the smaller of the LRU */
  nr[lru] = 0;
  nr[lru + LRU_ACTIVE] = 0;

  /*
   * Recalculate the other LRU scan count based on its original
   * scan target and the percentage scanning already complete
   */
  lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
  nr_scanned = targets[lru] - nr[lru];
  nr[lru] = targets[lru] * (100 - percentage) / 100;
  nr[lru] -= min(nr[lru], nr_scanned);

  lru += LRU_ACTIVE;
  nr_scanned = targets[lru] - nr[lru];
  nr[lru] = targets[lru] * (100 - percentage) / 100;
  nr[lru] -= min(nr[lru], nr_scanned);

  scan_adjusted = true;
 }
 blk_finish_plug(&plug);
 sc->nr_reclaimed += nr_reclaimed;

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

 throttle_vm_writeout(sc->gfp_mask);
}

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

 return false;
}

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

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

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

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

 /* If compaction would go ahead or the allocation would succeed, stop */
 for (z = 0; z <= sc->reclaim_idx; z++) {
  struct zone *zone = &pgdat->node_zones[z];
  if (!managed_zone(zone))
   continue;

  switch (compaction_suitable(zone, sc->order, 0, sc->reclaim_idx)) {
  case COMPACT_PARTIAL:
  case COMPACT_CONTINUE:
   return false;
  default:
   /* check next zone */
   ;
  }
 }
 return true;
}

static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
{
 struct reclaim_state *reclaim_state = current->reclaim_state;
 unsigned long nr_reclaimed, nr_scanned;
 bool reclaimable = false;

 do {
  struct mem_cgroup *root = sc->target_mem_cgroup;
  struct mem_cgroup_reclaim_cookie reclaim = {
   .pgdat = pgdat,
   .priority = sc->priority,
  };
  unsigned long node_lru_pages = 0;
  struct mem_cgroup *memcg;

  nr_reclaimed = sc->nr_reclaimed;
  nr_scanned = sc->nr_scanned;

  memcg = mem_cgroup_iter(root, NULL, &reclaim);
  do {
   unsigned long lru_pages;
   unsigned long reclaimed;
   unsigned long scanned;

   if (mem_cgroup_low(root, memcg)) {
    if (!sc->may_thrash)
     continue;
    mem_cgroup_events(memcg, MEMCG_LOW, 1);
   }

   reclaimed = sc->nr_reclaimed;
   scanned = sc->nr_scanned;

   shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
   node_lru_pages += lru_pages;

   if (memcg)
    shrink_slab(sc->gfp_mask, pgdat->node_id,
         memcg, sc->nr_scanned - scanned,
         lru_pages);

   /* Record the group's reclaim efficiency */
   vmpressure(sc->gfp_mask, memcg, false,
       sc->nr_scanned - scanned,
       sc->nr_reclaimed - reclaimed);

   /*
    * Direct reclaim and kswapd have to scan all memory
    * cgroups to fulfill the overall scan target for the
    * node.
    *
    * Limit reclaim, on the other hand, only cares about
    * nr_to_reclaim pages to be reclaimed and it will
    * retry with decreasing priority if one round over the
    * whole hierarchy is not sufficient.
    */
   if (!global_reclaim(sc) &&
     sc->nr_reclaimed >= sc->nr_to_reclaim) {
    mem_cgroup_iter_break(root, memcg);
    break;
   }
  } while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));

  /*
   * Shrink the slab caches in the same proportion that
   * the eligible LRU pages were scanned.
   */
  if (global_reclaim(sc))
   shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
        sc->nr_scanned - nr_scanned,
        node_lru_pages);

  if (reclaim_state) {
   sc->nr_reclaimed += reclaim_state->reclaimed_slab;
   reclaim_state->reclaimed_slab = 0;
  }

  /* Record the subtree's reclaim efficiency */
  vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
      sc->nr_scanned - nr_scanned,
      sc->nr_reclaimed - nr_reclaimed);

  if (sc->nr_reclaimed - nr_reclaimed)
   reclaimable = true;

 } while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
      sc->nr_scanned - nr_scanned, sc));

 return reclaimable;
}

/*
 * Returns true if compaction should go ahead for a high-order request, or
 * the high-order allocation would succeed without compaction.
 */
static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
{
 unsigned long watermark;
 bool watermark_ok;

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

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

 /*
  * If compaction is not ready to start and allocation is not likely
  * to succeed without it, then keep reclaiming.
  */
 if (compaction_suitable(zone, sc->order, 0, sc->reclaim_idx) == COMPACT_SKIPPED)
  return false;

 return watermark_ok;
}

/*
 * This is the direct reclaim path, for page-allocating processes.  We only
 * try to reclaim pages from zones which will satisfy the caller's allocation
 * request.
 *
 * 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(struct zonelist *zonelist, struct scan_control *sc)
{
 struct zoneref *z;
 struct zone *zone;
 unsigned long nr_soft_reclaimed;
 unsigned long nr_soft_scanned;
 gfp_t orig_mask;
 pg_data_t *last_pgdat = NULL;

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

 for_each_zone_zonelist_nodemask(zone, z, zonelist,
     sc->reclaim_idx, sc->nodemask) {
  /*
   * Take care memory controller reclaiming has small influence
   * to global LRU.
   */
  if (global_reclaim(sc)) {
   if (!cpuset_zone_allowed(zone,
       GFP_KERNEL | __GFP_HARDWALL))
    continue;

   if (sc->priority != DEF_PRIORITY &&
       !pgdat_reclaimable(zone->zone_pgdat))
    continue; /* Let kswapd poll it */

   /*
    * If we already have plenty of memory free for
    * compaction in this zone, don't free any more.
    * Even though compaction is invoked for any
    * non-zero order, only frequent costly order
    * reclamation is disruptive enough to become a
    * noticeable problem, like transparent huge
    * page allocations.
    */
   if (IS_ENABLED(CONFIG_COMPACTION) &&
       sc->order > PAGE_ALLOC_COSTLY_ORDER &&
       compaction_ready(zone, sc)) {
    sc->compaction_ready = true;
    continue;
   }

   /*
    * Shrink each node in the zonelist once. If the
    * zonelist is ordered by zone (not the default) then a
    * node may be shrunk multiple times but in that case
    * the user prefers lower zones being preserved.
    */
   if (zone->zone_pgdat == last_pgdat)
    continue;

   /*
    * This steals pages from memory cgroups over softlimit
    * and returns the number of reclaimed pages and
    * scanned pages. This works for global memory pressure
    * and balancing, not for a memcg's limit.
    */
   nr_soft_scanned = 0;
   nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone->zone_pgdat,
      sc->order, sc->gfp_mask,
      &nr_soft_scanned);
   sc->nr_reclaimed += nr_soft_reclaimed;
   sc->nr_scanned += nr_soft_scanned;
   /* need some check for avoid more shrink_zone() */
  }

  /* See comment about same check for global reclaim above */
  if (zone->zone_pgdat == last_pgdat)
   continue;
  last_pgdat = zone->zone_pgdat;
  shrink_node(zone->zone_pgdat, sc);
 }

 /*
  * Restore to original mask to avoid the impact on the caller if we
  * promoted it to __GFP_HIGHMEM.
  */
 sc->gfp_mask = orig_mask;
}

/*
 * 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 initial_priority = sc->priority;
 unsigned long total_scanned = 0;
 unsigned long writeback_threshold;
retry:
 delayacct_freepages_start();

 if (global_reclaim(sc))
  __count_zid_vm_events(ALLOCSTALL, sc->reclaim_idx, 1);

 do {
  vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
    sc->priority);
  sc->nr_scanned = 0;
  shrink_zones(zonelist, sc);

  total_scanned += sc->nr_scanned;
  if (sc->nr_reclaimed >= sc->nr_to_reclaim)
   break;

  if (sc->compaction_ready)
   break;

  /*
   * If we're getting trouble reclaiming, start doing
   * writepage even in laptop mode.
   */
  if (sc->priority < DEF_PRIORITY - 2)
   sc->may_writepage = 1;

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

 delayacct_freepages_end();

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

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

 /* Untapped cgroup reserves?  Don't OOM, retry. */
 if (!sc->may_thrash) {
  sc->priority = initial_priority;
  sc->may_thrash = 1;
  goto retry;
 }

 return 0;
}

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

 for (i = 0; i <= ZONE_NORMAL; i++) {
  zone = &pgdat->node_zones[i];
  if (!managed_zone(zone) ||
      pgdat_reclaimable_pages(pgdat) == 0)
   continue;

  pfmemalloc_reserve += min_wmark_pages(zone);
  free_pages += zone_page_state(zone, NR_FREE_PAGES);
 }

 /* If there are no reserves (unexpected config) then do not throttle */
 if (!pfmemalloc_reserve)
  return true;

 wmark_ok = free_pages > pfmemalloc_reserve / 2;

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

 return wmark_ok;
}

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

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

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

 /*
  * Check if the pfmemalloc reserves are ok by finding the first node
  * with a usable ZONE_NORMAL or lower zone. The expectation is that
  * GFP_KERNEL will be required for allocating network buffers when
  * swapping over the network so ZONE_HIGHMEM is unusable.
  *
  * Throttling is based on the first usable node and throttled processes
  * wait on a queue until kswapd makes progress and wakes them. There
  * is an affinity then between processes waking up and where reclaim
  * progress has been made assuming the process wakes on the same node.
  * More importantly, processes running on remote nodes will not compete
  * for remote pfmemalloc reserves and processes on different nodes
  * should make reasonable progress.
  */
 for_each_zone_zonelist_nodemask(zone, z, zonelist,
     gfp_zone(gfp_mask), nodemask) {
  if (zone_idx(zone) > ZONE_NORMAL)
   continue;

  /* Throttle based on the first usable node */
  pgdat = zone->zone_pgdat;
  if (pfmemalloc_watermark_ok(pgdat))
   goto out;
  break;
 }

 /* If no zone was usable by the allocation flags then do not throttle */
 if (!pgdat)
  goto out;

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

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

  goto check_pending;
 }

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

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

out:
 return false;
}

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

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

 trace_mm_vmscan_direct_reclaim_begin(order,
    sc.may_writepage,
    gfp_mask,
    sc.reclaim_idx);

 nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

 trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);

 return nr_reclaimed;
}

#ifdef CONFIG_MEMCG

unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
      gfp_t gfp_mask, bool noswap,
      pg_data_t *pgdat,
      unsigned long *nr_scanned)
{
 struct scan_control sc = {
  .nr_to_reclaim = SWAP_CLUSTER_MAX,
  .target_mem_cgroup = memcg,
  .may_writepage = !laptop_mode,
  .may_unmap = 1,
  .reclaim_idx = MAX_NR_ZONES - 1,
  .may_swap = !noswap,
 };
 unsigned long lru_pages;

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

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

 /*
  * 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_node from balance_pgdat
  * will pick up pages from other mem cgroup's as well. We hack
  * the priority and make it zero.
  */
 shrink_node_memcg(pgdat, memcg, &sc, &lru_pages);

 trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);

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

unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
        unsigned long nr_pages,
        gfp_t gfp_mask,
        bool may_swap)
{
 struct zonelist *zonelist;
 unsigned long nr_reclaimed;
 int nid;
 struct scan_control sc = {
  .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
  .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
    (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
  .reclaim_idx = MAX_NR_ZONES - 1,
  .target_mem_cgroup = memcg,
  .priority = DEF_PRIORITY,
  .may_writepage = !laptop_mode,
  .may_unmap = 1,
  .may_swap = may_swap,
 };

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

 zonelist = NODE_DATA(nid)->node_zonelists;

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

 nr_reclaimed = do_try_to_free_pages(zonelist, &sc);

 trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);

 return nr_reclaimed;
}
#endif

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

 if (!total_swap_pages)
  return;

 memcg = mem_cgroup_iter(NULL, NULL, NULL);
 do {
  struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);

  if (inactive_list_is_low(lruvec, false, sc))
   shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
        sc, LRU_ACTIVE_ANON);

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

static bool zone_balanced(struct zone *zone, int order, int classzone_idx)
{
 unsigned long mark = high_wmark_pages(zone);

 if (!zone_watermark_ok_safe(zone, order, mark, classzone_idx))
  return false;

 /*
  * If any eligible zone is balanced then the node is not considered
  * to be congested or dirty
  */
 clear_bit(PGDAT_CONGESTED, &zone->zone_pgdat->flags);
 clear_bit(PGDAT_DIRTY, &zone->zone_pgdat->flags);

 return true;
}

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

 /*
  * The throttled processes are normally woken up in balance_pgdat() as
  * soon as pfmemalloc_watermark_ok() is true. But there is a potential
  * race between when kswapd checks the watermarks and a process gets
  * throttled. There is also a potential race if processes get
  * throttled, kswapd wakes, a large process exits thereby balancing the
  * zones, which causes kswapd to exit balance_pgdat() before reaching
  * the wake up checks. If kswapd is going to sleep, no process should
  * be sleeping on pfmemalloc_wait, so wake them now if necessary. If
  * the wake up is premature, processes will wake kswapd and get
  * throttled again. The difference from wake ups in balance_pgdat() is
  * that here we are under prepare_to_wait().
  */
 if (waitqueue_active(&pgdat->pfmemalloc_wait))
  wake_up_all(&pgdat->pfmemalloc_wait);

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

  if (!managed_zone(zone))
   continue;

  if (!zone_balanced(zone, order, classzone_idx))
   return false;
 }

 return true;
}

/*
 * kswapd shrinks a node of pages that are at or below the highest usable
 * zone that is currently unbalanced.
 *
 * Returns true if kswapd scanned at least the requested number of pages to
 * reclaim or if the lack of progress was due to pages under writeback.
 * This is used to determine if the scanning priority needs to be raised.
 */
static bool kswapd_shrink_node(pg_data_t *pgdat,
          struct scan_control *sc)
{
 struct zone *zone;
 int z;

 /* Reclaim a number of pages proportional to the number of zones */
 sc->nr_to_reclaim = 0;
 for (z = 0; z <= sc->reclaim_idx; z++) {
  zone = pgdat->node_zones + z;
  if (!managed_zone(zone))
   continue;

  sc->nr_to_reclaim += max(high_wmark_pages(zone), SWAP_CLUSTER_MAX);
 }

 /*
  * Historically care was taken to put equal pressure on all zones but
  * now pressure is applied based on node LRU order.
  */
 shrink_node(pgdat, sc);

 /*
  * Fragmentation may mean that the system cannot be rebalanced for
  * high-order allocations. If twice the allocation size has been
  * reclaimed then recheck watermarks only at order-0 to prevent
  * excessive reclaim. Assume that a process requested a high-order
  * can direct reclaim/compact.
  */
 if (sc->order && sc->nr_reclaimed >= 2UL << sc->order)
  sc->order = 0;

 return sc->nr_scanned >= sc->nr_to_reclaim;
}

/*
 * For kswapd, balance_pgdat() will reclaim pages across a node from zones
 * that are eligible for use by the caller until at least one zone is
 * balanced.
 *
 * Returns the order kswapd finished reclaiming at.
 *
 * 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), any page is that zone
 * or lower is eligible for reclaim until at least one usable zone is
 * balanced.
 */
static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
{
 int i;
 unsigned long nr_soft_reclaimed;
 unsigned long nr_soft_scanned;
 struct zone *zone;
 struct scan_control sc = {
  .gfp_mask = GFP_KERNEL,
  .order = order,
  .priority = DEF_PRIORITY,
  .may_writepage = !laptop_mode,
  .may_unmap = 1,
  .may_swap = 1,
 };
 count_vm_event(PAGEOUTRUN);

 do {
  bool raise_priority = true;

  sc.nr_reclaimed = 0;
  sc.reclaim_idx = classzone_idx;

  /*
   * If the number of buffer_heads exceeds the maximum allowed
   * then consider reclaiming from all zones. This has a dual
   * purpose -- on 64-bit systems it is expected that
   * buffer_heads are stripped during active rotation. On 32-bit
   * systems, highmem pages can pin lowmem memory and shrinking
   * buffers can relieve lowmem pressure. Reclaim may still not
   * go ahead if all eligible zones for the original allocation
   * request are balanced to avoid excessive reclaim from kswapd.
   */
  if (buffer_heads_over_limit) {
   for (i = MAX_NR_ZONES - 1; i >= 0; i--) {
    zone = pgdat->node_zones + i;
    if (!managed_zone(zone))
     continue;

    sc.reclaim_idx = i;
    break;
   }
  }

  /*
   * Only reclaim if there are no eligible zones. Check from
   * high to low zone as allocations prefer higher zones.
   * Scanning from low to high zone would allow congestion to be
   * cleared during a very small window when a small low
   * zone was balanced even under extreme pressure when the
   * overall node may be congested. Note that sc.reclaim_idx
   * is not used as buffer_heads_over_limit may have adjusted
   * it.
   */
  for (i = classzone_idx; i >= 0; i--) {
   zone = pgdat->node_zones + i;
   if (!managed_zone(zone))
    continue;

   if (zone_balanced(zone, sc.order, classzone_idx))
    goto out;
  }

  /*
   * Do some background aging of the anon list, to give
   * pages a chance to be referenced before reclaiming. All
   * pages are rotated regardless of classzone as this is
   * about consistent aging.
   */
  age_active_anon(pgdat, &sc);

  /*
   * If we're getting trouble reclaiming, start doing writepage
   * even in laptop mode.
   */
  if (sc.priority < DEF_PRIORITY - 2 || !pgdat_reclaimable(pgdat))
   sc.may_writepage = 1;

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

  /*
   * There should be no need to raise the scanning priority if
   * enough pages are already being scanned that that high
   * watermark would be met at 100% efficiency.
   */
  if (kswapd_shrink_node(pgdat, &sc))
   raise_priority = false;

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

  /* Check if kswapd should be suspending */
  if (try_to_freeze() || kthread_should_stop())
   break;

  /*
   * Raise priority if scanning rate is too low or there was no
   * progress in reclaiming pages
   */
  if (raise_priority || !sc.nr_reclaimed)
   sc.priority--;
 } while (sc.priority >= 1);

out:
 /*
  * Return the order kswapd stopped reclaiming at as
  * prepare_kswapd_sleep() takes it into account. If another caller
  * entered the allocator slow path while kswapd was awake, order will
  * remain at the higher level.
  */
 return sc.order;
}

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

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

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

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

  /*
   * We have freed the memory, now we should compact it to make
   * allocation of the requested order possible.
   */
  wakeup_kcompactd(pgdat, alloc_order, classzone_idx);

  remaining = schedule_timeout(HZ/10);

  /*
   * If woken prematurely then reset kswapd_classzone_idx and
   * order. The values will either be from a wakeup request or
   * the previous request that slept prematurely.
   */
  if (remaining) {
   pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx, classzone_idx);
   pgdat->kswapd_order = max(pgdat->kswapd_order, reclaim_order);
  }

  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 (!remaining &&
     prepare_kswapd_sleep(pgdat, reclaim_order, classzone_idx)) {
  trace_mm_vmscan_kswapd_sleep(pgdat->node_id);

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

  if (!kthread_should_stop())
   schedule();

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

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

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

 lockdep_set_current_reclaim_state(GFP_KERNEL);

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

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

 pgdat->kswapd_order = alloc_order = reclaim_order = 0;
 pgdat->kswapd_classzone_idx = classzone_idx = 0;
 for ( ; ; ) {
  bool ret;

kswapd_try_sleep:
  kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
     classzone_idx);

  /* Read the new order and classzone_idx */
  alloc_order = reclaim_order = pgdat->kswapd_order;
  classzone_idx = pgdat->kswapd_classzone_idx;
  pgdat->kswapd_order = 0;
  pgdat->kswapd_classzone_idx = 0;

  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)
   continue;

  /*
   * Reclaim begins at the requested order but if a high-order
   * reclaim fails then kswapd falls back to reclaiming for
   * order-0. If that happens, kswapd will consider sleeping
   * for the order it finished reclaiming at (reclaim_order)
   * but kcompactd is woken to compact for the original
   * request (alloc_order).
   */
  trace_mm_vmscan_kswapd_wake(pgdat->node_id, classzone_idx,
      alloc_order);
  reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
  if (reclaim_order < alloc_order)
   goto kswapd_try_sleep;

  alloc_order = reclaim_order = pgdat->kswapd_order;
  classzone_idx = pgdat->kswapd_classzone_idx;
 }

 tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
 current->reclaim_state = NULL;
 lockdep_clear_current_reclaim_state();

 return 0;
}

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

 if (!managed_zone(zone))
  return;

 if (!cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
  return;
 pgdat = zone->zone_pgdat;
 pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx, classzone_idx);
 pgdat->kswapd_order = max(pgdat->kswapd_order, order);
 if (!waitqueue_active(&pgdat->kswapd_wait))
  return;

 /* Only wake kswapd if all zones are unbalanced */
 for (z = 0; z <= classzone_idx; z++) {
  zone = pgdat->node_zones + z;
  if (!managed_zone(zone))
   continue;
//wgz 这里需要注意的是只要有一个zone是balanced的，就不会唤醒kswapd线程，因为那样就有内存可以分配
//为何要这样，而不是只要有不balanced的就唤醒?平衡考虑?
  if (zone_balanced(zone, order, classzone_idx))
   return;
 }

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

#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 = {
  .nr_to_reclaim = nr_to_reclaim,
  .gfp_mask = GFP_HIGHUSER_MOVABLE,
  .reclaim_idx = MAX_NR_ZONES - 1,
  .priority = DEF_PRIORITY,
  .may_writepage = 1,
  .may_unmap = 1,
  .may_swap = 1,
  .hibernation_mode = 1,
 };
 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 cpu_callback(struct notifier_block *nfb, unsigned long action,
   void *hcpu)
{
 int nid;

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

   mask = cpumask_of_node(pgdat->node_id);

   if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
    /* One of our CPUs online: restore mask */
    set_cpus_allowed_ptr(pgdat->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);
  pr_err("Failed to start kswapd on node %d\n", nid);
  ret = PTR_ERR(pgdat->kswapd);
  pgdat->kswapd = NULL;
 }
 return ret;
}

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

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

static int __init kswapd_init(void)
{
 int nid;

 swap_setup();
    //每个node都有一个kswapd线程，还好我只有一个node wgz
 for_each_node_state(nid, N_MEMORY)
   kswapd_run(nid);
 hotcpu_notifier(cpu_callback, 0);
 return 0;
}

module_init(kswapd_init)

#ifdef CONFIG_NUMA
/*
 * Node reclaim mode
 *
 * If non-zero call node_reclaim when the number of free pages falls below
 * the watermarks.
 */
int node_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_UNMAP (1<<2) /* Unmap pages during reclaim */

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

/*
 * Percentage of pages in a zone that must be unmapped for node_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 node_unmapped_file_pages(struct pglist_data *pgdat)
{
 unsigned long file_mapped = node_page_state(pgdat, NR_FILE_MAPPED);
 unsigned long file_lru = node_page_state(pgdat, NR_INACTIVE_FILE) +
  node_page_state(pgdat, 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 unsigned long node_pagecache_reclaimable(struct pglist_data *pgdat)
{
 unsigned long nr_pagecache_reclaimable;
 unsigned long delta = 0;

 /*
  * If RECLAIM_UNMAP is set, then all file pages are considered
  * potentially reclaimable. Otherwise, we have to worry about
  * pages like swapcache and node_unmapped_file_pages() provides
  * a better estimate
  */
 if (node_reclaim_mode & RECLAIM_UNMAP)
  nr_pagecache_reclaimable = node_page_state(pgdat, NR_FILE_PAGES);
 else
  nr_pagecache_reclaimable = node_unmapped_file_pages(pgdat);

 /* If we can't clean pages, remove dirty pages from consideration */
 if (!(node_reclaim_mode & RECLAIM_WRITE))
  delta += node_page_state(pgdat, 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 node through reclaim.
 */
static int __node_reclaim(struct pglist_data *pgdat, 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 classzone_idx = gfp_zone(gfp_mask);
 struct scan_control sc = {
  .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
  .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
  .order = order,
  .priority = NODE_RECLAIM_PRIORITY,
  .may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
  .may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
  .may_swap = 1,
  .reclaim_idx = classzone_idx,
 };

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

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

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

int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
{
 int ret;

 /*
  * Node 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 node is overallocated. So we do not reclaim
  * if less than a specified percentage of the node is used by
  * unmapped file backed pages.
  */
 if (node_pagecache_reclaimable(pgdat) <= pgdat->min_unmapped_pages &&
     sum_zone_node_page_state(pgdat->node_id, NR_SLAB_RECLAIMABLE) <= pgdat->min_slab_pages)
  return NODE_RECLAIM_FULL;

 if (!pgdat_reclaimable(pgdat))
  return NODE_RECLAIM_FULL;

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

 /*
  * Only run node reclaim on the local node or on nodes 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.
  */
 if (node_state(pgdat->node_id, N_CPU) && pgdat->node_id != numa_node_id())
  return NODE_RECLAIM_NOSCAN;

 if (test_and_set_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags))
  return NODE_RECLAIM_NOSCAN;

 ret = __node_reclaim(pgdat, gfp_mask, order);
 clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags);

 if (!ret)
  count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);

 return ret;
}
#endif

/*
 * page_evictable - test whether a page is evictable
 * @page: the page to test
 *
 * Test whether page is evictable--i.e., should be placed on active/inactive
 * lists vs unevictable list.
 *
 * Reasons page might not be evictable:
 * (1) page's mapping marked unevictable
 * (2) page is part of an mlocked VMA
 *
 */
int page_evictable(struct page *page)
{
 return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
}
//CONFIG_SHMEM有定义:wgz
#ifdef CONFIG_SHMEM
/**
 * check_move_unevictable_pages - check pages for evictability and move to appropriate zone lru list
 * @pages: array of pages to check
 * @nr_pages: number of pages to check
 *
 * Checks pages for evictability and moves them to the appropriate lru list.
 *
 * This function is only used for SysV IPC SHM_UNLOCK.
 */
 //此函数将pages从unevictable队列中删除，放入其他合适的队列中 wgz
void check_move_unevictable_pages(struct page **pages, int nr_pages)
{
 struct lruvec *lruvec;
 struct pglist_data *pgdat = NULL;
 int pgscanned = 0;
 int pgrescued = 0;
 int i;

 for (i = 0; i < nr_pages; i++) {
  struct page *page = pages[i];
  struct pglist_data *pagepgdat = page_pgdat(page);

  pgscanned++;
//每次lock，unlock开销很大吗? wgz
  if (pagepgdat != pgdat) {
   if (pgdat)
    spin_unlock_irq(&pgdat->lru_lock);
   pgdat = pagepgdat;
   spin_lock_irq(&pgdat->lru_lock);
  }
  lruvec = mem_cgroup_page_lruvec(page, pgdat);

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

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

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

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