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vmscan.c

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

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

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

#include <linux/swapops.h>

#include "internal.h"

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

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

      int may_writepage;

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

      /* This context's SWAP_CLUSTER_MAX. If freeing memory for
       * suspend, we effectively ignore SWAP_CLUSTER_MAX.
       * In this context, it doesn't matter that we scan the
       * whole list at once. */
      int swap_cluster_max;

      int swappiness;

      int all_unreclaimable;

      int order;
};

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

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

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

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

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

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

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

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

      if (scanned == 0)
            scanned = SWAP_CLUSTER_MAX;

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

      list_for_each_entry(shrinker, &shrinker_list, list) {
            unsigned long long delta;
            unsigned long total_scan;
            unsigned long max_pass = (*shrinker->shrink)(0, gfp_mask);

            delta = (4 * scanned) / shrinker->seeks;
            delta *= max_pass;
            do_div(delta, lru_pages + 1);
            shrinker->nr += delta;
            if (shrinker->nr < 0) {
                  printk(KERN_ERR "%s: nr=%ld\n",
                              __FUNCTION__, shrinker->nr);
                  shrinker->nr = max_pass;
            }

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

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

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

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

                  cond_resched();
            }

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

/* Called without lock on whether page is mapped, so answer is unstable */
static inline int page_mapping_inuse(struct page *page)
{
      struct address_space *mapping;

      /* Page is in somebody's page tables. */
      if (page_mapped(page))
            return 1;

      /* Be more reluctant to reclaim swapcache than pagecache */
      if (PageSwapCache(page))
            return 1;

      mapping = page_mapping(page);
      if (!mapping)
            return 0;

      /* File is mmap'd by somebody? */
      return mapping_mapped(mapping);
}

static inline int is_page_cache_freeable(struct page *page)
{
      return page_count(page) - !!PagePrivate(page) == 2;
}

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

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

/* Request for sync pageout. */
enum pageout_io {
      PAGEOUT_IO_ASYNC,
      PAGEOUT_IO_SYNC,
};

/* 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,
                                    enum pageout_io sync_writeback)
{
      /*
       * 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() 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.
       * See swapfile.c:page_queue_congested().
       */
      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 (PagePrivate(page)) {
                  if (try_to_free_buffers(page)) {
                        ClearPageDirty(page);
                        printk("%s: orphaned page\n", __FUNCTION__);
                        return PAGE_CLEAN;
                  }
            }
            return PAGE_KEEP;
      }
      if (mapping->a_ops->writepage == NULL)
            return PAGE_ACTIVATE;
      if (!may_write_to_queue(mapping->backing_dev_info))
            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,
                  .nonblocking = 1,
                  .for_reclaim = 1,
            };

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

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

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

      return PAGE_CLEAN;
}

/*
 * 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)
{
      BUG_ON(!PageLocked(page));
      BUG_ON(mapping != page_mapping(page));

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

      if (PageSwapCache(page)) {
            swp_entry_t swap = { .val = page_private(page) };
            __delete_from_swap_cache(page);
            write_unlock_irq(&mapping->tree_lock);
            swap_free(swap);
            __put_page(page); /* The pagecache ref */
            return 1;
      }

      __remove_from_page_cache(page);
      write_unlock_irq(&mapping->tree_lock);
      __put_page(page);
      return 1;

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

/*
 * shrink_page_list() returns the number of reclaimed pages
 */
static unsigned long shrink_page_list(struct list_head *page_list,
                              struct scan_control *sc,
                              enum pageout_io sync_writeback)
{
      LIST_HEAD(ret_pages);
      struct pagevec freed_pvec;
      int pgactivate = 0;
      unsigned long nr_reclaimed = 0;

      cond_resched();

      pagevec_init(&freed_pvec, 1);
      while (!list_empty(page_list)) {
            struct address_space *mapping;
            struct page *page;
            int may_enter_fs;
            int referenced;

            cond_resched();

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

            if (TestSetPageLocked(page))
                  goto keep;

            VM_BUG_ON(PageActive(page));

            sc->nr_scanned++;

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

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

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

            if (PageWriteback(page)) {
                  /*
                   * Synchronous reclaim is performed in two passes,
                   * first an asynchronous pass over the list to
                   * start parallel writeback, and a second synchronous
                   * pass to wait for the IO to complete.  Wait here
                   * for any page for which writeback has already
                   * started.
                   */
                  if (sync_writeback == PAGEOUT_IO_SYNC && may_enter_fs)
                        wait_on_page_writeback(page);
                  else
                        goto keep_locked;
            }

            referenced = page_referenced(page, 1);
            /* In active use or really unfreeable?  Activate it. */
            if (sc->order <= PAGE_ALLOC_COSTLY_ORDER &&
                              referenced && page_mapping_inuse(page))
                  goto activate_locked;

#ifdef CONFIG_SWAP
            /*
             * Anonymous process memory has backing store?
             * Try to allocate it some swap space here.
             */
            if (PageAnon(page) && !PageSwapCache(page))
                  if (!add_to_swap(page, GFP_ATOMIC))
                        goto activate_locked;
#endif /* CONFIG_SWAP */

            mapping = page_mapping(page);

            /*
             * The page is mapped into the page tables of one or more
             * processes. Try to unmap it here.
             */
            if (page_mapped(page) && mapping) {
                  switch (try_to_unmap(page, 0)) {
                  case SWAP_FAIL:
                        goto activate_locked;
                  case SWAP_AGAIN:
                        goto keep_locked;
                  case SWAP_SUCCESS:
                        ; /* try to free the page below */
                  }
            }

            if (PageDirty(page)) {
                  if (sc->order <= PAGE_ALLOC_COSTLY_ORDER && referenced)
                        goto keep_locked;
                  if (!may_enter_fs)
                        goto keep_locked;
                  if (!sc->may_writepage)
                        goto keep_locked;

                  /* Page is dirty, try to write it out here */
                  switch (pageout(page, mapping, sync_writeback)) {
                  case PAGE_KEEP:
                        goto keep_locked;
                  case PAGE_ACTIVATE:
                        goto activate_locked;
                  case PAGE_SUCCESS:
                        if (PageWriteback(page) || PageDirty(page))
                              goto keep;
                        /*
                         * A synchronous write - probably a ramdisk.  Go
                         * ahead and try to reclaim the page.
                         */
                        if (TestSetPageLocked(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 (PagePrivate(page)) {
                  if (!try_to_release_page(page, sc->gfp_mask))
                        goto activate_locked;
                  if (!mapping && page_count(page) == 1)
                        goto free_it;
            }

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

free_it:
            unlock_page(page);
            nr_reclaimed++;
            if (!pagevec_add(&freed_pvec, page))
                  __pagevec_release_nonlru(&freed_pvec);
            continue;

activate_locked:
            SetPageActive(page);
            pgactivate++;
keep_locked:
            unlock_page(page);
keep:
            list_add(&page->lru, &ret_pages);
            VM_BUG_ON(PageLRU(page));
      }
      list_splice(&ret_pages, page_list);
      if (pagevec_count(&freed_pvec))
            __pagevec_release_nonlru(&freed_pvec);
      count_vm_events(PGACTIVATE, pgactivate);
      return nr_reclaimed;
}

/* LRU Isolation modes. */
#define ISOLATE_INACTIVE 0    /* Isolate inactive pages. */
#define ISOLATE_ACTIVE 1      /* Isolate active pages. */
#define ISOLATE_BOTH 2        /* Isolate both active and inactive pages. */

/*
 * 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.
 */
static int __isolate_lru_page(struct page *page, int mode)
{
      int ret = -EINVAL;

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

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

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

      return ret;
}

/*
 * zone->lru_lock is heavily contended.  Some of the functions that
 * shrink the lists perform better by taking out a batch of pages
 * and working on them outside the LRU lock.
 *
 * For pagecache intensive workloads, this function is the hottest
 * spot in the kernel (apart from copy_*_user functions).
 *
 * Appropriate locks must be held before calling this function.
 *
 * @nr_to_scan:   The number of pages to look through on the list.
 * @src:    The LRU list to pull pages off.
 * @dst:    The temp list to put pages on to.
 * @scanned:      The number of pages that were scanned.
 * @order:  The caller's attempted allocation order
 * @mode:   One of the LRU isolation modes
 *
 * returns how many pages were moved onto *@dst.
 */
static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
            struct list_head *src, struct list_head *dst,
            unsigned long *scanned, int order, int mode)
{
      unsigned long nr_taken = 0;
      unsigned long scan;

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

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

            VM_BUG_ON(!PageLRU(page));

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

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

            default:
                  BUG();
            }

            if (!order)
                  continue;

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

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

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

                  cursor_page = pfn_to_page(pfn);
                  /* Check that we have not crossed a zone boundary. */
                  if (unlikely(page_zone_id(cursor_page) != zone_id))
                        continue;
                  switch (__isolate_lru_page(cursor_page, mode)) {
                  case 0:
                        list_move(&cursor_page->lru, dst);
                        nr_taken++;
                        scan++;
                        break;

                  case -EBUSY:
                        /* else it is being freed elsewhere */
                        list_move(&cursor_page->lru, src);
                  default:
                        break;
                  }
            }
      }

      *scanned = scan;
      return nr_taken;
}

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

      list_for_each_entry(page, page_list, lru)
            if (PageActive(page)) {
                  ClearPageActive(page);
                  nr_active++;
            }

      return nr_active;
}

/*
 * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
 * of reclaimed pages
 */
static unsigned long shrink_inactive_list(unsigned long max_scan,
                        struct zone *zone, struct scan_control *sc)
{
      LIST_HEAD(page_list);
      struct pagevec pvec;
      unsigned long nr_scanned = 0;
      unsigned long nr_reclaimed = 0;

      pagevec_init(&pvec, 1);

      lru_add_drain();
      spin_lock_irq(&zone->lru_lock);
      do {
            struct page *page;
            unsigned long nr_taken;
            unsigned long nr_scan;
            unsigned long nr_freed;
            unsigned long nr_active;

            nr_taken = isolate_lru_pages(sc->swap_cluster_max,
                       &zone->inactive_list,
                       &page_list, &nr_scan, sc->order,
                       (sc->order > PAGE_ALLOC_COSTLY_ORDER)?
                                   ISOLATE_BOTH : ISOLATE_INACTIVE);
            nr_active = clear_active_flags(&page_list);
            __count_vm_events(PGDEACTIVATE, nr_active);

            __mod_zone_page_state(zone, NR_ACTIVE, -nr_active);
            __mod_zone_page_state(zone, NR_INACTIVE,
                                    -(nr_taken - nr_active));
            zone->pages_scanned += nr_scan;
            spin_unlock_irq(&zone->lru_lock);

            nr_scanned += nr_scan;
            nr_freed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);

            /*
             * If we are direct reclaiming for contiguous pages and we do
             * not reclaim everything in the list, try again and wait
             * for IO to complete. This will stall high-order allocations
             * but that should be acceptable to the caller
             */
            if (nr_freed < nr_taken && !current_is_kswapd() &&
                              sc->order > PAGE_ALLOC_COSTLY_ORDER) {
                  congestion_wait(WRITE, HZ/10);

                  /*
                   * The attempt at page out may have made some
                   * of the pages active, mark them inactive again.
                   */
                  nr_active = clear_active_flags(&page_list);
                  count_vm_events(PGDEACTIVATE, nr_active);

                  nr_freed += shrink_page_list(&page_list, sc,
                                          PAGEOUT_IO_SYNC);
            }

            nr_reclaimed += nr_freed;
            local_irq_disable();
            if (current_is_kswapd()) {
                  __count_zone_vm_events(PGSCAN_KSWAPD, zone, nr_scan);
                  __count_vm_events(KSWAPD_STEAL, nr_freed);
            } else
                  __count_zone_vm_events(PGSCAN_DIRECT, zone, nr_scan);
            __count_zone_vm_events(PGSTEAL, zone, nr_freed);

            if (nr_taken == 0)
                  goto done;

            spin_lock(&zone->lru_lock);
            /*
             * Put back any unfreeable pages.
             */
            while (!list_empty(&page_list)) {
                  page = lru_to_page(&page_list);
                  VM_BUG_ON(PageLRU(page));
                  SetPageLRU(page);
                  list_del(&page->lru);
                  if (PageActive(page))
                        add_page_to_active_list(zone, page);
                  else
                        add_page_to_inactive_list(zone, page);
                  if (!pagevec_add(&pvec, page)) {
                        spin_unlock_irq(&zone->lru_lock);
                        __pagevec_release(&pvec);
                        spin_lock_irq(&zone->lru_lock);
                  }
            }
      } while (nr_scanned < max_scan);
      spin_unlock(&zone->lru_lock);
done:
      local_irq_enable();
      pagevec_release(&pvec);
      return nr_reclaimed;
}

/*
 * We are about to scan this zone at a certain priority level.  If that priority
 * level is smaller (ie: more urgent) than the previous priority, then note
 * that priority level within the zone.  This is done so that when the next
 * process comes in to scan this zone, it will immediately start out at this
 * priority level rather than having to build up its own scanning priority.
 * Here, this priority affects only the reclaim-mapped threshold.
 */
static inline void note_zone_scanning_priority(struct zone *zone, int priority)
{
      if (priority < zone->prev_priority)
            zone->prev_priority = priority;
}

static inline int zone_is_near_oom(struct zone *zone)
{
      return zone->pages_scanned >= (zone_page_state(zone, NR_ACTIVE)
                        + zone_page_state(zone, NR_INACTIVE))*3;
}

/*
 * This moves pages from the active list to the inactive list.
 *
 * We move them the other way if the page is referenced by one or more
 * processes, from rmap.
 *
 * If the pages are mostly unmapped, the processing is fast and it is
 * appropriate to hold zone->lru_lock across the whole operation.  But if
 * the pages are mapped, the processing is slow (page_referenced()) so we
 * should drop zone->lru_lock around each page.  It's impossible to balance
 * this, so instead we remove the pages from the LRU while processing them.
 * It is safe to rely on PG_active against the non-LRU pages in here because
 * nobody will play with that bit on a non-LRU page.
 *
 * The downside is that we have to touch page->_count against each page.
 * But we had to alter page->flags anyway.
 */
static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
                        struct scan_control *sc, int priority)
{
      unsigned long pgmoved;
      int pgdeactivate = 0;
      unsigned long pgscanned;
      LIST_HEAD(l_hold);      /* The pages which were snipped off */
      LIST_HEAD(l_inactive);  /* Pages to go onto the inactive_list */
      LIST_HEAD(l_active);    /* Pages to go onto the active_list */
      struct page *page;
      struct pagevec pvec;
      int reclaim_mapped = 0;

      if (sc->may_swap) {
            long mapped_ratio;
            long distress;
            long swap_tendency;
            long imbalance;

            if (zone_is_near_oom(zone))
                  goto force_reclaim_mapped;

            /*
             * `distress' is a measure of how much trouble we're having
             * reclaiming pages.  0 -> no problems.  100 -> great trouble.
             */
            distress = 100 >> min(zone->prev_priority, priority);

            /*
             * The point of this algorithm is to decide when to start
             * reclaiming mapped memory instead of just pagecache.  Work out
             * how much memory
             * is mapped.
             */
            mapped_ratio = ((global_page_state(NR_FILE_MAPPED) +
                        global_page_state(NR_ANON_PAGES)) * 100) /
                              vm_total_pages;

            /*
             * Now decide how much we really want to unmap some pages.  The
             * mapped ratio is downgraded - just because there's a lot of
             * mapped memory doesn't necessarily mean that page reclaim
             * isn't succeeding.
             *
             * The distress ratio is important - we don't want to start
             * going oom.
             *
             * A 100% value of vm_swappiness overrides this algorithm
             * altogether.
             */
            swap_tendency = mapped_ratio / 2 + distress + sc->swappiness;

            /*
             * If there's huge imbalance between active and inactive
             * (think active 100 times larger than inactive) we should
             * become more permissive, or the system will take too much
             * cpu before it start swapping during memory pressure.
             * Distress is about avoiding early-oom, this is about
             * making swappiness graceful despite setting it to low
             * values.
             *
             * Avoid div by zero with nr_inactive+1, and max resulting
             * value is vm_total_pages.
             */
            imbalance  = zone_page_state(zone, NR_ACTIVE);
            imbalance /= zone_page_state(zone, NR_INACTIVE) + 1;

            /*
             * Reduce the effect of imbalance if swappiness is low,
             * this means for a swappiness very low, the imbalance
             * must be much higher than 100 for this logic to make
             * the difference.
             *
             * Max temporary value is vm_total_pages*100.
             */
            imbalance *= (vm_swappiness + 1);
            imbalance /= 100;

            /*
             * If not much of the ram is mapped, makes the imbalance
             * less relevant, it's high priority we refill the inactive
             * list with mapped pages only in presence of high ratio of
             * mapped pages.
             *
             * Max temporary value is vm_total_pages*100.
             */
            imbalance *= mapped_ratio;
            imbalance /= 100;

            /* apply imbalance feedback to swap_tendency */
            swap_tendency += imbalance;

            /*
             * Now use this metric to decide whether to start moving mapped
             * memory onto the inactive list.
             */
            if (swap_tendency >= 100)
force_reclaim_mapped:
                  reclaim_mapped = 1;
      }

      lru_add_drain();
      spin_lock_irq(&zone->lru_lock);
      pgmoved = isolate_lru_pages(nr_pages, &zone->active_list,
                      &l_hold, &pgscanned, sc->order, ISOLATE_ACTIVE);
      zone->pages_scanned += pgscanned;
      __mod_zone_page_state(zone, NR_ACTIVE, -pgmoved);
      spin_unlock_irq(&zone->lru_lock);

      while (!list_empty(&l_hold)) {
            cond_resched();
            page = lru_to_page(&l_hold);
            list_del(&page->lru);
            if (page_mapped(page)) {
                  if (!reclaim_mapped ||
                      (total_swap_pages == 0 && PageAnon(page)) ||
                      page_referenced(page, 0)) {
                        list_add(&page->lru, &l_active);
                        continue;
                  }
            }
            list_add(&page->lru, &l_inactive);
      }

      pagevec_init(&pvec, 1);
      pgmoved = 0;
      spin_lock_irq(&zone->lru_lock);
      while (!list_empty(&l_inactive)) {
            page = lru_to_page(&l_inactive);
            prefetchw_prev_lru_page(page, &l_inactive, flags);
            VM_BUG_ON(PageLRU(page));
            SetPageLRU(page);
            VM_BUG_ON(!PageActive(page));
            ClearPageActive(page);

            list_move(&page->lru, &zone->inactive_list);
            pgmoved++;
            if (!pagevec_add(&pvec, page)) {
                  __mod_zone_page_state(zone, NR_INACTIVE, pgmoved);
                  spin_unlock_irq(&zone->lru_lock);
                  pgdeactivate += pgmoved;
                  pgmoved = 0;
                  if (buffer_heads_over_limit)
                        pagevec_strip(&pvec);
                  __pagevec_release(&pvec);
                  spin_lock_irq(&zone->lru_lock);
            }
      }
      __mod_zone_page_state(zone, NR_INACTIVE, pgmoved);
      pgdeactivate += pgmoved;
      if (buffer_heads_over_limit) {
            spin_unlock_irq(&zone->lru_lock);
            pagevec_strip(&pvec);
            spin_lock_irq(&zone->lru_lock);
      }

      pgmoved = 0;
      while (!list_empty(&l_active)) {
            page = lru_to_page(&l_active);
            prefetchw_prev_lru_page(page, &l_active, flags);
            VM_BUG_ON(PageLRU(page));
            SetPageLRU(page);
            VM_BUG_ON(!PageActive(page));
            list_move(&page->lru, &zone->active_list);
            pgmoved++;
            if (!pagevec_add(&pvec, page)) {
                  __mod_zone_page_state(zone, NR_ACTIVE, pgmoved);
                  pgmoved = 0;
                  spin_unlock_irq(&zone->lru_lock);
                  __pagevec_release(&pvec);
                  spin_lock_irq(&zone->lru_lock);
            }
      }
      __mod_zone_page_state(zone, NR_ACTIVE, pgmoved);

      __count_zone_vm_events(PGREFILL, zone, pgscanned);
      __count_vm_events(PGDEACTIVATE, pgdeactivate);
      spin_unlock_irq(&zone->lru_lock);

      pagevec_release(&pvec);
}

/*
 * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
 */
static unsigned long shrink_zone(int priority, struct zone *zone,
                        struct scan_control *sc)
{
      unsigned long nr_active;
      unsigned long nr_inactive;
      unsigned long nr_to_scan;
      unsigned long nr_reclaimed = 0;

      /*
       * Add one to `nr_to_scan' just to make sure that the kernel will
       * slowly sift through the active list.
       */
      zone->nr_scan_active +=
            (zone_page_state(zone, NR_ACTIVE) >> priority) + 1;
      nr_active = zone->nr_scan_active;
      if (nr_active >= sc->swap_cluster_max)
            zone->nr_scan_active = 0;
      else
            nr_active = 0;

      zone->nr_scan_inactive +=
            (zone_page_state(zone, NR_INACTIVE) >> priority) + 1;
      nr_inactive = zone->nr_scan_inactive;
      if (nr_inactive >= sc->swap_cluster_max)
            zone->nr_scan_inactive = 0;
      else
            nr_inactive = 0;

      while (nr_active || nr_inactive) {
            if (nr_active) {
                  nr_to_scan = min(nr_active,
                              (unsigned long)sc->swap_cluster_max);
                  nr_active -= nr_to_scan;
                  shrink_active_list(nr_to_scan, zone, sc, priority);
            }

            if (nr_inactive) {
                  nr_to_scan = min(nr_inactive,
                              (unsigned long)sc->swap_cluster_max);
                  nr_inactive -= nr_to_scan;
                  nr_reclaimed += shrink_inactive_list(nr_to_scan, zone,
                                                sc);
            }
      }

      throttle_vm_writeout(sc->gfp_mask);
      return nr_reclaimed;
}

/*
 * This is the direct reclaim path, for page-allocating processes.  We only
 * try to reclaim pages from zones which will satisfy the caller's allocation
 * request.
 *
 * We reclaim from a zone even if that zone is over pages_high.  Because:
 * a) The caller may be trying to free *extra* pages to satisfy a higher-order
 *    allocation or
 * b) The zones may be over pages_high but they must go *over* pages_high to
 *    satisfy the `incremental min' zone defense algorithm.
 *
 * Returns the number of reclaimed pages.
 *
 * If a zone is deemed to be full of pinned pages then just give it a light
 * scan then give up on it.
 */
static unsigned long shrink_zones(int priority, struct zone **zones,
                              struct scan_control *sc)
{
      unsigned long nr_reclaimed = 0;
      int i;

      sc->all_unreclaimable = 1;
      for (i = 0; zones[i] != NULL; i++) {
            struct zone *zone = zones[i];

            if (!populated_zone(zone))
                  continue;

            if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                  continue;

            note_zone_scanning_priority(zone, priority);

            if (zone_is_all_unreclaimable(zone) && priority != DEF_PRIORITY)
                  continue;   /* Let kswapd poll it */

            sc->all_unreclaimable = 0;

            nr_reclaimed += shrink_zone(priority, zone, sc);
      }
      return nr_reclaimed;
}
 
/*
 * 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 pdflush 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.
 */
unsigned long try_to_free_pages(struct zone **zones, int order, gfp_t gfp_mask)
{
      int priority;
      int ret = 0;
      unsigned long total_scanned = 0;
      unsigned long nr_reclaimed = 0;
      struct reclaim_state *reclaim_state = current->reclaim_state;
      unsigned long lru_pages = 0;
      int i;
      struct scan_control sc = {
            .gfp_mask = gfp_mask,
            .may_writepage = !laptop_mode,
            .swap_cluster_max = SWAP_CLUSTER_MAX,
            .may_swap = 1,
            .swappiness = vm_swappiness,
            .order = order,
      };

      count_vm_event(ALLOCSTALL);

      for (i = 0; zones[i] != NULL; i++) {
            struct zone *zone = zones[i];

            if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                  continue;

            lru_pages += zone_page_state(zone, NR_ACTIVE)
                        + zone_page_state(zone, NR_INACTIVE);
      }

      for (priority = DEF_PRIORITY; priority >= 0; priority--) {
            sc.nr_scanned = 0;
            if (!priority)
                  disable_swap_token();
            nr_reclaimed += shrink_zones(priority, zones, &sc);
            shrink_slab(sc.nr_scanned, gfp_mask, lru_pages);
            if (reclaim_state) {
                  nr_reclaimed += reclaim_state->reclaimed_slab;
                  reclaim_state->reclaimed_slab = 0;
            }
            total_scanned += sc.nr_scanned;
            if (nr_reclaimed >= sc.swap_cluster_max) {
                  ret = 1;
                  goto out;
            }

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

            /* Take a nap, wait for some writeback to complete */
            if (sc.nr_scanned && priority < DEF_PRIORITY - 2)
                  congestion_wait(WRITE, HZ/10);
      }
      /* top priority shrink_caches still had more to do? don't OOM, then */
      if (!sc.all_unreclaimable)
            ret = 1;
out:
      /*
       * Now that we've scanned all the zones at this priority level, note
       * that level within the zone so that the next thread which performs
       * scanning of this zone will immediately start out at this priority
       * level.  This affects only the decision whether or not to bring
       * mapped pages onto the inactive list.
       */
      if (priority < 0)
            priority = 0;
      for (i = 0; zones[i] != NULL; i++) {
            struct zone *zone = zones[i];

            if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                  continue;

            zone->prev_priority = priority;
      }
      return ret;
}

/*
 * For kswapd, balance_pgdat() will work across all this node's zones until
 * they are all at pages_high.
 *
 * Returns the number of pages which were actually freed.
 *
 * There is special handling here for zones which are full of pinned pages.
 * This can happen if the pages are all mlocked, or if they are all used by
 * device drivers (say, ZONE_DMA).  Or if they are all in use by hugetlb.
 * What we do is to detect the case where all pages in the zone have been
 * scanned twice and there has been zero successful reclaim.  Mark the zone as
 * dead and from now on, only perform a short scan.  Basically we're polling
 * the zone for when the problem goes away.
 *
 * kswapd scans the zones in the highmem->normal->dma direction.  It skips
 * zones which have free_pages > pages_high, but once a zone is found to have
 * free_pages <= pages_high, we scan that zone and the lower zones regardless
 * of the number of free pages in the lower zones.  This interoperates with
 * the page allocator fallback scheme to ensure that aging of pages is balanced
 * across the zones.
 */
static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
{
      int all_zones_ok;
      int priority;
      int i;
      unsigned long total_scanned;
      unsigned long nr_reclaimed;
      struct reclaim_state *reclaim_state = current->reclaim_state;
      struct scan_control sc = {
            .gfp_mask = GFP_KERNEL,
            .may_swap = 1,
            .swap_cluster_max = SWAP_CLUSTER_MAX,
            .swappiness = vm_swappiness,
            .order = order,
      };
      /*
       * temp_priority is used to remember the scanning priority at which
       * this zone was successfully refilled to free_pages == pages_high.
       */
      int temp_priority[MAX_NR_ZONES];

loop_again:
      total_scanned = 0;
      nr_reclaimed = 0;
      sc.may_writepage = !laptop_mode;
      count_vm_event(PAGEOUTRUN);

      for (i = 0; i < pgdat->nr_zones; i++)
            temp_priority[i] = DEF_PRIORITY;

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

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

            all_zones_ok = 1;

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

                  if (!populated_zone(zone))
                        continue;

                  if (zone_is_all_unreclaimable(zone) &&
                      priority != DEF_PRIORITY)
                        continue;

                  if (!zone_watermark_ok(zone, order, zone->pages_high,
                                     0, 0)) {
                        end_zone = i;
                        break;
                  }
            }
            if (i < 0)
                  goto out;

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

                  lru_pages += zone_page_state(zone, NR_ACTIVE)
                              + zone_page_state(zone, NR_INACTIVE);
            }

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

                  if (!populated_zone(zone))
                        continue;

                  if (zone_is_all_unreclaimable(zone) &&
                              priority != DEF_PRIORITY)
                        continue;

                  if (!zone_watermark_ok(zone, order, zone->pages_high,
                                     end_zone, 0))
                        all_zones_ok = 0;
                  temp_priority[i] = priority;
                  sc.nr_scanned = 0;
                  note_zone_scanning_priority(zone, priority);
                  /*
                   * We put equal pressure on every zone, unless one
                   * zone has way too many pages free already.
                   */
                  if (!zone_watermark_ok(zone, order, 8*zone->pages_high,
                                    end_zone, 0))
                        nr_reclaimed += shrink_zone(priority, zone, &sc);
                  reclaim_state->reclaimed_slab = 0;
                  nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,
                                    lru_pages);
                  nr_reclaimed += reclaim_state->reclaimed_slab;
                  total_scanned += sc.nr_scanned;
                  if (zone_is_all_unreclaimable(zone))
                        continue;
                  if (nr_slab == 0 && zone->pages_scanned >=
                        (zone_page_state(zone, NR_ACTIVE)
                        + zone_page_state(zone, NR_INACTIVE)) * 6)
                              zone_set_flag(zone,
                                          ZONE_ALL_UNRECLAIMABLE);
                  /*
                   * If we've done a decent amount of scanning and
                   * the reclaim ratio is low, start doing writepage
                   * even in laptop mode
                   */
                  if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
                      total_scanned > nr_reclaimed + nr_reclaimed / 2)
                        sc.may_writepage = 1;
            }
            if (all_zones_ok)
                  break;            /* kswapd: all done */
            /*
             * OK, kswapd is getting into trouble.  Take a nap, then take
             * another pass across the zones.
             */
            if (total_scanned && priority < DEF_PRIORITY - 2)
                  congestion_wait(WRITE, HZ/10);

            /*
             * We do this so kswapd doesn't build up large priorities for
             * example when it is freeing in parallel with allocators. It
             * matches the direct reclaim path behaviour in terms of impact
             * on zone->*_priority.
             */
            if (nr_reclaimed >= SWAP_CLUSTER_MAX)
                  break;
      }
out:
      /*
       * Note within each zone the priority level at which this zone was
       * brought into a happy state.  So that the next thread which scans this
       * zone will start out at that priority level.
       */
      for (i = 0; i < pgdat->nr_zones; i++) {
            struct zone *zone = pgdat->node_zones + i;

            zone->prev_priority = temp_priority[i];
      }
      if (!all_zones_ok) {
            cond_resched();

            try_to_freeze();

            goto loop_again;
      }

      return nr_reclaimed;
}

/*
 * The background pageout daemon, started as a kernel thread
 * from the init process. 
 *
 * This basically trickles out pages so that we have _some_
 * free memory available even if there is no other activity
 * that frees anything up. This is needed for things like routing
 * etc, where we otherwise might have all activity going on in
 * asynchronous contexts that cannot page things out.
 *
 * If there are applications that are active memory-allocators
 * (most normal use), this basically shouldn't matter.
 */
static int kswapd(void *p)
{
      unsigned long order;
      pg_data_t *pgdat = (pg_data_t*)p;
      struct task_struct *tsk = current;
      DEFINE_WAIT(wait);
      struct reclaim_state reclaim_state = {
            .reclaimed_slab = 0,
      };
      cpumask_t cpumask;

      cpumask = node_to_cpumask(pgdat->node_id);
      if (!cpus_empty(cpumask))
            set_cpus_allowed(tsk, cpumask);
      current->reclaim_state = &reclaim_state;

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

      order = 0;
      for ( ; ; ) {
            unsigned long new_order;

            prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
            new_order = pgdat->kswapd_max_order;
            pgdat->kswapd_max_order = 0;
            if (order < new_order) {
                  /*
                   * Don't sleep if someone wants a larger 'order'
                   * allocation
                   */
                  order = new_order;
            } else {
                  if (!freezing(current))
                        schedule();

                  order = pgdat->kswapd_max_order;
            }
            finish_wait(&pgdat->kswapd_wait, &wait);

            if (!try_to_freeze()) {
                  /* We can speed up thawing tasks if we don't call
                   * balance_pgdat after returning from the refrigerator
                   */
                  balance_pgdat(pgdat, order);
            }
      }
      return 0;
}

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

      if (!populated_zone(zone))
            return;

      pgdat = zone->zone_pgdat;
      if (zone_watermark_ok(zone, order, zone->pages_low, 0, 0))
            return;
      if (pgdat->kswapd_max_order < order)
            pgdat->kswapd_max_order = order;
      if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
            return;
      if (!waitqueue_active(&pgdat->kswapd_wait))
            return;
      wake_up_interruptible(&pgdat->kswapd_wait);
}

#ifdef CONFIG_PM
/*
 * Helper function for shrink_all_memory().  Tries to reclaim 'nr_pages' pages
 * from LRU lists system-wide, for given pass and priority, and returns the
 * number of reclaimed pages
 *
 * For pass > 3 we also try to shrink the LRU lists that contain a few pages
 */
static unsigned long shrink_all_zones(unsigned long nr_pages, int prio,
                              int pass, struct scan_control *sc)
{
      struct zone *zone;
      unsigned long nr_to_scan, ret = 0;

      for_each_zone(zone) {

            if (!populated_zone(zone))
                  continue;

            if (zone_is_all_unreclaimable(zone) && prio != DEF_PRIORITY)
                  continue;

            /* For pass = 0 we don't shrink the active list */
            if (pass > 0) {
                  zone->nr_scan_active +=
                        (zone_page_state(zone, NR_ACTIVE) >> prio) + 1;
                  if (zone->nr_scan_active >= nr_pages || pass > 3) {
                        zone->nr_scan_active = 0;
                        nr_to_scan = min(nr_pages,
                              zone_page_state(zone, NR_ACTIVE));
                        shrink_active_list(nr_to_scan, zone, sc, prio);
                  }
            }

            zone->nr_scan_inactive +=
                  (zone_page_state(zone, NR_INACTIVE) >> prio) + 1;
            if (zone->nr_scan_inactive >= nr_pages || pass > 3) {
                  zone->nr_scan_inactive = 0;
                  nr_to_scan = min(nr_pages,
                        zone_page_state(zone, NR_INACTIVE));
                  ret += shrink_inactive_list(nr_to_scan, zone, sc);
                  if (ret >= nr_pages)
                        return ret;
            }
      }

      return ret;
}

static unsigned long count_lru_pages(void)
{
      return global_page_state(NR_ACTIVE) + global_page_state(NR_INACTIVE);
}

/*
 * Try to free `nr_pages' 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_pages)
{
      unsigned long lru_pages, nr_slab;
      unsigned long ret = 0;
      int pass;
      struct reclaim_state reclaim_state;
      struct scan_control sc = {
            .gfp_mask = GFP_KERNEL,
            .may_swap = 0,
            .swap_cluster_max = nr_pages,
            .may_writepage = 1,
            .swappiness = vm_swappiness,
      };

      current->reclaim_state = &reclaim_state;

      lru_pages = count_lru_pages();
      nr_slab = global_page_state(NR_SLAB_RECLAIMABLE);
      /* If slab caches are huge, it's better to hit them first */
      while (nr_slab >= lru_pages) {
            reclaim_state.reclaimed_slab = 0;
            shrink_slab(nr_pages, sc.gfp_mask, lru_pages);
            if (!reclaim_state.reclaimed_slab)
                  break;

            ret += reclaim_state.reclaimed_slab;
            if (ret >= nr_pages)
                  goto out;

            nr_slab -= reclaim_state.reclaimed_slab;
      }

      /*
       * We try to shrink LRUs in 5 passes:
       * 0 = Reclaim from inactive_list only
       * 1 = Reclaim from active list but don't reclaim mapped
       * 2 = 2nd pass of type 1
       * 3 = Reclaim mapped (normal reclaim)
       * 4 = 2nd pass of type 3
       */
      for (pass = 0; pass < 5; pass++) {
            int prio;

            /* Force reclaiming mapped pages in the passes #3 and #4 */
            if (pass > 2) {
                  sc.may_swap = 1;
                  sc.swappiness = 100;
            }

            for (prio = DEF_PRIORITY; prio >= 0; prio--) {
                  unsigned long nr_to_scan = nr_pages - ret;

                  sc.nr_scanned = 0;
                  ret += shrink_all_zones(nr_to_scan, prio, pass, &sc);
                  if (ret >= nr_pages)
                        goto out;

                  reclaim_state.reclaimed_slab = 0;
                  shrink_slab(sc.nr_scanned, sc.gfp_mask,
                              count_lru_pages());
                  ret += reclaim_state.reclaimed_slab;
                  if (ret >= nr_pages)
                        goto out;

                  if (sc.nr_scanned && prio < DEF_PRIORITY - 2)
                        congestion_wait(WRITE, HZ / 10);
            }
      }

      /*
       * If ret = 0, we could not shrink LRUs, but there may be something
       * in slab caches
       */
      if (!ret) {
            do {
                  reclaim_state.reclaimed_slab = 0;
                  shrink_slab(nr_pages, sc.gfp_mask, count_lru_pages());
                  ret += reclaim_state.reclaimed_slab;
            } while (ret < nr_pages && reclaim_state.reclaimed_slab > 0);
      }

out:
      current->reclaim_state = NULL;

      return ret;
}
#endif

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

      if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN) {
            for_each_node_state(nid, N_HIGH_MEMORY) {
                  pgdat = NODE_DATA(nid);
                  mask = node_to_cpumask(pgdat->node_id);
                  if (any_online_cpu(mask) != NR_CPUS)
                        /* One of our CPUs online: restore mask */
                        set_cpus_allowed(pgdat->kswapd, mask);
            }
      }
      return NOTIFY_OK;
}

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

      if (pgdat->kswapd)
            return 0;

      pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
      if (IS_ERR(pgdat->kswapd)) {
            /* failure at boot is fatal */
            BUG_ON(system_state == SYSTEM_BOOTING);
            printk("Failed to start kswapd on node %d\n",nid);
            ret = -1;
      }
      return ret;
}

static int __init kswapd_init(void)
{
      int nid;

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

module_init(kswapd_init)

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

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

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

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

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

/*
 * Try to free up some pages from this zone through reclaim.
 */
static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
{
      /* Minimum pages needed in order to stay on node */
      const unsigned long nr_pages = 1 << order;
      struct task_struct *p = current;
      struct reclaim_state reclaim_state;
      int priority;
      unsigned long nr_reclaimed = 0;
      struct scan_control sc = {
            .may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
            .may_swap = !!(zone_reclaim_mode & RECLAIM_SWAP),
            .swap_cluster_max = max_t(unsigned long, nr_pages,
                              SWAP_CLUSTER_MAX),
            .gfp_mask = gfp_mask,
            .swappiness = vm_swappiness,
      };
      unsigned long slab_reclaimable;

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

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

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

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

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

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

      /*
       * Zone reclaim reclaims unmapped file backed pages and
       * slab pages if we are over the defined limits.
       *
       * A small portion of unmapped file backed pages is needed for
       * file I/O otherwise pages read by file I/O will be immediately
       * thrown out if the zone is overallocated. So we do not reclaim
       * if less than a specified percentage of the zone is used by
       * unmapped file backed pages.
       */
      if (zone_page_state(zone, NR_FILE_PAGES) -
          zone_page_state(zone, NR_FILE_MAPPED) <= zone->min_unmapped_pages
          && zone_page_state(zone, NR_SLAB_RECLAIMABLE)
                  <= zone->min_slab_pages)
            return 0;

      if (zone_is_all_unreclaimable(zone))
            return 0;

      /*
       * Do not scan if the allocation should not be delayed.
       */
      if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
                  return 0;

      /*
       * Only run zone reclaim on the local zone or on zones that do not
       * have associated processors. This will favor the local processor
       * over remote processors and spread off node memory allocations
       * as wide as possible.
       */
      node_id = zone_to_nid(zone);
      if (node_state(node_id, N_CPU) && node_id != numa_node_id())
            return 0;

      if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
            return 0;
      ret = __zone_reclaim(zone, gfp_mask, order);
      zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);

      return ret;
}
#endif

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