/*
   *  linux/mm/swap.c
   *
   *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
   */
  
  /*

   * This file contains the default values for the operation of the

   * Linux VM subsystem. Fine-tuning documentation can be found in
   * Documentation/sysctl/vm.txt.
   * Started 18.12.91
   * Swap aging added 23.2.95, Stephen Tweedie.
   * Buffermem limits added 12.3.98, Rik van Riel.
   */
  
  #include <linux/mm.h>
  #include <linux/sched.h>
  #include <linux/kernel_stat.h>
  #include <linux/swap.h>
  #include <linux/mman.h>
  #include <linux/pagemap.h>
  #include <linux/pagevec.h>
  #include <linux/init.h>
  #include <linux/module.h>
  #include <linux/mm_inline.h>
  #include <linux/buffer_head.h>	/* for try_to_release_page() */

  #include <linux/percpu_counter.h>
  #include <linux/percpu.h>
  #include <linux/cpu.h>
  #include <linux/notifier.h>

  #include <linux/backing-dev.h>

  #include <linux/memcontrol.h>

  #include <linux/gfp.h>

  #include "internal.h"

  /* How many pages do we try to swap or page in/out together? */
  int page_cluster;

  static DEFINE_PER_CPU(struct pagevec[NR_LRU_LISTS], lru_add_pvecs);

  static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);

  static DEFINE_PER_CPU(struct pagevec, lru_deactivate_pvecs);

  /*
   * This path almost never happens for VM activity - pages are normally
   * freed via pagevecs.  But it gets used by networking.
   */

  static void __page_cache_release(struct page *page)

  {
  	if (PageLRU(page)) {
  		unsigned long flags;
  		struct zone *zone = page_zone(page);
  
  		spin_lock_irqsave(&zone->lru_lock, flags);
  		VM_BUG_ON(!PageLRU(page));
  		__ClearPageLRU(page);
  		del_page_from_lru(zone, page);
  		spin_unlock_irqrestore(&zone->lru_lock, flags);
  	}

  }
  
  static void __put_single_page(struct page *page)
  {
  	__page_cache_release(page);

  	free_hot_cold_page(page, 0);

  }

  static void __put_compound_page(struct page *page)

  {

  	compound_page_dtor *dtor;

  	__page_cache_release(page);
  	dtor = get_compound_page_dtor(page);
  	(*dtor)(page);
  }
  
  static void put_compound_page(struct page *page)
  {
  	if (unlikely(PageTail(page))) {
  		/* __split_huge_page_refcount can run under us */
  		struct page *page_head = page->first_page;
  		smp_rmb();
  		/*
  		 * If PageTail is still set after smp_rmb() we can be sure
  		 * that the page->first_page we read wasn't a dangling pointer.
  		 * See __split_huge_page_refcount() smp_wmb().
  		 */
  		if (likely(PageTail(page) && get_page_unless_zero(page_head))) {
  			unsigned long flags;
  			/*
  			 * Verify that our page_head wasn't converted
  			 * to a a regular page before we got a
  			 * reference on it.
  			 */
  			if (unlikely(!PageHead(page_head))) {
  				/* PageHead is cleared after PageTail */
  				smp_rmb();
  				VM_BUG_ON(PageTail(page));
  				goto out_put_head;
  			}
  			/*
  			 * Only run compound_lock on a valid PageHead,
  			 * after having it pinned with
  			 * get_page_unless_zero() above.
  			 */
  			smp_mb();
  			/* page_head wasn't a dangling pointer */
  			flags = compound_lock_irqsave(page_head);
  			if (unlikely(!PageTail(page))) {
  				/* __split_huge_page_refcount run before us */
  				compound_unlock_irqrestore(page_head, flags);
  				VM_BUG_ON(PageHead(page_head));
  			out_put_head:
  				if (put_page_testzero(page_head))
  					__put_single_page(page_head);
  			out_put_single:
  				if (put_page_testzero(page))
  					__put_single_page(page);
  				return;
  			}
  			VM_BUG_ON(page_head != page->first_page);
  			/*
  			 * We can release the refcount taken by
  			 * get_page_unless_zero now that
  			 * split_huge_page_refcount is blocked on the
  			 * compound_lock.
  			 */
  			if (put_page_testzero(page_head))
  				VM_BUG_ON(1);
  			/* __split_huge_page_refcount will wait now */
  			VM_BUG_ON(atomic_read(&page->_count) <= 0);
  			atomic_dec(&page->_count);
  			VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
  			compound_unlock_irqrestore(page_head, flags);

  			if (put_page_testzero(page_head)) {
  				if (PageHead(page_head))
  					__put_compound_page(page_head);
  				else
  					__put_single_page(page_head);
  			}

  		} else {
  			/* page_head is a dangling pointer */
  			VM_BUG_ON(PageTail(page));
  			goto out_put_single;
  		}
  	} else if (put_page_testzero(page)) {
  		if (PageHead(page))
  			__put_compound_page(page);
  		else
  			__put_single_page(page);

  	}

  }
  
  void put_page(struct page *page)
  {
  	if (unlikely(PageCompound(page)))
  		put_compound_page(page);
  	else if (put_page_testzero(page))

  		__put_single_page(page);

  }
  EXPORT_SYMBOL(put_page);

  /**

   * put_pages_list() - release a list of pages
   * @pages: list of pages threaded on page->lru

   *
   * Release a list of pages which are strung together on page.lru.  Currently
   * used by read_cache_pages() and related error recovery code.

   */
  void put_pages_list(struct list_head *pages)
  {
  	while (!list_empty(pages)) {
  		struct page *victim;
  
  		victim = list_entry(pages->prev, struct page, lru);
  		list_del(&victim->lru);
  		page_cache_release(victim);
  	}
  }
  EXPORT_SYMBOL(put_pages_list);

  static void pagevec_lru_move_fn(struct pagevec *pvec,
  				void (*move_fn)(struct page *page, void *arg),
  				void *arg)

  {
  	int i;

  	struct zone *zone = NULL;

  	unsigned long flags = 0;

  
  	for (i = 0; i < pagevec_count(pvec); i++) {
  		struct page *page = pvec->pages[i];
  		struct zone *pagezone = page_zone(page);
  
  		if (pagezone != zone) {
  			if (zone)

  				spin_unlock_irqrestore(&zone->lru_lock, flags);

  			zone = pagezone;

  			spin_lock_irqsave(&zone->lru_lock, flags);

  		}

  
  		(*move_fn)(page, arg);

  	}
  	if (zone)

  		spin_unlock_irqrestore(&zone->lru_lock, flags);

  	release_pages(pvec->pages, pvec->nr, pvec->cold);
  	pagevec_reinit(pvec);

  }

  static void pagevec_move_tail_fn(struct page *page, void *arg)
  {
  	int *pgmoved = arg;
  	struct zone *zone = page_zone(page);
  
  	if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
  		enum lru_list lru = page_lru_base_type(page);
  		list_move_tail(&page->lru, &zone->lru[lru].list);
  		mem_cgroup_rotate_reclaimable_page(page);
  		(*pgmoved)++;
  	}
  }
  
  /*
   * pagevec_move_tail() must be called with IRQ disabled.
   * Otherwise this may cause nasty races.
   */
  static void pagevec_move_tail(struct pagevec *pvec)
  {
  	int pgmoved = 0;
  
  	pagevec_lru_move_fn(pvec, pagevec_move_tail_fn, &pgmoved);
  	__count_vm_events(PGROTATED, pgmoved);
  }

  /*

   * Writeback is about to end against a page which has been marked for immediate
   * reclaim.  If it still appears to be reclaimable, move it to the tail of the

   * inactive list.

   */

  void rotate_reclaimable_page(struct page *page)

  {

  	if (!PageLocked(page) && !PageDirty(page) && !PageActive(page) &&

  	    !PageUnevictable(page) && PageLRU(page)) {

  		struct pagevec *pvec;
  		unsigned long flags;
  
  		page_cache_get(page);
  		local_irq_save(flags);
  		pvec = &__get_cpu_var(lru_rotate_pvecs);
  		if (!pagevec_add(pvec, page))
  			pagevec_move_tail(pvec);
  		local_irq_restore(flags);
  	}

  }

  static void update_page_reclaim_stat(struct zone *zone, struct page *page,
  				     int file, int rotated)
  {
  	struct zone_reclaim_stat *reclaim_stat = &zone->reclaim_stat;
  	struct zone_reclaim_stat *memcg_reclaim_stat;
  
  	memcg_reclaim_stat = mem_cgroup_get_reclaim_stat_from_page(page);
  
  	reclaim_stat->recent_scanned[file]++;
  	if (rotated)
  		reclaim_stat->recent_rotated[file]++;
  
  	if (!memcg_reclaim_stat)
  		return;
  
  	memcg_reclaim_stat->recent_scanned[file]++;
  	if (rotated)
  		memcg_reclaim_stat->recent_rotated[file]++;
  }

  static void __activate_page(struct page *page, void *arg)

  {
  	struct zone *zone = page_zone(page);

  	if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {

  		int file = page_is_file_cache(page);
  		int lru = page_lru_base_type(page);
  		del_page_from_lru_list(zone, page, lru);

  		SetPageActive(page);
  		lru += LRU_ACTIVE;
  		add_page_to_lru_list(zone, page, lru);
  		__count_vm_event(PGACTIVATE);

  		update_page_reclaim_stat(zone, page, file, 1);

  	}

  }
  
  #ifdef CONFIG_SMP
  static DEFINE_PER_CPU(struct pagevec, activate_page_pvecs);
  
  static void activate_page_drain(int cpu)
  {
  	struct pagevec *pvec = &per_cpu(activate_page_pvecs, cpu);
  
  	if (pagevec_count(pvec))
  		pagevec_lru_move_fn(pvec, __activate_page, NULL);
  }
  
  void activate_page(struct page *page)
  {
  	if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
  		struct pagevec *pvec = &get_cpu_var(activate_page_pvecs);
  
  		page_cache_get(page);
  		if (!pagevec_add(pvec, page))
  			pagevec_lru_move_fn(pvec, __activate_page, NULL);
  		put_cpu_var(activate_page_pvecs);
  	}
  }
  
  #else
  static inline void activate_page_drain(int cpu)
  {
  }
  
  void activate_page(struct page *page)
  {
  	struct zone *zone = page_zone(page);
  
  	spin_lock_irq(&zone->lru_lock);
  	__activate_page(page, NULL);

  	spin_unlock_irq(&zone->lru_lock);
  }

  #endif

  
  /*
   * Mark a page as having seen activity.
   *
   * inactive,unreferenced	->	inactive,referenced
   * inactive,referenced		->	active,unreferenced
   * active,unreferenced		->	active,referenced
   */

  void mark_page_accessed(struct page *page)

  {

  	if (!PageActive(page) && !PageUnevictable(page) &&
  			PageReferenced(page) && PageLRU(page)) {

  		activate_page(page);
  		ClearPageReferenced(page);
  	} else if (!PageReferenced(page)) {
  		SetPageReferenced(page);
  	}
  }
  
  EXPORT_SYMBOL(mark_page_accessed);

  void __lru_cache_add(struct page *page, enum lru_list lru)

  {

  	struct pagevec *pvec = &get_cpu_var(lru_add_pvecs)[lru];

  
  	page_cache_get(page);
  	if (!pagevec_add(pvec, page))

  		____pagevec_lru_add(pvec, lru);

  	put_cpu_var(lru_add_pvecs);
  }

  EXPORT_SYMBOL(__lru_cache_add);

  /**
   * lru_cache_add_lru - add a page to a page list
   * @page: the page to be added to the LRU.
   * @lru: the LRU list to which the page is added.
   */
  void lru_cache_add_lru(struct page *page, enum lru_list lru)

  {

  	if (PageActive(page)) {

  		VM_BUG_ON(PageUnevictable(page));

  		ClearPageActive(page);

  	} else if (PageUnevictable(page)) {
  		VM_BUG_ON(PageActive(page));
  		ClearPageUnevictable(page);

  	}

  	VM_BUG_ON(PageLRU(page) || PageActive(page) || PageUnevictable(page));

  	__lru_cache_add(page, lru);

  }

  /**
   * add_page_to_unevictable_list - add a page to the unevictable list
   * @page:  the page to be added to the unevictable list
   *
   * Add page directly to its zone's unevictable list.  To avoid races with
   * tasks that might be making the page evictable, through eg. munlock,
   * munmap or exit, while it's not on the lru, we want to add the page
   * while it's locked or otherwise "invisible" to other tasks.  This is
   * difficult to do when using the pagevec cache, so bypass that.
   */
  void add_page_to_unevictable_list(struct page *page)
  {
  	struct zone *zone = page_zone(page);
  
  	spin_lock_irq(&zone->lru_lock);
  	SetPageUnevictable(page);
  	SetPageLRU(page);
  	add_page_to_lru_list(zone, page, LRU_UNEVICTABLE);
  	spin_unlock_irq(&zone->lru_lock);
  }

  /*

   * If the page can not be invalidated, it is moved to the
   * inactive list to speed up its reclaim.  It is moved to the
   * head of the list, rather than the tail, to give the flusher
   * threads some time to write it out, as this is much more
   * effective than the single-page writeout from reclaim.

   *
   * If the page isn't page_mapped and dirty/writeback, the page
   * could reclaim asap using PG_reclaim.
   *
   * 1. active, mapped page -> none
   * 2. active, dirty/writeback page -> inactive, head, PG_reclaim
   * 3. inactive, mapped page -> none
   * 4. inactive, dirty/writeback page -> inactive, head, PG_reclaim
   * 5. inactive, clean -> inactive, tail
   * 6. Others -> none
   *
   * In 4, why it moves inactive's head, the VM expects the page would
   * be write it out by flusher threads as this is much more effective
   * than the single-page writeout from reclaim.

   */

  static void lru_deactivate_fn(struct page *page, void *arg)

  {
  	int lru, file;

  	bool active;

  	struct zone *zone = page_zone(page);

  	if (!PageLRU(page))

  		return;

  	if (PageUnevictable(page))
  		return;

  	/* Some processes are using the page */
  	if (page_mapped(page))
  		return;

  	active = PageActive(page);

  	file = page_is_file_cache(page);
  	lru = page_lru_base_type(page);

  	del_page_from_lru_list(zone, page, lru + active);

  	ClearPageActive(page);
  	ClearPageReferenced(page);
  	add_page_to_lru_list(zone, page, lru);

  	if (PageWriteback(page) || PageDirty(page)) {
  		/*
  		 * PG_reclaim could be raced with end_page_writeback
  		 * It can make readahead confusing.  But race window
  		 * is _really_ small and  it's non-critical problem.
  		 */
  		SetPageReclaim(page);
  	} else {
  		/*
  		 * The page's writeback ends up during pagevec
  		 * We moves tha page into tail of inactive.
  		 */
  		list_move_tail(&page->lru, &zone->lru[lru].list);
  		mem_cgroup_rotate_reclaimable_page(page);
  		__count_vm_event(PGROTATED);
  	}
  
  	if (active)
  		__count_vm_event(PGDEACTIVATE);

  	update_page_reclaim_stat(zone, page, file, 0);
  }

  /*

   * Drain pages out of the cpu's pagevecs.
   * Either "cpu" is the current CPU, and preemption has already been
   * disabled; or "cpu" is being hot-unplugged, and is already dead.
   */
  static void drain_cpu_pagevecs(int cpu)

  {

  	struct pagevec *pvecs = per_cpu(lru_add_pvecs, cpu);

  	struct pagevec *pvec;

  	int lru;

  	for_each_lru(lru) {
  		pvec = &pvecs[lru - LRU_BASE];
  		if (pagevec_count(pvec))
  			____pagevec_lru_add(pvec, lru);
  	}

  
  	pvec = &per_cpu(lru_rotate_pvecs, cpu);
  	if (pagevec_count(pvec)) {
  		unsigned long flags;
  
  		/* No harm done if a racing interrupt already did this */
  		local_irq_save(flags);
  		pagevec_move_tail(pvec);
  		local_irq_restore(flags);
  	}

  
  	pvec = &per_cpu(lru_deactivate_pvecs, cpu);
  	if (pagevec_count(pvec))

  		pagevec_lru_move_fn(pvec, lru_deactivate_fn, NULL);

  
  	activate_page_drain(cpu);

  }
  
  /**
   * deactivate_page - forcefully deactivate a page
   * @page: page to deactivate
   *
   * This function hints the VM that @page is a good reclaim candidate,
   * for example if its invalidation fails due to the page being dirty
   * or under writeback.
   */
  void deactivate_page(struct page *page)
  {

  	/*
  	 * In a workload with many unevictable page such as mprotect, unevictable
  	 * page deactivation for accelerating reclaim is pointless.
  	 */
  	if (PageUnevictable(page))
  		return;

  	if (likely(get_page_unless_zero(page))) {
  		struct pagevec *pvec = &get_cpu_var(lru_deactivate_pvecs);
  
  		if (!pagevec_add(pvec, page))

  			pagevec_lru_move_fn(pvec, lru_deactivate_fn, NULL);

  		put_cpu_var(lru_deactivate_pvecs);
  	}

  }
  
  void lru_add_drain(void)
  {

  	drain_cpu_pagevecs(get_cpu());

  	put_cpu();

  }

  static void lru_add_drain_per_cpu(struct work_struct *dummy)

  {
  	lru_add_drain();
  }
  
  /*
   * Returns 0 for success
   */
  int lru_add_drain_all(void)
  {

  	return schedule_on_each_cpu(lru_add_drain_per_cpu);

  }

  /*

   * Batched page_cache_release().  Decrement the reference count on all the
   * passed pages.  If it fell to zero then remove the page from the LRU and
   * free it.
   *
   * Avoid taking zone->lru_lock if possible, but if it is taken, retain it
   * for the remainder of the operation.
   *

   * The locking in this function is against shrink_inactive_list(): we recheck
   * the page count inside the lock to see whether shrink_inactive_list()
   * grabbed the page via the LRU.  If it did, give up: shrink_inactive_list()
   * will free it.

   */
  void release_pages(struct page **pages, int nr, int cold)
  {
  	int i;
  	struct pagevec pages_to_free;
  	struct zone *zone = NULL;

  	unsigned long uninitialized_var(flags);

  
  	pagevec_init(&pages_to_free, cold);
  	for (i = 0; i < nr; i++) {
  		struct page *page = pages[i];

  		if (unlikely(PageCompound(page))) {
  			if (zone) {

  				spin_unlock_irqrestore(&zone->lru_lock, flags);

  				zone = NULL;
  			}
  			put_compound_page(page);
  			continue;
  		}

  		if (!put_page_testzero(page))

  			continue;

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

  			if (pagezone != zone) {
  				if (zone)

  					spin_unlock_irqrestore(&zone->lru_lock,
  									flags);

  				zone = pagezone;

  				spin_lock_irqsave(&zone->lru_lock, flags);

  			}

  			VM_BUG_ON(!PageLRU(page));

  			__ClearPageLRU(page);

  			del_page_from_lru(zone, page);

  		}
  
  		if (!pagevec_add(&pages_to_free, page)) {
  			if (zone) {

  				spin_unlock_irqrestore(&zone->lru_lock, flags);

  				zone = NULL;

  			}

  			__pagevec_free(&pages_to_free);
  			pagevec_reinit(&pages_to_free);
    		}

  	}
  	if (zone)

  		spin_unlock_irqrestore(&zone->lru_lock, flags);

  
  	pagevec_free(&pages_to_free);
  }

  EXPORT_SYMBOL(release_pages);

  
  /*
   * The pages which we're about to release may be in the deferred lru-addition
   * queues.  That would prevent them from really being freed right now.  That's
   * OK from a correctness point of view but is inefficient - those pages may be
   * cache-warm and we want to give them back to the page allocator ASAP.
   *
   * So __pagevec_release() will drain those queues here.  __pagevec_lru_add()
   * and __pagevec_lru_add_active() call release_pages() directly to avoid
   * mutual recursion.
   */
  void __pagevec_release(struct pagevec *pvec)
  {
  	lru_add_drain();
  	release_pages(pvec->pages, pagevec_count(pvec), pvec->cold);
  	pagevec_reinit(pvec);
  }

  EXPORT_SYMBOL(__pagevec_release);

  /* used by __split_huge_page_refcount() */
  void lru_add_page_tail(struct zone* zone,
  		       struct page *page, struct page *page_tail)
  {
  	int active;
  	enum lru_list lru;
  	const int file = 0;
  	struct list_head *head;
  
  	VM_BUG_ON(!PageHead(page));
  	VM_BUG_ON(PageCompound(page_tail));
  	VM_BUG_ON(PageLRU(page_tail));
  	VM_BUG_ON(!spin_is_locked(&zone->lru_lock));
  
  	SetPageLRU(page_tail);
  
  	if (page_evictable(page_tail, NULL)) {
  		if (PageActive(page)) {
  			SetPageActive(page_tail);
  			active = 1;
  			lru = LRU_ACTIVE_ANON;
  		} else {
  			active = 0;
  			lru = LRU_INACTIVE_ANON;
  		}
  		update_page_reclaim_stat(zone, page_tail, file, active);
  		if (likely(PageLRU(page)))
  			head = page->lru.prev;
  		else
  			head = &zone->lru[lru].list;
  		__add_page_to_lru_list(zone, page_tail, lru, head);
  	} else {
  		SetPageUnevictable(page_tail);
  		add_page_to_lru_list(zone, page_tail, LRU_UNEVICTABLE);
  	}
  }

  static void ____pagevec_lru_add_fn(struct page *page, void *arg)
  {
  	enum lru_list lru = (enum lru_list)arg;
  	struct zone *zone = page_zone(page);
  	int file = is_file_lru(lru);
  	int active = is_active_lru(lru);
  
  	VM_BUG_ON(PageActive(page));
  	VM_BUG_ON(PageUnevictable(page));
  	VM_BUG_ON(PageLRU(page));
  
  	SetPageLRU(page);
  	if (active)
  		SetPageActive(page);
  	update_page_reclaim_stat(zone, page, file, active);
  	add_page_to_lru_list(zone, page, lru);
  }

  /*

   * Add the passed pages to the LRU, then drop the caller's refcount
   * on them.  Reinitialises the caller's pagevec.
   */

  void ____pagevec_lru_add(struct pagevec *pvec, enum lru_list lru)

  {

  	VM_BUG_ON(is_unevictable_lru(lru));

  	pagevec_lru_move_fn(pvec, ____pagevec_lru_add_fn, (void *)lru);

  }

  EXPORT_SYMBOL(____pagevec_lru_add);

  
  /*
   * Try to drop buffers from the pages in a pagevec
   */
  void pagevec_strip(struct pagevec *pvec)
  {
  	int i;
  
  	for (i = 0; i < pagevec_count(pvec); i++) {
  		struct page *page = pvec->pages[i];

  		if (page_has_private(page) && trylock_page(page)) {
  			if (page_has_private(page))

  				try_to_release_page(page, 0);

  			unlock_page(page);
  		}
  	}
  }
  
  /**
   * pagevec_lookup - gang pagecache lookup
   * @pvec:	Where the resulting pages are placed
   * @mapping:	The address_space to search
   * @start:	The starting page index
   * @nr_pages:	The maximum number of pages
   *
   * pagevec_lookup() will search for and return a group of up to @nr_pages pages
   * in the mapping.  The pages are placed in @pvec.  pagevec_lookup() takes a
   * reference against the pages in @pvec.
   *
   * The search returns a group of mapping-contiguous pages with ascending
   * indexes.  There may be holes in the indices due to not-present pages.
   *
   * pagevec_lookup() returns the number of pages which were found.
   */
  unsigned pagevec_lookup(struct pagevec *pvec, struct address_space *mapping,
  		pgoff_t start, unsigned nr_pages)
  {
  	pvec->nr = find_get_pages(mapping, start, nr_pages, pvec->pages);
  	return pagevec_count(pvec);
  }

  EXPORT_SYMBOL(pagevec_lookup);

  unsigned pagevec_lookup_tag(struct pagevec *pvec, struct address_space *mapping,
  		pgoff_t *index, int tag, unsigned nr_pages)
  {
  	pvec->nr = find_get_pages_tag(mapping, index, tag,
  					nr_pages, pvec->pages);
  	return pagevec_count(pvec);
  }

  EXPORT_SYMBOL(pagevec_lookup_tag);

  /*
   * Perform any setup for the swap system
   */
  void __init swap_setup(void)
  {

  	unsigned long megs = totalram_pages >> (20 - PAGE_SHIFT);

  #ifdef CONFIG_SWAP
  	bdi_init(swapper_space.backing_dev_info);
  #endif

  	/* Use a smaller cluster for small-memory machines */
  	if (megs < 16)
  		page_cluster = 2;
  	else
  		page_cluster = 3;
  	/*
  	 * Right now other parts of the system means that we
  	 * _really_ don't want to cluster much more
  	 */

  }