swap.c

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/*
 *  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/export.h>
#include <linux/mm_inline.h>
#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)) {
        struct zone *zone = page_zone(page);
        struct lruvec *lruvec;
        unsigned long flags;

        spin_lock_irqsave(&zone->lru_lock, flags);
        lruvec = mem_cgroup_page_lruvec(page, zone);
        VM_BUG_ON(!PageLRU(page));
        __ClearPageLRU(page);
        del_page_from_lru_list(page, lruvec, page_off_lru(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 = compound_trans_head(page);

        if (likely(page != page_head &&
               get_page_unless_zero(page_head))) {
            unsigned long flags;

            /*
             * THP can not break up slab pages so avoid taking
             * compound_lock().  Slab performs non-atomic bit ops
             * on page->flags for better performance.  In particular
             * slab_unlock() in slub used to be a hot path.  It is
             * still hot on arches that do not support
             * this_cpu_cmpxchg_double().
             */
            if (PageSlab(page_head)) {
                if (PageTail(page)) {
                    if (put_page_testzero(page_head))
                        VM_BUG_ON(1);

                    atomic_dec(&page->_mapcount);
                    goto skip_lock_tail;
                } else
                    goto skip_lock;
            }
            /*
             * page_head wasn't a dangling pointer but it
             * may not be a head page anymore by the time
             * we obtain the lock. That is ok as long as it
             * can't be freed from under us.
             */
            flags = compound_lock_irqsave(page_head);
            if (unlikely(!PageTail(page))) {
                /* __split_huge_page_refcount run before us */
                compound_unlock_irqrestore(page_head, flags);
skip_lock:
                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(page_mapcount(page) <= 0);
            atomic_dec(&page->_mapcount);
            VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
            VM_BUG_ON(atomic_read(&page->_count) != 0);
            compound_unlock_irqrestore(page_head, flags);

skip_lock_tail:
            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);

/*
 * This function is exported but must not be called by anything other
 * than get_page(). It implements the slow path of get_page().
 */
bool __get_page_tail(struct page *page)
{
    /*
     * This takes care of get_page() if run on a tail page
     * returned by one of the get_user_pages/follow_page variants.
     * get_user_pages/follow_page itself doesn't need the compound
     * lock because it runs __get_page_tail_foll() under the
     * proper PT lock that already serializes against
     * split_huge_page().
     */
    unsigned long flags;
    bool got = false;
    struct page *page_head = compound_trans_head(page);

    if (likely(page != page_head && get_page_unless_zero(page_head))) {

        /* Ref to put_compound_page() comment. */
        if (PageSlab(page_head)) {
            if (likely(PageTail(page))) {
                __get_page_tail_foll(page, false);
                return true;
            } else {
                put_page(page_head);
                return false;
            }
        }

        /*
         * page_head wasn't a dangling pointer but it
         * may not be a head page anymore by the time
         * we obtain the lock. That is ok as long as it
         * can't be freed from under us.
         */
        flags = compound_lock_irqsave(page_head);
        /* here __split_huge_page_refcount won't run anymore */
        if (likely(PageTail(page))) {
            __get_page_tail_foll(page, false);
            got = true;
        }
        compound_unlock_irqrestore(page_head, flags);
        if (unlikely(!got))
            put_page(page_head);
    }
    return got;
}
EXPORT_SYMBOL(__get_page_tail);

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);

/*
 * get_kernel_pages() - pin kernel pages in memory
 * @kiov:   An array of struct kvec structures
 * @nr_segs:    number of segments to pin
 * @write:  pinning for read/write, currently ignored
 * @pages:  array that receives pointers to the pages pinned.
 *      Should be at least nr_segs long.
 *
 * Returns number of pages pinned. This may be fewer than the number
 * requested. If nr_pages is 0 or negative, returns 0. If no pages
 * were pinned, returns -errno. Each page returned must be released
 * with a put_page() call when it is finished with.
 */
int get_kernel_pages(const struct kvec *kiov, int nr_segs, int write,
        struct page **pages)
{
    int seg;

    for (seg = 0; seg < nr_segs; seg++) {
        if (WARN_ON(kiov[seg].iov_len != PAGE_SIZE))
            return seg;

        pages[seg] = kmap_to_page(kiov[seg].iov_base);
        page_cache_get(pages[seg]);
    }

    return seg;
}
EXPORT_SYMBOL_GPL(get_kernel_pages);

/*
 * get_kernel_page() - pin a kernel page in memory
 * @start:  starting kernel address
 * @write:  pinning for read/write, currently ignored
 * @pages:  array that receives pointer to the page pinned.
 *      Must be at least nr_segs long.
 *
 * Returns 1 if page is pinned. If the page was not pinned, returns
 * -errno. The page returned must be released with a put_page() call
 * when it is finished with.
 */
int get_kernel_page(unsigned long start, int write, struct page **pages)
{
    const struct kvec kiov = {
        .iov_base = (void *)start,
        .iov_len = PAGE_SIZE
    };

    return get_kernel_pages(&kiov, 1, write, pages);
}
EXPORT_SYMBOL_GPL(get_kernel_page);

static void pagevec_lru_move_fn(struct pagevec *pvec,
    void (*move_fn)(struct page *page, struct lruvec *lruvec, void *arg),
    void *arg)
{
    int i;
    struct zone *zone = NULL;
    struct lruvec *lruvec;
    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);
        }

        lruvec = mem_cgroup_page_lruvec(page, zone);
        (*move_fn)(page, lruvec, 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, struct lruvec *lruvec,
                 void *arg)
{
    int *pgmoved = arg;

    if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
        enum lru_list lru = page_lru_base_type(page);
        list_move_tail(&page->lru, &lruvec->lists[lru]);
        (*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 lruvec *lruvec,
                     int file, int rotated)
{
    struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;

    reclaim_stat->recent_scanned[file]++;
    if (rotated)
        reclaim_stat->recent_rotated[file]++;
}

static void __activate_page(struct page *page, struct lruvec *lruvec,
                void *arg)
{
    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(page, lruvec, lru);
        SetPageActive(page);
        lru += LRU_ACTIVE;
        add_page_to_lru_list(page, lruvec, lru);

        __count_vm_event(PGACTIVATE);
        update_page_reclaim_stat(lruvec, 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, mem_cgroup_page_lruvec(page, zone), 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);

/*
 * Order of operations is important: flush the pagevec when it's already
 * full, not when adding the last page, to make sure that last page is
 * not added to the LRU directly when passed to this function. Because
 * mark_page_accessed() (called after this when writing) only activates
 * pages that are on the LRU, linear writes in subpage chunks would see
 * every PAGEVEC_SIZE page activated, which is unexpected.
 */
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_space(pvec))
        __pagevec_lru_add(pvec, lru);
    pagevec_add(pvec, page);
    put_cpu_var(lru_add_pvecs);
}
EXPORT_SYMBOL(__lru_cache_add);

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);
}

void add_page_to_unevictable_list(struct page *page)
{
    struct zone *zone = page_zone(page);
    struct lruvec *lruvec;

    spin_lock_irq(&zone->lru_lock);
    lruvec = mem_cgroup_page_lruvec(page, zone);
    SetPageUnevictable(page);
    SetPageLRU(page);
    add_page_to_lru_list(page, lruvec, 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, struct lruvec *lruvec,
                  void *arg)
{
    int lru, file;
    bool active;

    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(page, lruvec, lru + active);
    ClearPageActive(page);
    ClearPageReferenced(page);
    add_page_to_lru_list(page, lruvec, 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, &lruvec->lists[lru]);
        __count_vm_event(PGROTATED);
    }

    if (active)
        __count_vm_event(PGDEACTIVATE);
    update_page_reclaim_stat(lruvec, 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.
 */
void lru_add_drain_cpu(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);
}

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)
{
    lru_add_drain_cpu(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;
    LIST_HEAD(pages_to_free);
    struct zone *zone = NULL;
    struct lruvec *lruvec;
    unsigned long uninitialized_var(flags);

    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);
            }

            lruvec = mem_cgroup_page_lruvec(page, zone);
            VM_BUG_ON(!PageLRU(page));
            __ClearPageLRU(page);
            del_page_from_lru_list(page, lruvec, page_off_lru(page));
        }

        list_add(&page->lru, &pages_to_free);
    }
    if (zone)
        spin_unlock_irqrestore(&zone->lru_lock, flags);

    free_hot_cold_page_list(&pages_to_free, cold);
}
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);

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
/* used by __split_huge_page_refcount() */
void lru_add_page_tail(struct page *page, struct page *page_tail,
               struct lruvec *lruvec)
{
    int uninitialized_var(active);
    enum lru_list lru;
    const int file = 0;

    VM_BUG_ON(!PageHead(page));
    VM_BUG_ON(PageCompound(page_tail));
    VM_BUG_ON(PageLRU(page_tail));
    VM_BUG_ON(NR_CPUS != 1 &&
          !spin_is_locked(&lruvec_zone(lruvec)->lru_lock));

    SetPageLRU(page_tail);

    if (page_evictable(page_tail)) {
        if (PageActive(page)) {
            SetPageActive(page_tail);
            active = 1;
            lru = LRU_ACTIVE_ANON;
        } else {
            active = 0;
            lru = LRU_INACTIVE_ANON;
        }
    } else {
        SetPageUnevictable(page_tail);
        lru = LRU_UNEVICTABLE;
    }

    if (likely(PageLRU(page)))
        list_add_tail(&page_tail->lru, &page->lru);
    else {
        struct list_head *list_head;
        /*
         * Head page has not yet been counted, as an hpage,
         * so we must account for each subpage individually.
         *
         * Use the standard add function to put page_tail on the list,
         * but then correct its position so they all end up in order.
         */
        add_page_to_lru_list(page_tail, lruvec, lru);
        list_head = page_tail->lru.prev;
        list_move_tail(&page_tail->lru, list_head);
    }

    if (!PageUnevictable(page))
        update_page_reclaim_stat(lruvec, file, active);
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
                 void *arg)
{
    enum lru_list lru = (enum lru_list)arg;
    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);
    add_page_to_lru_list(page, lruvec, lru);
    update_page_reclaim_stat(lruvec, file, active);
}

/*
 * 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);

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
     */
}