[PATCH] swap: freeing update swap_list.next

[linux-2.6] / mm / swapfile.c

/*
 *  linux/mm/swapfile.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *  Swap reorganised 29.12.95, Stephen Tweedie
 */

#include <linux/config.h>
#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/mman.h>
#include <linux/slab.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/vmalloc.h>
#include <linux/pagemap.h>
#include <linux/namei.h>
#include <linux/shm.h>
#include <linux/blkdev.h>
#include <linux/writeback.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/rmap.h>
#include <linux/security.h>
#include <linux/backing-dev.h>
#include <linux/syscalls.h>

#include <asm/pgtable.h>
#include <asm/tlbflush.h>
#include <linux/swapops.h>

DEFINE_SPINLOCK(swaplock);
unsigned int nr_swapfiles;
long total_swap_pages;
static int swap_overflow;

EXPORT_SYMBOL(total_swap_pages);

static const char Bad_file[] = "Bad swap file entry ";
static const char Unused_file[] = "Unused swap file entry ";
static const char Bad_offset[] = "Bad swap offset entry ";
static const char Unused_offset[] = "Unused swap offset entry ";

struct swap_list_t swap_list = {-1, -1};

struct swap_info_struct swap_info[MAX_SWAPFILES];

static DECLARE_MUTEX(swapon_sem);

/*
 * We need this because the bdev->unplug_fn can sleep and we cannot
 * hold swap_list_lock while calling the unplug_fn. And swap_list_lock
 * cannot be turned into a semaphore.
 */
static DECLARE_RWSEM(swap_unplug_sem);

#define SWAPFILE_CLUSTER 256

void swap_unplug_io_fn(struct backing_dev_info *unused_bdi, struct page *page)
{
        swp_entry_t entry;

        down_read(&swap_unplug_sem);
        entry.val = page->private;
        if (PageSwapCache(page)) {
                struct block_device *bdev = swap_info[swp_type(entry)].bdev;
                struct backing_dev_info *bdi;

                /*
                 * If the page is removed from swapcache from under us (with a
                 * racy try_to_unuse/swapoff) we need an additional reference
                 * count to avoid reading garbage from page->private above. If
                 * the WARN_ON triggers during a swapoff it maybe the race
                 * condition and it's harmless. However if it triggers without
                 * swapoff it signals a problem.
                 */
                WARN_ON(page_count(page) <= 1);

                bdi = bdev->bd_inode->i_mapping->backing_dev_info;
                blk_run_backing_dev(bdi, page);
        }
        up_read(&swap_unplug_sem);
}

static inline unsigned long scan_swap_map(struct swap_info_struct *si)
{
        unsigned long offset;
        /* 
         * We try to cluster swap pages by allocating them
         * sequentially in swap.  Once we've allocated
         * SWAPFILE_CLUSTER pages this way, however, we resort to
         * first-free allocation, starting a new cluster.  This
         * prevents us from scattering swap pages all over the entire
         * swap partition, so that we reduce overall disk seek times
         * between swap pages.  -- sct */
        if (si->cluster_nr) {
                while (si->cluster_next <= si->highest_bit) {
                        offset = si->cluster_next++;
                        if (si->swap_map[offset])
                                continue;
                        si->cluster_nr--;
                        goto got_page;
                }
        }
        si->cluster_nr = SWAPFILE_CLUSTER;

        /* try to find an empty (even not aligned) cluster. */
        offset = si->lowest_bit;
 check_next_cluster:
        if (offset+SWAPFILE_CLUSTER-1 <= si->highest_bit)
        {
                unsigned long nr;
                for (nr = offset; nr < offset+SWAPFILE_CLUSTER; nr++)
                        if (si->swap_map[nr])
                        {
                                offset = nr+1;
                                goto check_next_cluster;
                        }
                /* We found a completly empty cluster, so start
                 * using it.
                 */
                goto got_page;
        }
        /* No luck, so now go finegrined as usual. -Andrea */
        for (offset = si->lowest_bit; offset <= si->highest_bit ; offset++) {
                if (si->swap_map[offset])
                        continue;
                si->lowest_bit = offset+1;
        got_page:
                if (offset == si->lowest_bit)
                        si->lowest_bit++;
                if (offset == si->highest_bit)
                        si->highest_bit--;
                if (si->lowest_bit > si->highest_bit) {
                        si->lowest_bit = si->max;
                        si->highest_bit = 0;
                }
                si->swap_map[offset] = 1;
                si->inuse_pages++;
                nr_swap_pages--;
                si->cluster_next = offset+1;
                return offset;
        }
        si->lowest_bit = si->max;
        si->highest_bit = 0;
        return 0;
}

swp_entry_t get_swap_page(void)
{
        struct swap_info_struct * p;
        unsigned long offset;
        swp_entry_t entry;
        int type, wrapped = 0;

        entry.val = 0;  /* Out of memory */
        swap_list_lock();
        type = swap_list.next;
        if (type < 0)
                goto out;
        if (nr_swap_pages <= 0)
                goto out;

        while (1) {
                p = &swap_info[type];
                if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
                        swap_device_lock(p);
                        offset = scan_swap_map(p);
                        swap_device_unlock(p);
                        if (offset) {
                                entry = swp_entry(type,offset);
                                type = swap_info[type].next;
                                if (type < 0 ||
                                        p->prio != swap_info[type].prio) {
                                                swap_list.next = swap_list.head;
                                } else {
                                        swap_list.next = type;
                                }
                                goto out;
                        }
                }
                type = p->next;
                if (!wrapped) {
                        if (type < 0 || p->prio != swap_info[type].prio) {
                                type = swap_list.head;
                                wrapped = 1;
                        }
                } else
                        if (type < 0)
                                goto out;       /* out of swap space */
        }
out:
        swap_list_unlock();
        return entry;
}

static struct swap_info_struct * swap_info_get(swp_entry_t entry)
{
        struct swap_info_struct * p;
        unsigned long offset, type;

        if (!entry.val)
                goto out;
        type = swp_type(entry);
        if (type >= nr_swapfiles)
                goto bad_nofile;
        p = & swap_info[type];
        if (!(p->flags & SWP_USED))
                goto bad_device;
        offset = swp_offset(entry);
        if (offset >= p->max)
                goto bad_offset;
        if (!p->swap_map[offset])
                goto bad_free;
        swap_list_lock();
        swap_device_lock(p);
        return p;

bad_free:
        printk(KERN_ERR "swap_free: %s%08lx\n", Unused_offset, entry.val);
        goto out;
bad_offset:
        printk(KERN_ERR "swap_free: %s%08lx\n", Bad_offset, entry.val);
        goto out;
bad_device:
        printk(KERN_ERR "swap_free: %s%08lx\n", Unused_file, entry.val);
        goto out;
bad_nofile:
        printk(KERN_ERR "swap_free: %s%08lx\n", Bad_file, entry.val);
out:
        return NULL;
}       

static void swap_info_put(struct swap_info_struct * p)
{
        swap_device_unlock(p);
        swap_list_unlock();
}

static int swap_entry_free(struct swap_info_struct *p, unsigned long offset)
{
        int count = p->swap_map[offset];

        if (count < SWAP_MAP_MAX) {
                count--;
                p->swap_map[offset] = count;
                if (!count) {
                        if (offset < p->lowest_bit)
                                p->lowest_bit = offset;
                        if (offset > p->highest_bit)
                                p->highest_bit = offset;
                        if (p->prio > swap_info[swap_list.next].prio)
                                swap_list.next = p - swap_info;
                        nr_swap_pages++;
                        p->inuse_pages--;
                }
        }
        return count;
}

/*
 * Caller has made sure that the swapdevice corresponding to entry
 * is still around or has not been recycled.
 */
void swap_free(swp_entry_t entry)
{
        struct swap_info_struct * p;

        p = swap_info_get(entry);
        if (p) {
                swap_entry_free(p, swp_offset(entry));
                swap_info_put(p);
        }
}

/*
 * How many references to page are currently swapped out?
 */
static inline int page_swapcount(struct page *page)
{
        int count = 0;
        struct swap_info_struct *p;
        swp_entry_t entry;

        entry.val = page->private;
        p = swap_info_get(entry);
        if (p) {
                /* Subtract the 1 for the swap cache itself */
                count = p->swap_map[swp_offset(entry)] - 1;
                swap_info_put(p);
        }
        return count;
}

/*
 * We can use this swap cache entry directly
 * if there are no other references to it.
 */
int can_share_swap_page(struct page *page)
{
        int count;

        BUG_ON(!PageLocked(page));
        count = page_mapcount(page);
        if (count <= 1 && PageSwapCache(page))
                count += page_swapcount(page);
        return count == 1;
}

/*
 * Work out if there are any other processes sharing this
 * swap cache page. Free it if you can. Return success.
 */
int remove_exclusive_swap_page(struct page *page)
{
        int retval;
        struct swap_info_struct * p;
        swp_entry_t entry;

        BUG_ON(PagePrivate(page));
        BUG_ON(!PageLocked(page));

        if (!PageSwapCache(page))
                return 0;
        if (PageWriteback(page))
                return 0;
        if (page_count(page) != 2) /* 2: us + cache */
                return 0;

        entry.val = page->private;
        p = swap_info_get(entry);
        if (!p)
                return 0;

        /* Is the only swap cache user the cache itself? */
        retval = 0;
        if (p->swap_map[swp_offset(entry)] == 1) {
                /* Recheck the page count with the swapcache lock held.. */
                write_lock_irq(&swapper_space.tree_lock);
                if ((page_count(page) == 2) && !PageWriteback(page)) {
                        __delete_from_swap_cache(page);
                        SetPageDirty(page);
                        retval = 1;
                }
                write_unlock_irq(&swapper_space.tree_lock);
        }
        swap_info_put(p);

        if (retval) {
                swap_free(entry);
                page_cache_release(page);
        }

        return retval;
}

/*
 * Free the swap entry like above, but also try to
 * free the page cache entry if it is the last user.
 */
void free_swap_and_cache(swp_entry_t entry)
{
        struct swap_info_struct * p;
        struct page *page = NULL;

        p = swap_info_get(entry);
        if (p) {
                if (swap_entry_free(p, swp_offset(entry)) == 1)
                        page = find_trylock_page(&swapper_space, entry.val);
                swap_info_put(p);
        }
        if (page) {
                int one_user;

                BUG_ON(PagePrivate(page));
                page_cache_get(page);
                one_user = (page_count(page) == 2);
                /* Only cache user (+us), or swap space full? Free it! */
                if (!PageWriteback(page) && (one_user || vm_swap_full())) {
                        delete_from_swap_cache(page);
                        SetPageDirty(page);
                }
                unlock_page(page);
                page_cache_release(page);
        }
}

/*
 * Always set the resulting pte to be nowrite (the same as COW pages
 * after one process has exited).  We don't know just how many PTEs will
 * share this swap entry, so be cautious and let do_wp_page work out
 * what to do if a write is requested later.
 *
 * vma->vm_mm->page_table_lock is held.
 */
static void unuse_pte(struct vm_area_struct *vma, pte_t *pte,
                unsigned long addr, swp_entry_t entry, struct page *page)
{
        inc_mm_counter(vma->vm_mm, rss);
        get_page(page);
        set_pte_at(vma->vm_mm, addr, pte,
                   pte_mkold(mk_pte(page, vma->vm_page_prot)));
        page_add_anon_rmap(page, vma, addr);
        swap_free(entry);
        /*
         * Move the page to the active list so it is not
         * immediately swapped out again after swapon.
         */
        activate_page(page);
}

static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
                                unsigned long addr, unsigned long end,
                                swp_entry_t entry, struct page *page)
{
        pte_t *pte;
        pte_t swp_pte = swp_entry_to_pte(entry);

        pte = pte_offset_map(pmd, addr);
        do {
                /*
                 * swapoff spends a _lot_ of time in this loop!
                 * Test inline before going to call unuse_pte.
                 */
                if (unlikely(pte_same(*pte, swp_pte))) {
                        unuse_pte(vma, pte, addr, entry, page);
                        pte_unmap(pte);
                        return 1;
                }
        } while (pte++, addr += PAGE_SIZE, addr != end);
        pte_unmap(pte - 1);
        return 0;
}

static inline int unuse_pmd_range(struct vm_area_struct *vma, pud_t *pud,
                                unsigned long addr, unsigned long end,
                                swp_entry_t entry, struct page *page)
{
        pmd_t *pmd;
        unsigned long next;

        pmd = pmd_offset(pud, addr);
        do {
                next = pmd_addr_end(addr, end);
                if (pmd_none_or_clear_bad(pmd))
                        continue;
                if (unuse_pte_range(vma, pmd, addr, next, entry, page))
                        return 1;
        } while (pmd++, addr = next, addr != end);
        return 0;
}

static inline int unuse_pud_range(struct vm_area_struct *vma, pgd_t *pgd,
                                unsigned long addr, unsigned long end,
                                swp_entry_t entry, struct page *page)
{
        pud_t *pud;
        unsigned long next;

        pud = pud_offset(pgd, addr);
        do {
                next = pud_addr_end(addr, end);
                if (pud_none_or_clear_bad(pud))
                        continue;
                if (unuse_pmd_range(vma, pud, addr, next, entry, page))
                        return 1;
        } while (pud++, addr = next, addr != end);
        return 0;
}

static int unuse_vma(struct vm_area_struct *vma,
                                swp_entry_t entry, struct page *page)
{
        pgd_t *pgd;
        unsigned long addr, end, next;

        if (page->mapping) {
                addr = page_address_in_vma(page, vma);
                if (addr == -EFAULT)
                        return 0;
                else
                        end = addr + PAGE_SIZE;
        } else {
                addr = vma->vm_start;
                end = vma->vm_end;
        }

        pgd = pgd_offset(vma->vm_mm, addr);
        do {
                next = pgd_addr_end(addr, end);
                if (pgd_none_or_clear_bad(pgd))
                        continue;
                if (unuse_pud_range(vma, pgd, addr, next, entry, page))
                        return 1;
        } while (pgd++, addr = next, addr != end);
        return 0;
}

static int unuse_mm(struct mm_struct *mm,
                                swp_entry_t entry, struct page *page)
{
        struct vm_area_struct *vma;

        if (!down_read_trylock(&mm->mmap_sem)) {
                /*
                 * Activate page so shrink_cache is unlikely to unmap its
                 * ptes while lock is dropped, so swapoff can make progress.
                 */
                activate_page(page);
                unlock_page(page);
                down_read(&mm->mmap_sem);
                lock_page(page);
        }
        spin_lock(&mm->page_table_lock);
        for (vma = mm->mmap; vma; vma = vma->vm_next) {
                if (vma->anon_vma && unuse_vma(vma, entry, page))
                        break;
        }
        spin_unlock(&mm->page_table_lock);
        up_read(&mm->mmap_sem);
        /*
         * Currently unuse_mm cannot fail, but leave error handling
         * at call sites for now, since we change it from time to time.
         */
        return 0;
}

/*
 * Scan swap_map from current position to next entry still in use.
 * Recycle to start on reaching the end, returning 0 when empty.
 */
static unsigned int find_next_to_unuse(struct swap_info_struct *si,
                                        unsigned int prev)
{
        unsigned int max = si->max;
        unsigned int i = prev;
        int count;

        /*
         * No need for swap_device_lock(si) here: we're just looking
         * for whether an entry is in use, not modifying it; false
         * hits are okay, and sys_swapoff() has already prevented new
         * allocations from this area (while holding swap_list_lock()).
         */
        for (;;) {
                if (++i >= max) {
                        if (!prev) {
                                i = 0;
                                break;
                        }
                        /*
                         * No entries in use at top of swap_map,
                         * loop back to start and recheck there.
                         */
                        max = prev + 1;
                        prev = 0;
                        i = 1;
                }
                count = si->swap_map[i];
                if (count && count != SWAP_MAP_BAD)
                        break;
        }
        return i;
}

/*
 * We completely avoid races by reading each swap page in advance,
 * and then search for the process using it.  All the necessary
 * page table adjustments can then be made atomically.
 */
static int try_to_unuse(unsigned int type)
{
        struct swap_info_struct * si = &swap_info[type];
        struct mm_struct *start_mm;
        unsigned short *swap_map;
        unsigned short swcount;
        struct page *page;
        swp_entry_t entry;
        unsigned int i = 0;
        int retval = 0;
        int reset_overflow = 0;
        int shmem;

        /*
         * When searching mms for an entry, a good strategy is to
         * start at the first mm we freed the previous entry from
         * (though actually we don't notice whether we or coincidence
         * freed the entry).  Initialize this start_mm with a hold.
         *
         * A simpler strategy would be to start at the last mm we
         * freed the previous entry from; but that would take less
         * advantage of mmlist ordering, which clusters forked mms
         * together, child after parent.  If we race with dup_mmap(), we
         * prefer to resolve parent before child, lest we miss entries
         * duplicated after we scanned child: using last mm would invert
         * that.  Though it's only a serious concern when an overflowed
         * swap count is reset from SWAP_MAP_MAX, preventing a rescan.
         */
        start_mm = &init_mm;
        atomic_inc(&init_mm.mm_users);

        /*
         * Keep on scanning until all entries have gone.  Usually,
         * one pass through swap_map is enough, but not necessarily:
         * there are races when an instance of an entry might be missed.
         */
        while ((i = find_next_to_unuse(si, i)) != 0) {
                if (signal_pending(current)) {
                        retval = -EINTR;
                        break;
                }

                /* 
                 * Get a page for the entry, using the existing swap
                 * cache page if there is one.  Otherwise, get a clean
                 * page and read the swap into it. 
                 */
                swap_map = &si->swap_map[i];
                entry = swp_entry(type, i);
                page = read_swap_cache_async(entry, NULL, 0);
                if (!page) {
                        /*
                         * Either swap_duplicate() failed because entry
                         * has been freed independently, and will not be
                         * reused since sys_swapoff() already disabled
                         * allocation from here, or alloc_page() failed.
                         */
                        if (!*swap_map)
                                continue;
                        retval = -ENOMEM;
                        break;
                }

                /*
                 * Don't hold on to start_mm if it looks like exiting.
                 */
                if (atomic_read(&start_mm->mm_users) == 1) {
                        mmput(start_mm);
                        start_mm = &init_mm;
                        atomic_inc(&init_mm.mm_users);
                }

                /*
                 * Wait for and lock page.  When do_swap_page races with
                 * try_to_unuse, do_swap_page can handle the fault much
                 * faster than try_to_unuse can locate the entry.  This
                 * apparently redundant "wait_on_page_locked" lets try_to_unuse
                 * defer to do_swap_page in such a case - in some tests,
                 * do_swap_page and try_to_unuse repeatedly compete.
                 */
                wait_on_page_locked(page);
                wait_on_page_writeback(page);
                lock_page(page);
                wait_on_page_writeback(page);

                /*
                 * Remove all references to entry.
                 * Whenever we reach init_mm, there's no address space
                 * to search, but use it as a reminder to search shmem.
                 */
                shmem = 0;
                swcount = *swap_map;
                if (swcount > 1) {
                        if (start_mm == &init_mm)
                                shmem = shmem_unuse(entry, page);
                        else
                                retval = unuse_mm(start_mm, entry, page);
                }
                if (*swap_map > 1) {
                        int set_start_mm = (*swap_map >= swcount);
                        struct list_head *p = &start_mm->mmlist;
                        struct mm_struct *new_start_mm = start_mm;
                        struct mm_struct *prev_mm = start_mm;
                        struct mm_struct *mm;

                        atomic_inc(&new_start_mm->mm_users);
                        atomic_inc(&prev_mm->mm_users);
                        spin_lock(&mmlist_lock);
                        while (*swap_map > 1 && !retval &&
                                        (p = p->next) != &start_mm->mmlist) {
                                mm = list_entry(p, struct mm_struct, mmlist);
                                if (atomic_inc_return(&mm->mm_users) == 1) {
                                        atomic_dec(&mm->mm_users);
                                        continue;
                                }
                                spin_unlock(&mmlist_lock);
                                mmput(prev_mm);
                                prev_mm = mm;

                                cond_resched();

                                swcount = *swap_map;
                                if (swcount <= 1)
                                        ;
                                else if (mm == &init_mm) {
                                        set_start_mm = 1;
                                        shmem = shmem_unuse(entry, page);
                                } else
                                        retval = unuse_mm(mm, entry, page);
                                if (set_start_mm && *swap_map < swcount) {
                                        mmput(new_start_mm);
                                        atomic_inc(&mm->mm_users);
                                        new_start_mm = mm;
                                        set_start_mm = 0;
                                }
                                spin_lock(&mmlist_lock);
                        }
                        spin_unlock(&mmlist_lock);
                        mmput(prev_mm);
                        mmput(start_mm);
                        start_mm = new_start_mm;
                }
                if (retval) {
                        unlock_page(page);
                        page_cache_release(page);
                        break;
                }

                /*
                 * How could swap count reach 0x7fff when the maximum
                 * pid is 0x7fff, and there's no way to repeat a swap
                 * page within an mm (except in shmem, where it's the
                 * shared object which takes the reference count)?
                 * We believe SWAP_MAP_MAX cannot occur in Linux 2.4.
                 *
                 * If that's wrong, then we should worry more about
                 * exit_mmap() and do_munmap() cases described above:
                 * we might be resetting SWAP_MAP_MAX too early here.
                 * We know "Undead"s can happen, they're okay, so don't
                 * report them; but do report if we reset SWAP_MAP_MAX.
                 */
                if (*swap_map == SWAP_MAP_MAX) {
                        swap_device_lock(si);
                        *swap_map = 1;
                        swap_device_unlock(si);
                        reset_overflow = 1;
                }

                /*
                 * If a reference remains (rare), we would like to leave
                 * the page in the swap cache; but try_to_unmap could
                 * then re-duplicate the entry once we drop page lock,
                 * so we might loop indefinitely; also, that page could
                 * not be swapped out to other storage meanwhile.  So:
                 * delete from cache even if there's another reference,
                 * after ensuring that the data has been saved to disk -
                 * since if the reference remains (rarer), it will be
                 * read from disk into another page.  Splitting into two
                 * pages would be incorrect if swap supported "shared
                 * private" pages, but they are handled by tmpfs files.
                 *
                 * Note shmem_unuse already deleted a swappage from
                 * the swap cache, unless the move to filepage failed:
                 * in which case it left swappage in cache, lowered its
                 * swap count to pass quickly through the loops above,
                 * and now we must reincrement count to try again later.
                 */
                if ((*swap_map > 1) && PageDirty(page) && PageSwapCache(page)) {
                        struct writeback_control wbc = {
                                .sync_mode = WB_SYNC_NONE,
                        };

                        swap_writepage(page, &wbc);
                        lock_page(page);
                        wait_on_page_writeback(page);
                }
                if (PageSwapCache(page)) {
                        if (shmem)
                                swap_duplicate(entry);
                        else
                                delete_from_swap_cache(page);
                }

                /*
                 * So we could skip searching mms once swap count went
                 * to 1, we did not mark any present ptes as dirty: must
                 * mark page dirty so shrink_list will preserve it.
                 */
                SetPageDirty(page);
                unlock_page(page);
                page_cache_release(page);

                /*
                 * Make sure that we aren't completely killing
                 * interactive performance.
                 */
                cond_resched();
        }

        mmput(start_mm);
        if (reset_overflow) {
                printk(KERN_WARNING "swapoff: cleared swap entry overflow\n");
                swap_overflow = 0;
        }
        return retval;
}

/*
 * After a successful try_to_unuse, if no swap is now in use, we know we
 * can empty the mmlist.  swap_list_lock must be held on entry and exit.
 * Note that mmlist_lock nests inside swap_list_lock, and an mm must be
 * added to the mmlist just after page_duplicate - before would be racy.
 */
static void drain_mmlist(void)
{
        struct list_head *p, *next;
        unsigned int i;

        for (i = 0; i < nr_swapfiles; i++)
                if (swap_info[i].inuse_pages)
                        return;
        spin_lock(&mmlist_lock);
        list_for_each_safe(p, next, &init_mm.mmlist)
                list_del_init(p);
        spin_unlock(&mmlist_lock);
}

/*
 * Use this swapdev's extent info to locate the (PAGE_SIZE) block which
 * corresponds to page offset `offset'.
 */
sector_t map_swap_page(struct swap_info_struct *sis, pgoff_t offset)
{
        struct swap_extent *se = sis->curr_swap_extent;
        struct swap_extent *start_se = se;

        for ( ; ; ) {
                struct list_head *lh;

                if (se->start_page <= offset &&
                                offset < (se->start_page + se->nr_pages)) {
                        return se->start_block + (offset - se->start_page);
                }
                lh = se->list.next;
                if (lh == &sis->extent_list)
                        lh = lh->next;
                se = list_entry(lh, struct swap_extent, list);
                sis->curr_swap_extent = se;
                BUG_ON(se == start_se);         /* It *must* be present */
        }
}

/*
 * Free all of a swapdev's extent information
 */
static void destroy_swap_extents(struct swap_info_struct *sis)
{
        while (!list_empty(&sis->extent_list)) {
                struct swap_extent *se;

                se = list_entry(sis->extent_list.next,
                                struct swap_extent, list);
                list_del(&se->list);
                kfree(se);
        }
}

/*
 * Add a block range (and the corresponding page range) into this swapdev's
 * extent list.  The extent list is kept sorted in page order.
 *
 * This function rather assumes that it is called in ascending page order.
 */
static int
add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
                unsigned long nr_pages, sector_t start_block)
{
        struct swap_extent *se;
        struct swap_extent *new_se;
        struct list_head *lh;

        lh = sis->extent_list.prev;     /* The highest page extent */
        if (lh != &sis->extent_list) {
                se = list_entry(lh, struct swap_extent, list);
                BUG_ON(se->start_page + se->nr_pages != start_page);
                if (se->start_block + se->nr_pages == start_block) {
                        /* Merge it */
                        se->nr_pages += nr_pages;
                        return 0;
                }
        }

        /*
         * No merge.  Insert a new extent, preserving ordering.
         */
        new_se = kmalloc(sizeof(*se), GFP_KERNEL);
        if (new_se == NULL)
                return -ENOMEM;
        new_se->start_page = start_page;
        new_se->nr_pages = nr_pages;
        new_se->start_block = start_block;

        list_add_tail(&new_se->list, &sis->extent_list);
        return 1;
}

/*
 * A `swap extent' is a simple thing which maps a contiguous range of pages
 * onto a contiguous range of disk blocks.  An ordered list of swap extents
 * is built at swapon time and is then used at swap_writepage/swap_readpage
 * time for locating where on disk a page belongs.
 *
 * If the swapfile is an S_ISBLK block device, a single extent is installed.
 * This is done so that the main operating code can treat S_ISBLK and S_ISREG
 * swap files identically.
 *
 * Whether the swapdev is an S_ISREG file or an S_ISBLK blockdev, the swap
 * extent list operates in PAGE_SIZE disk blocks.  Both S_ISREG and S_ISBLK
 * swapfiles are handled *identically* after swapon time.
 *
 * For S_ISREG swapfiles, setup_swap_extents() will walk all the file's blocks
 * and will parse them into an ordered extent list, in PAGE_SIZE chunks.  If
 * some stray blocks are found which do not fall within the PAGE_SIZE alignment
 * requirements, they are simply tossed out - we will never use those blocks
 * for swapping.
 *
 * For S_ISREG swapfiles we set S_SWAPFILE across the life of the swapon.  This
 * prevents root from shooting her foot off by ftruncating an in-use swapfile,
 * which will scribble on the fs.
 *
 * The amount of disk space which a single swap extent represents varies.
 * Typically it is in the 1-4 megabyte range.  So we can have hundreds of
 * extents in the list.  To avoid much list walking, we cache the previous
 * search location in `curr_swap_extent', and start new searches from there.
 * This is extremely effective.  The average number of iterations in
 * map_swap_page() has been measured at about 0.3 per page.  - akpm.
 */
static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
{
        struct inode *inode;
        unsigned blocks_per_page;
        unsigned long page_no;
        unsigned blkbits;
        sector_t probe_block;
        sector_t last_block;
        sector_t lowest_block = -1;
        sector_t highest_block = 0;
        int nr_extents = 0;
        int ret;

        inode = sis->swap_file->f_mapping->host;
        if (S_ISBLK(inode->i_mode)) {
                ret = add_swap_extent(sis, 0, sis->max, 0);
                *span = sis->pages;
                goto done;
        }

        blkbits = inode->i_blkbits;
        blocks_per_page = PAGE_SIZE >> blkbits;

        /*
         * Map all the blocks into the extent list.  This code doesn't try
         * to be very smart.
         */
        probe_block = 0;
        page_no = 0;
        last_block = i_size_read(inode) >> blkbits;
        while ((probe_block + blocks_per_page) <= last_block &&
                        page_no < sis->max) {
                unsigned block_in_page;
                sector_t first_block;

                first_block = bmap(inode, probe_block);
                if (first_block == 0)
                        goto bad_bmap;

                /*
                 * It must be PAGE_SIZE aligned on-disk
                 */
                if (first_block & (blocks_per_page - 1)) {
                        probe_block++;
                        goto reprobe;
                }

                for (block_in_page = 1; block_in_page < blocks_per_page;
                                        block_in_page++) {
                        sector_t block;

                        block = bmap(inode, probe_block + block_in_page);
                        if (block == 0)
                                goto bad_bmap;
                        if (block != first_block + block_in_page) {
                                /* Discontiguity */
                                probe_block++;
                                goto reprobe;
                        }
                }

                first_block >>= (PAGE_SHIFT - blkbits);
                if (page_no) {  /* exclude the header page */
                        if (first_block < lowest_block)
                                lowest_block = first_block;
                        if (first_block > highest_block)
                                highest_block = first_block;
                }

                /*
                 * We found a PAGE_SIZE-length, PAGE_SIZE-aligned run of blocks
                 */
                ret = add_swap_extent(sis, page_no, 1, first_block);
                if (ret < 0)
                        goto out;
                nr_extents += ret;
                page_no++;
                probe_block += blocks_per_page;
reprobe:
                continue;
        }
        ret = nr_extents;
        *span = 1 + highest_block - lowest_block;
        if (page_no == 0)
                page_no = 1;    /* force Empty message */
        sis->max = page_no;
        sis->pages = page_no - 1;
        sis->highest_bit = page_no - 1;
done:
        sis->curr_swap_extent = list_entry(sis->extent_list.prev,
                                        struct swap_extent, list);
        goto out;
bad_bmap:
        printk(KERN_ERR "swapon: swapfile has holes\n");
        ret = -EINVAL;
out:
        return ret;
}

#if 0   /* We don't need this yet */
#include <linux/backing-dev.h>
int page_queue_congested(struct page *page)
{
        struct backing_dev_info *bdi;

        BUG_ON(!PageLocked(page));      /* It pins the swap_info_struct */

        if (PageSwapCache(page)) {
                swp_entry_t entry = { .val = page->private };
                struct swap_info_struct *sis;

                sis = get_swap_info_struct(swp_type(entry));
                bdi = sis->bdev->bd_inode->i_mapping->backing_dev_info;
        } else
                bdi = page->mapping->backing_dev_info;
        return bdi_write_congested(bdi);
}
#endif

asmlinkage long sys_swapoff(const char __user * specialfile)
{
        struct swap_info_struct * p = NULL;
        unsigned short *swap_map;
        struct file *swap_file, *victim;
        struct address_space *mapping;
        struct inode *inode;
        char * pathname;
        int i, type, prev;
        int err;
        
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        pathname = getname(specialfile);
        err = PTR_ERR(pathname);
        if (IS_ERR(pathname))
                goto out;

        victim = filp_open(pathname, O_RDWR|O_LARGEFILE, 0);
        putname(pathname);
        err = PTR_ERR(victim);
        if (IS_ERR(victim))
                goto out;

        mapping = victim->f_mapping;
        prev = -1;
        swap_list_lock();
        for (type = swap_list.head; type >= 0; type = swap_info[type].next) {
                p = swap_info + type;
                if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
                        if (p->swap_file->f_mapping == mapping)
                                break;
                }
                prev = type;
        }
        if (type < 0) {
                err = -EINVAL;
                swap_list_unlock();
                goto out_dput;
        }
        if (!security_vm_enough_memory(p->pages))
                vm_unacct_memory(p->pages);
        else {
                err = -ENOMEM;
                swap_list_unlock();
                goto out_dput;
        }
        if (prev < 0) {
                swap_list.head = p->next;
        } else {
                swap_info[prev].next = p->next;
        }
        if (type == swap_list.next) {
                /* just pick something that's safe... */
                swap_list.next = swap_list.head;
        }
        nr_swap_pages -= p->pages;
        total_swap_pages -= p->pages;
        p->flags &= ~SWP_WRITEOK;
        swap_list_unlock();
        current->flags |= PF_SWAPOFF;
        err = try_to_unuse(type);
        current->flags &= ~PF_SWAPOFF;

        /* wait for any unplug function to finish */
        down_write(&swap_unplug_sem);
        up_write(&swap_unplug_sem);

        if (err) {
                /* re-insert swap space back into swap_list */
                swap_list_lock();
                for (prev = -1, i = swap_list.head; i >= 0; prev = i, i = swap_info[i].next)
                        if (p->prio >= swap_info[i].prio)
                                break;
                p->next = i;
                if (prev < 0)
                        swap_list.head = swap_list.next = p - swap_info;
                else
                        swap_info[prev].next = p - swap_info;
                nr_swap_pages += p->pages;
                total_swap_pages += p->pages;
                p->flags |= SWP_WRITEOK;
                swap_list_unlock();
                goto out_dput;
        }
        destroy_swap_extents(p);
        down(&swapon_sem);
        swap_list_lock();
        drain_mmlist();
        swap_device_lock(p);
        swap_file = p->swap_file;
        p->swap_file = NULL;
        p->max = 0;
        swap_map = p->swap_map;
        p->swap_map = NULL;
        p->flags = 0;
        swap_device_unlock(p);
        swap_list_unlock();
        up(&swapon_sem);
        vfree(swap_map);
        inode = mapping->host;
        if (S_ISBLK(inode->i_mode)) {
                struct block_device *bdev = I_BDEV(inode);
                set_blocksize(bdev, p->old_block_size);
                bd_release(bdev);
        } else {
                down(&inode->i_sem);
                inode->i_flags &= ~S_SWAPFILE;
                up(&inode->i_sem);
        }
        filp_close(swap_file, NULL);
        err = 0;

out_dput:
        filp_close(victim, NULL);
out:
        return err;
}

#ifdef CONFIG_PROC_FS
/* iterator */
static void *swap_start(struct seq_file *swap, loff_t *pos)
{
        struct swap_info_struct *ptr = swap_info;
        int i;
        loff_t l = *pos;

        down(&swapon_sem);

        for (i = 0; i < nr_swapfiles; i++, ptr++) {
                if (!(ptr->flags & SWP_USED) || !ptr->swap_map)
                        continue;
                if (!l--)
                        return ptr;
        }

        return NULL;
}

static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
{
        struct swap_info_struct *ptr = v;
        struct swap_info_struct *endptr = swap_info + nr_swapfiles;

        for (++ptr; ptr < endptr; ptr++) {
                if (!(ptr->flags & SWP_USED) || !ptr->swap_map)
                        continue;
                ++*pos;
                return ptr;
        }

        return NULL;
}

static void swap_stop(struct seq_file *swap, void *v)
{
        up(&swapon_sem);
}

static int swap_show(struct seq_file *swap, void *v)
{
        struct swap_info_struct *ptr = v;
        struct file *file;
        int len;

        if (v == swap_info)
                seq_puts(swap, "Filename\t\t\t\tType\t\tSize\tUsed\tPriority\n");

        file = ptr->swap_file;
        len = seq_path(swap, file->f_vfsmnt, file->f_dentry, " \t\n\\");
        seq_printf(swap, "%*s%s\t%u\t%u\t%d\n",
                       len < 40 ? 40 - len : 1, " ",
                       S_ISBLK(file->f_dentry->d_inode->i_mode) ?
                                "partition" : "file\t",
                       ptr->pages << (PAGE_SHIFT - 10),
                       ptr->inuse_pages << (PAGE_SHIFT - 10),
                       ptr->prio);
        return 0;
}

static struct seq_operations swaps_op = {
        .start =        swap_start,
        .next =         swap_next,
        .stop =         swap_stop,
        .show =         swap_show
};

static int swaps_open(struct inode *inode, struct file *file)
{
        return seq_open(file, &swaps_op);
}

static struct file_operations proc_swaps_operations = {
        .open           = swaps_open,
        .read           = seq_read,
        .llseek         = seq_lseek,
        .release        = seq_release,
};

static int __init procswaps_init(void)
{
        struct proc_dir_entry *entry;

        entry = create_proc_entry("swaps", 0, NULL);
        if (entry)
                entry->proc_fops = &proc_swaps_operations;
        return 0;
}
__initcall(procswaps_init);
#endif /* CONFIG_PROC_FS */

/*
 * Written 01/25/92 by Simmule Turner, heavily changed by Linus.
 *
 * The swapon system call
 */
asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
{
        struct swap_info_struct * p;
        char *name = NULL;
        struct block_device *bdev = NULL;
        struct file *swap_file = NULL;
        struct address_space *mapping;
        unsigned int type;
        int i, prev;
        int error;
        static int least_priority;
        union swap_header *swap_header = NULL;
        int swap_header_version;
        unsigned int nr_good_pages = 0;
        int nr_extents = 0;
        sector_t span;
        unsigned long maxpages = 1;
        int swapfilesize;
        unsigned short *swap_map;
        struct page *page = NULL;
        struct inode *inode = NULL;
        int did_down = 0;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        swap_list_lock();
        p = swap_info;
        for (type = 0 ; type < nr_swapfiles ; type++,p++)
                if (!(p->flags & SWP_USED))
                        break;
        error = -EPERM;
        /*
         * Test if adding another swap device is possible. There are
         * two limiting factors: 1) the number of bits for the swap
         * type swp_entry_t definition and 2) the number of bits for
         * the swap type in the swap ptes as defined by the different
         * architectures. To honor both limitations a swap entry
         * with swap offset 0 and swap type ~0UL is created, encoded
         * to a swap pte, decoded to a swp_entry_t again and finally
         * the swap type part is extracted. This will mask all bits
         * from the initial ~0UL that can't be encoded in either the
         * swp_entry_t or the architecture definition of a swap pte.
         */
        if (type > swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))) {
                swap_list_unlock();
                goto out;
        }
        if (type >= nr_swapfiles)
                nr_swapfiles = type+1;
        INIT_LIST_HEAD(&p->extent_list);
        p->flags = SWP_USED;
        p->swap_file = NULL;
        p->old_block_size = 0;
        p->swap_map = NULL;
        p->lowest_bit = 0;
        p->highest_bit = 0;
        p->cluster_nr = 0;
        p->inuse_pages = 0;
        spin_lock_init(&p->sdev_lock);
        p->next = -1;
        if (swap_flags & SWAP_FLAG_PREFER) {
                p->prio =
                  (swap_flags & SWAP_FLAG_PRIO_MASK)>>SWAP_FLAG_PRIO_SHIFT;
        } else {
                p->prio = --least_priority;
        }
        swap_list_unlock();
        name = getname(specialfile);
        error = PTR_ERR(name);
        if (IS_ERR(name)) {
                name = NULL;
                goto bad_swap_2;
        }
        swap_file = filp_open(name, O_RDWR|O_LARGEFILE, 0);
        error = PTR_ERR(swap_file);
        if (IS_ERR(swap_file)) {
                swap_file = NULL;
                goto bad_swap_2;
        }

        p->swap_file = swap_file;
        mapping = swap_file->f_mapping;
        inode = mapping->host;

        error = -EBUSY;
        for (i = 0; i < nr_swapfiles; i++) {
                struct swap_info_struct *q = &swap_info[i];

                if (i == type || !q->swap_file)
                        continue;
                if (mapping == q->swap_file->f_mapping)
                        goto bad_swap;
        }

        error = -EINVAL;
        if (S_ISBLK(inode->i_mode)) {
                bdev = I_BDEV(inode);
                error = bd_claim(bdev, sys_swapon);
                if (error < 0) {
                        bdev = NULL;
                        goto bad_swap;
                }
                p->old_block_size = block_size(bdev);
                error = set_blocksize(bdev, PAGE_SIZE);
                if (error < 0)
                        goto bad_swap;
                p->bdev = bdev;
        } else if (S_ISREG(inode->i_mode)) {
                p->bdev = inode->i_sb->s_bdev;
                down(&inode->i_sem);
                did_down = 1;
                if (IS_SWAPFILE(inode)) {
                        error = -EBUSY;
                        goto bad_swap;
                }
        } else {
                goto bad_swap;
        }

        swapfilesize = i_size_read(inode) >> PAGE_SHIFT;

        /*
         * Read the swap header.
         */
        if (!mapping->a_ops->readpage) {
                error = -EINVAL;
                goto bad_swap;
        }
        page = read_cache_page(mapping, 0,
                        (filler_t *)mapping->a_ops->readpage, swap_file);
        if (IS_ERR(page)) {
                error = PTR_ERR(page);
                goto bad_swap;
        }
        wait_on_page_locked(page);
        if (!PageUptodate(page))
                goto bad_swap;
        kmap(page);
        swap_header = page_address(page);

        if (!memcmp("SWAP-SPACE",swap_header->magic.magic,10))
                swap_header_version = 1;
        else if (!memcmp("SWAPSPACE2",swap_header->magic.magic,10))
                swap_header_version = 2;
        else {
                printk("Unable to find swap-space signature\n");
                error = -EINVAL;
                goto bad_swap;
        }
        
        switch (swap_header_version) {
        case 1:
                printk(KERN_ERR "version 0 swap is no longer supported. "
                        "Use mkswap -v1 %s\n", name);
                error = -EINVAL;
                goto bad_swap;
        case 2:
                /* Check the swap header's sub-version and the size of
                   the swap file and bad block lists */
                if (swap_header->info.version != 1) {
                        printk(KERN_WARNING
                               "Unable to handle swap header version %d\n",
                               swap_header->info.version);
                        error = -EINVAL;
                        goto bad_swap;
                }

                p->lowest_bit  = 1;
                /*
                 * Find out how many pages are allowed for a single swap
                 * device. There are two limiting factors: 1) the number of
                 * bits for the swap offset in the swp_entry_t type and
                 * 2) the number of bits in the a swap pte as defined by
                 * the different architectures. In order to find the
                 * largest possible bit mask a swap entry with swap type 0
                 * and swap offset ~0UL is created, encoded to a swap pte,
                 * decoded to a swp_entry_t again and finally the swap
                 * offset is extracted. This will mask all the bits from
                 * the initial ~0UL mask that can't be encoded in either
                 * the swp_entry_t or the architecture definition of a
                 * swap pte.
                 */
                maxpages = swp_offset(pte_to_swp_entry(swp_entry_to_pte(swp_entry(0,~0UL)))) - 1;
                if (maxpages > swap_header->info.last_page)
                        maxpages = swap_header->info.last_page;
                p->highest_bit = maxpages - 1;

                error = -EINVAL;
                if (!maxpages)
                        goto bad_swap;
                if (swap_header->info.nr_badpages && S_ISREG(inode->i_mode))
                        goto bad_swap;
                if (swap_header->info.nr_badpages > MAX_SWAP_BADPAGES)
                        goto bad_swap;
                
                /* OK, set up the swap map and apply the bad block list */
                if (!(p->swap_map = vmalloc(maxpages * sizeof(short)))) {
                        error = -ENOMEM;
                        goto bad_swap;
                }

                error = 0;
                memset(p->swap_map, 0, maxpages * sizeof(short));
                for (i=0; i<swap_header->info.nr_badpages; i++) {
                        int page = swap_header->info.badpages[i];
                        if (page <= 0 || page >= swap_header->info.last_page)
                                error = -EINVAL;
                        else
                                p->swap_map[page] = SWAP_MAP_BAD;
                }
                nr_good_pages = swap_header->info.last_page -
                                swap_header->info.nr_badpages -
                                1 /* header page */;
                if (error) 
                        goto bad_swap;
        }

        if (swapfilesize && maxpages > swapfilesize) {
                printk(KERN_WARNING
                       "Swap area shorter than signature indicates\n");
                error = -EINVAL;
                goto bad_swap;
        }
        if (nr_good_pages) {
                p->swap_map[0] = SWAP_MAP_BAD;
                p->max = maxpages;
                p->pages = nr_good_pages;
                nr_extents = setup_swap_extents(p, &span);
                if (nr_extents < 0) {
                        error = nr_extents;
                        goto bad_swap;
                }
                nr_good_pages = p->pages;
        }
        if (!nr_good_pages) {
                printk(KERN_WARNING "Empty swap-file\n");
                error = -EINVAL;
                goto bad_swap;
        }

        down(&swapon_sem);
        swap_list_lock();
        swap_device_lock(p);
        p->flags = SWP_ACTIVE;
        nr_swap_pages += nr_good_pages;
        total_swap_pages += nr_good_pages;

        printk(KERN_INFO "Adding %uk swap on %s.  "
                        "Priority:%d extents:%d across:%lluk\n",
                nr_good_pages<<(PAGE_SHIFT-10), name, p->prio,
                nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10));

        /* insert swap space into swap_list: */
        prev = -1;
        for (i = swap_list.head; i >= 0; i = swap_info[i].next) {
                if (p->prio >= swap_info[i].prio) {
                        break;
                }
                prev = i;
        }
        p->next = i;
        if (prev < 0) {
                swap_list.head = swap_list.next = p - swap_info;
        } else {
                swap_info[prev].next = p - swap_info;
        }
        swap_device_unlock(p);
        swap_list_unlock();
        up(&swapon_sem);
        error = 0;
        goto out;
bad_swap:
        if (bdev) {
                set_blocksize(bdev, p->old_block_size);
                bd_release(bdev);
        }
        destroy_swap_extents(p);
bad_swap_2:
        swap_list_lock();
        swap_map = p->swap_map;
        p->swap_file = NULL;
        p->swap_map = NULL;
        p->flags = 0;
        if (!(swap_flags & SWAP_FLAG_PREFER))
                ++least_priority;
        swap_list_unlock();
        vfree(swap_map);
        if (swap_file)
                filp_close(swap_file, NULL);
out:
        if (page && !IS_ERR(page)) {
                kunmap(page);
                page_cache_release(page);
        }
        if (name)
                putname(name);
        if (did_down) {
                if (!error)
                        inode->i_flags |= S_SWAPFILE;
                up(&inode->i_sem);
        }
        return error;
}

void si_swapinfo(struct sysinfo *val)
{
        unsigned int i;
        unsigned long nr_to_be_unused = 0;

        swap_list_lock();
        for (i = 0; i < nr_swapfiles; i++) {
                if (!(swap_info[i].flags & SWP_USED) ||
                     (swap_info[i].flags & SWP_WRITEOK))
                        continue;
                nr_to_be_unused += swap_info[i].inuse_pages;
        }
        val->freeswap = nr_swap_pages + nr_to_be_unused;
        val->totalswap = total_swap_pages + nr_to_be_unused;
        swap_list_unlock();
}

/*
 * Verify that a swap entry is valid and increment its swap map count.
 *
 * Note: if swap_map[] reaches SWAP_MAP_MAX the entries are treated as
 * "permanent", but will be reclaimed by the next swapoff.
 */
int swap_duplicate(swp_entry_t entry)
{
        struct swap_info_struct * p;
        unsigned long offset, type;
        int result = 0;

        type = swp_type(entry);
        if (type >= nr_swapfiles)
                goto bad_file;
        p = type + swap_info;
        offset = swp_offset(entry);

        swap_device_lock(p);
        if (offset < p->max && p->swap_map[offset]) {
                if (p->swap_map[offset] < SWAP_MAP_MAX - 1) {
                        p->swap_map[offset]++;
                        result = 1;
                } else if (p->swap_map[offset] <= SWAP_MAP_MAX) {
                        if (swap_overflow++ < 5)
                                printk(KERN_WARNING "swap_dup: swap entry overflow\n");
                        p->swap_map[offset] = SWAP_MAP_MAX;
                        result = 1;
                }
        }
        swap_device_unlock(p);
out:
        return result;

bad_file:
        printk(KERN_ERR "swap_dup: %s%08lx\n", Bad_file, entry.val);
        goto out;
}

struct swap_info_struct *
get_swap_info_struct(unsigned type)
{
        return &swap_info[type];
}

/*
 * swap_device_lock prevents swap_map being freed. Don't grab an extra
 * reference on the swaphandle, it doesn't matter if it becomes unused.
 */
int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
{
        int ret = 0, i = 1 << page_cluster;
        unsigned long toff;
        struct swap_info_struct *swapdev = swp_type(entry) + swap_info;

        if (!page_cluster)      /* no readahead */
                return 0;
        toff = (swp_offset(entry) >> page_cluster) << page_cluster;
        if (!toff)              /* first page is swap header */
                toff++, i--;
        *offset = toff;

        swap_device_lock(swapdev);
        do {
                /* Don't read-ahead past the end of the swap area */
                if (toff >= swapdev->max)
                        break;
                /* Don't read in free or bad pages */
                if (!swapdev->swap_map[toff])
                        break;
                if (swapdev->swap_map[toff] == SWAP_MAP_BAD)
                        break;
                toff++;
                ret++;
        } while (--i);
        swap_device_unlock(swapdev);
        return ret;
}