swap_state.c

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/*
 *  linux/mm/swap_state.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *  Swap reorganised 29.12.95, Stephen Tweedie
 *
 *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
 */

#include <linux/mm.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/swapctl.h>
#include <linux/init.h>
#include <linux/pagemap.h>

#include <asm/pgtable.h>

/* 
 * Keep a reserved false inode which we will use to mark pages in the
 * page cache are acting as swap cache instead of file cache. 
 *
 * We only need a unique pointer to satisfy the page cache, but we'll
 * reserve an entire zeroed inode structure for the purpose just to
 * ensure that any mistaken dereferences of this structure cause a
 * kernel oops.
 */
struct inode swapper_inode;

#ifdef SWAP_CACHE_INFO
unsigned long swap_cache_add_total = 0;
unsigned long swap_cache_del_total = 0;
unsigned long swap_cache_find_total = 0;
unsigned long swap_cache_find_success = 0;

void show_swap_cache_info(void)
{
        printk("Swap cache: add %ld, delete %ld, find %ld/%ld\n",
                swap_cache_add_total, 
                swap_cache_del_total,
                swap_cache_find_success, swap_cache_find_total);
}
#endif

int add_to_swap_cache(struct page *page, unsigned long entry)
{
#ifdef SWAP_CACHE_INFO
        swap_cache_add_total++;
#endif
#ifdef DEBUG_SWAP
        printk("DebugVM: add_to_swap_cache(%08lx count %d, entry %08lx)\n",
               page_address(page), atomic_read(&page->count), entry);
#endif
        if (PageTestandSetSwapCache(page)) {
                printk(KERN_ERR "swap_cache: replacing non-empty entry %08lx "
                       "on page %08lx\n",
                       page->offset, page_address(page));
                return 0;
        }
        if (page->inode) {
                printk(KERN_ERR "swap_cache: replacing page-cached entry "
                       "on page %08lx\n", page_address(page));
                return 0;
        }
        atomic_inc(&page->count);
        page->flags = page->flags & ~((1 << PG_uptodate) | (1 << PG_error) | (1 << PG_referenced));
        page->inode = &swapper_inode;
        page->offset = entry;
        add_page_to_hash_queue(page, &swapper_inode, entry);
        add_page_to_inode_queue(&swapper_inode, page);
        return 1;
}

/*
 * 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(unsigned long entry)
{
        struct swap_info_struct * p;
        unsigned long offset, type;
        int result = 0;

        if (!entry)
                goto out;
        type = SWP_TYPE(entry);
        if (type & SHM_SWP_TYPE)
                goto out;
        if (type >= nr_swapfiles)
                goto bad_file;
        p = type + swap_info;
        offset = SWP_OFFSET(entry);
        if (offset >= p->max)
                goto bad_offset;
        if (!p->swap_map[offset])
                goto bad_unused;
        /*
         * Entry is valid, so increment the map count.
         */
        if (p->swap_map[offset] < SWAP_MAP_MAX)
                p->swap_map[offset]++;
        else {
                static int overflow = 0;
                if (overflow++ < 5)
                        printk(KERN_WARNING
                                "swap_duplicate: entry %08lx map count=%d\n",
                                entry, p->swap_map[offset]);
                p->swap_map[offset] = SWAP_MAP_MAX;
        }
        result = 1;
#ifdef DEBUG_SWAP
        printk("DebugVM: swap_duplicate(entry %08lx, count now %d)\n",
               entry, p->swap_map[offset]);
#endif
out:
        return result;

bad_file:
        printk(KERN_ERR
                "swap_duplicate: entry %08lx, nonexistent swap file\n", entry);
        goto out;
bad_offset:
        printk(KERN_ERR
                "swap_duplicate: entry %08lx, offset exceeds max\n", entry);
        goto out;
bad_unused:
        printk(KERN_ERR
                "swap_duplicate at %8p: entry %08lx, unused page\n", 
               __builtin_return_address(0), entry);
        goto out;
}

int swap_count(unsigned long entry)
{
        struct swap_info_struct * p;
        unsigned long offset, type;
        int retval = 0;

        if (!entry)
                goto bad_entry;
        type = SWP_TYPE(entry);
        if (type & SHM_SWP_TYPE)
                goto out;
        if (type >= nr_swapfiles)
                goto bad_file;
        p = type + swap_info;
        offset = SWP_OFFSET(entry);
        if (offset >= p->max)
                goto bad_offset;
        if (!p->swap_map[offset])
                goto bad_unused;
        retval = p->swap_map[offset];
#ifdef DEBUG_SWAP
        printk("DebugVM: swap_count(entry %08lx, count %d)\n",
               entry, retval);
#endif
out:
        return retval;

bad_entry:
        printk(KERN_ERR "swap_count: null entry!\n");
        goto out;
bad_file:
        printk(KERN_ERR
               "swap_count: entry %08lx, nonexistent swap file!\n", entry);
        goto out;
bad_offset:
        printk(KERN_ERR
               "swap_count: entry %08lx, offset exceeds max!\n", entry);
        goto out;
bad_unused:
        printk(KERN_ERR
               "swap_count at %8p: entry %08lx, unused page!\n", 
               __builtin_return_address(0), entry);
        goto out;
}

static inline void remove_from_swap_cache(struct page *page)
{
        if (!page->inode) {
                printk ("VM: Removing swap cache page with zero inode hash "
                        "on page %08lx\n", page_address(page));
                return;
        }
        if (page->inode != &swapper_inode) {
                printk ("VM: Removing swap cache page with wrong inode hash "
                        "on page %08lx\n", page_address(page));
        }

#ifdef DEBUG_SWAP
        printk("DebugVM: remove_from_swap_cache(%08lx count %d)\n",
               page_address(page), atomic_read(&page->count));
#endif
        PageClearSwapCache (page);
        remove_inode_page(page);
}


/*
 * This must be called only on pages that have
 * been verified to be in the swap cache.
 */
void delete_from_swap_cache(struct page *page)
{
        long entry = page->offset;

#ifdef SWAP_CACHE_INFO
        swap_cache_del_total++;
#endif
#ifdef DEBUG_SWAP
        printk("DebugVM: delete_from_swap_cache(%08lx count %d, "
               "entry %08lx)\n",
               page_address(page), atomic_read(&page->count), entry);
#endif
        remove_from_swap_cache (page);
        swap_free (entry);
}

/* 
 * Perform a free_page(), also freeing any swap cache associated with
 * this page if it is the last user of the page. 
 */

void free_page_and_swap_cache(unsigned long addr)
{
        struct page *page = mem_map + MAP_NR(addr);

        /* 
         * If we are the only user, then free up the swap cache. 
         */
        if (PageSwapCache(page) && !is_page_shared(page)) {
                delete_from_swap_cache(page);
        }
        
        __free_page(page);
}


/*
 * Lookup a swap entry in the swap cache.  We need to be careful about
 * locked pages.  A found page will be returned with its refcount
 * incremented.
 */

struct page * lookup_swap_cache(unsigned long entry)
{
        struct page *found;

#ifdef SWAP_CACHE_INFO
        swap_cache_find_total++;
#endif
        while (1) {
                found = find_page(&swapper_inode, entry);
                if (!found)
                        return 0;
                if (found->inode != &swapper_inode || !PageSwapCache(found))
                        goto out_bad;
                if (!PageLocked(found)) {
#ifdef SWAP_CACHE_INFO
                        swap_cache_find_success++;
#endif
                        return found;
                }
                __free_page(found);
                __wait_on_page(found);
        }

out_bad:
        printk (KERN_ERR "VM: Found a non-swapper swap page!\n");
        __free_page(found);
        return 0;
}

/* 
 * Locate a page of swap in physical memory, reserving swap cache space
 * and reading the disk if it is not already cached.  If wait==0, we are
 * only doing readahead, so don't worry if the page is already locked.
 *
 * A failure return means that either the page allocation failed or that
 * the swap entry is no longer in use.
 */

struct page * read_swap_cache_async(unsigned long entry, int wait)
{
        struct page *found_page = 0, *new_page;
        unsigned long new_page_addr;
        
#ifdef DEBUG_SWAP
        printk("DebugVM: read_swap_cache_async entry %08lx%s\n",
               entry, wait ? ", wait" : "");
#endif
        /*
         * Make sure the swap entry is still in use.
         */
        if (!swap_duplicate(entry))     /* Account for the swap cache */
                goto out;
        /*
         * Look for the page in the swap cache.
         */
        found_page = lookup_swap_cache(entry);
        if (found_page)
                goto out_free_swap;

        new_page_addr = __get_free_page(GFP_USER);
        if (!new_page_addr)
                goto out_free_swap;     /* Out of memory */
        new_page = mem_map + MAP_NR(new_page_addr);

        /*
         * Check the swap cache again, in case we stalled above.
         */
        found_page = lookup_swap_cache(entry);
        if (found_page)
                goto out_free_page;
        /* 
         * Add it to the swap cache and read its contents.
         */
        if (!add_to_swap_cache(new_page, entry))
                goto out_free_page;

        set_bit(PG_locked, &new_page->flags);
        rw_swap_page(READ, entry, (char *) new_page_addr, wait);
#ifdef DEBUG_SWAP
        printk("DebugVM: read_swap_cache_async created "
               "entry %08lx at %p\n",
               entry, (char *) page_address(new_page));
#endif
        return new_page;

out_free_page:
        __free_page(new_page);
out_free_swap:
        swap_free(entry);
out:
        return found_page;
}

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