cache.c

Go to the documentation of this file.

/*
 * Squashfs - a compressed read only filesystem for Linux
 *
 * Copyright (c) 2002, 2003, 2004, 2005, 2006, 2007, 2008
 * Phillip Lougher <[email protected]>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2,
 * or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 *
 * cache.c
 */

/*
 * Blocks in Squashfs are compressed.  To avoid repeatedly decompressing
 * recently accessed data Squashfs uses two small metadata and fragment caches.
 *
 * This file implements a generic cache implementation used for both caches,
 * plus functions layered ontop of the generic cache implementation to
 * access the metadata and fragment caches.
 *
 * To avoid out of memory and fragmentation issues with vmalloc the cache
 * uses sequences of kmalloced PAGE_CACHE_SIZE buffers.
 *
 * It should be noted that the cache is not used for file datablocks, these
 * are decompressed and cached in the page-cache in the normal way.  The
 * cache is only used to temporarily cache fragment and metadata blocks
 * which have been read as as a result of a metadata (i.e. inode or
 * directory) or fragment access.  Because metadata and fragments are packed
 * together into blocks (to gain greater compression) the read of a particular
 * piece of metadata or fragment will retrieve other metadata/fragments which
 * have been packed with it, these because of locality-of-reference may be read
 * in the near future. Temporarily caching them ensures they are available for
 * near future access without requiring an additional read and decompress.
 */

#include <linux/fs.h>
#include <linux/vfs.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/sched.h>
#include <linux/spinlock.h>
#include <linux/wait.h>
#include <linux/pagemap.h>

#include "squashfs_fs.h"
#include "squashfs_fs_sb.h"
#include "squashfs.h"

/*
 * Look-up block in cache, and increment usage count.  If not in cache, read
 * and decompress it from disk.
 */
struct squashfs_cache_entry *squashfs_cache_get(struct super_block *sb,
    struct squashfs_cache *cache, u64 block, int length)
{
    int i, n;
    struct squashfs_cache_entry *entry;

    spin_lock(&cache->lock);

    while (1) {
        for (i = cache->curr_blk, n = 0; n < cache->entries; n++) {
            if (cache->entry[i].block == block) {
                cache->curr_blk = i;
                break;
            }
            i = (i + 1) % cache->entries;
        }

        if (n == cache->entries) {
            /*
             * Block not in cache, if all cache entries are used
             * go to sleep waiting for one to become available.
             */
            if (cache->unused == 0) {
                cache->num_waiters++;
                spin_unlock(&cache->lock);
                wait_event(cache->wait_queue, cache->unused);
                spin_lock(&cache->lock);
                cache->num_waiters--;
                continue;
            }

            /*
             * At least one unused cache entry.  A simple
             * round-robin strategy is used to choose the entry to
             * be evicted from the cache.
             */
            i = cache->next_blk;
            for (n = 0; n < cache->entries; n++) {
                if (cache->entry[i].refcount == 0)
                    break;
                i = (i + 1) % cache->entries;
            }

            cache->next_blk = (i + 1) % cache->entries;
            entry = &cache->entry[i];

            /*
             * Initialise chosen cache entry, and fill it in from
             * disk.
             */
            cache->unused--;
            entry->block = block;
            entry->refcount = 1;
            entry->pending = 1;
            entry->num_waiters = 0;
            entry->error = 0;
            spin_unlock(&cache->lock);

            entry->length = squashfs_read_data(sb, entry->data,
                block, length, &entry->next_index,
                cache->block_size, cache->pages);

            spin_lock(&cache->lock);

            if (entry->length < 0)
                entry->error = entry->length;

            entry->pending = 0;

            /*
             * While filling this entry one or more other processes
             * have looked it up in the cache, and have slept
             * waiting for it to become available.
             */
            if (entry->num_waiters) {
                spin_unlock(&cache->lock);
                wake_up_all(&entry->wait_queue);
            } else
                spin_unlock(&cache->lock);

            goto out;
        }

        /*
         * Block already in cache.  Increment refcount so it doesn't
         * get reused until we're finished with it, if it was
         * previously unused there's one less cache entry available
         * for reuse.
         */
        entry = &cache->entry[i];
        if (entry->refcount == 0)
            cache->unused--;
        entry->refcount++;

        /*
         * If the entry is currently being filled in by another process
         * go to sleep waiting for it to become available.
         */
        if (entry->pending) {
            entry->num_waiters++;
            spin_unlock(&cache->lock);
            wait_event(entry->wait_queue, !entry->pending);
        } else
            spin_unlock(&cache->lock);

        goto out;
    }

out:
    TRACE("Got %s %d, start block %lld, refcount %d, error %d\n",
        cache->name, i, entry->block, entry->refcount, entry->error);

    if (entry->error)
        ERROR("Unable to read %s cache entry [%llx]\n", cache->name,
                            block);
    return entry;
}


/*
 * Release cache entry, once usage count is zero it can be reused.
 */
void squashfs_cache_put(struct squashfs_cache_entry *entry)
{
    struct squashfs_cache *cache = entry->cache;

    spin_lock(&cache->lock);
    entry->refcount--;
    if (entry->refcount == 0) {
        cache->unused++;
        /*
         * If there's any processes waiting for a block to become
         * available, wake one up.
         */
        if (cache->num_waiters) {
            spin_unlock(&cache->lock);
            wake_up(&cache->wait_queue);
            return;
        }
    }
    spin_unlock(&cache->lock);
}

/*
 * Delete cache reclaiming all kmalloced buffers.
 */
void squashfs_cache_delete(struct squashfs_cache *cache)
{
    int i, j;

    if (cache == NULL)
        return;

    for (i = 0; i < cache->entries; i++) {
        if (cache->entry[i].data) {
            for (j = 0; j < cache->pages; j++)
                kfree(cache->entry[i].data[j]);
            kfree(cache->entry[i].data);
        }
    }

    kfree(cache->entry);
    kfree(cache);
}


/*
 * Initialise cache allocating the specified number of entries, each of
 * size block_size.  To avoid vmalloc fragmentation issues each entry
 * is allocated as a sequence of kmalloced PAGE_CACHE_SIZE buffers.
 */
struct squashfs_cache *squashfs_cache_init(char *name, int entries,
    int block_size)
{
    int i, j;
    struct squashfs_cache *cache = kzalloc(sizeof(*cache), GFP_KERNEL);

    if (cache == NULL) {
        ERROR("Failed to allocate %s cache\n", name);
        return NULL;
    }

    cache->entry = kcalloc(entries, sizeof(*(cache->entry)), GFP_KERNEL);
    if (cache->entry == NULL) {
        ERROR("Failed to allocate %s cache\n", name);
        goto cleanup;
    }

    cache->curr_blk = 0;
    cache->next_blk = 0;
    cache->unused = entries;
    cache->entries = entries;
    cache->block_size = block_size;
    cache->pages = block_size >> PAGE_CACHE_SHIFT;
    cache->pages = cache->pages ? cache->pages : 1;
    cache->name = name;
    cache->num_waiters = 0;
    spin_lock_init(&cache->lock);
    init_waitqueue_head(&cache->wait_queue);

    for (i = 0; i < entries; i++) {
        struct squashfs_cache_entry *entry = &cache->entry[i];

        init_waitqueue_head(&cache->entry[i].wait_queue);
        entry->cache = cache;
        entry->block = SQUASHFS_INVALID_BLK;
        entry->data = kcalloc(cache->pages, sizeof(void *), GFP_KERNEL);
        if (entry->data == NULL) {
            ERROR("Failed to allocate %s cache entry\n", name);
            goto cleanup;
        }

        for (j = 0; j < cache->pages; j++) {
            entry->data[j] = kmalloc(PAGE_CACHE_SIZE, GFP_KERNEL);
            if (entry->data[j] == NULL) {
                ERROR("Failed to allocate %s buffer\n", name);
                goto cleanup;
            }
        }
    }

    return cache;

cleanup:
    squashfs_cache_delete(cache);
    return NULL;
}


/*
 * Copy up to length bytes from cache entry to buffer starting at offset bytes
 * into the cache entry.  If there's not length bytes then copy the number of
 * bytes available.  In all cases return the number of bytes copied.
 */
int squashfs_copy_data(void *buffer, struct squashfs_cache_entry *entry,
        int offset, int length)
{
    int remaining = length;

    if (length == 0)
        return 0;
    else if (buffer == NULL)
        return min(length, entry->length - offset);

    while (offset < entry->length) {
        void *buff = entry->data[offset / PAGE_CACHE_SIZE]
                + (offset % PAGE_CACHE_SIZE);
        int bytes = min_t(int, entry->length - offset,
                PAGE_CACHE_SIZE - (offset % PAGE_CACHE_SIZE));

        if (bytes >= remaining) {
            memcpy(buffer, buff, remaining);
            remaining = 0;
            break;
        }

        memcpy(buffer, buff, bytes);
        buffer += bytes;
        remaining -= bytes;
        offset += bytes;
    }

    return length - remaining;
}


/*
 * Read length bytes from metadata position <block, offset> (block is the
 * start of the compressed block on disk, and offset is the offset into
 * the block once decompressed).  Data is packed into consecutive blocks,
 * and length bytes may require reading more than one block.
 */
int squashfs_read_metadata(struct super_block *sb, void *buffer,
        u64 *block, int *offset, int length)
{
    struct squashfs_sb_info *msblk = sb->s_fs_info;
    int bytes, res = length;
    struct squashfs_cache_entry *entry;

    TRACE("Entered squashfs_read_metadata [%llx:%x]\n", *block, *offset);

    while (length) {
        entry = squashfs_cache_get(sb, msblk->block_cache, *block, 0);
        if (entry->error) {
            res = entry->error;
            goto error;
        } else if (*offset >= entry->length) {
            res = -EIO;
            goto error;
        }

        bytes = squashfs_copy_data(buffer, entry, *offset, length);
        if (buffer)
            buffer += bytes;
        length -= bytes;
        *offset += bytes;

        if (*offset == entry->length) {
            *block = entry->next_index;
            *offset = 0;
        }

        squashfs_cache_put(entry);
    }

    return res;

error:
    squashfs_cache_put(entry);
    return res;
}


/*
 * Look-up in the fragmment cache the fragment located at <start_block> in the
 * filesystem.  If necessary read and decompress it from disk.
 */
struct squashfs_cache_entry *squashfs_get_fragment(struct super_block *sb,
                u64 start_block, int length)
{
    struct squashfs_sb_info *msblk = sb->s_fs_info;

    return squashfs_cache_get(sb, msblk->fragment_cache, start_block,
        length);
}


/*
 * Read and decompress the datablock located at <start_block> in the
 * filesystem.  The cache is used here to avoid duplicating locking and
 * read/decompress code.
 */
struct squashfs_cache_entry *squashfs_get_datablock(struct super_block *sb,
                u64 start_block, int length)
{
    struct squashfs_sb_info *msblk = sb->s_fs_info;

    return squashfs_cache_get(sb, msblk->read_page, start_block, length);
}


/*
 * Read a filesystem table (uncompressed sequence of bytes) from disk
 */
void *squashfs_read_table(struct super_block *sb, u64 block, int length)
{
    int pages = (length + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
    int i, res;
    void *table, *buffer, **data;

    table = buffer = kmalloc(length, GFP_KERNEL);
    if (table == NULL)
        return ERR_PTR(-ENOMEM);

    data = kcalloc(pages, sizeof(void *), GFP_KERNEL);
    if (data == NULL) {
        res = -ENOMEM;
        goto failed;
    }

    for (i = 0; i < pages; i++, buffer += PAGE_CACHE_SIZE)
        data[i] = buffer;

    res = squashfs_read_data(sb, data, block, length |
        SQUASHFS_COMPRESSED_BIT_BLOCK, NULL, length, pages);

    kfree(data);

    if (res < 0)
        goto failed;

    return table;

failed:
    kfree(table);
    return ERR_PTR(res);
}