linux/fs/squashfs/cache.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Squashfs - a compressed read only filesystem for Linux
 *
 * Copyright (c) 2002, 2003, 2004, 2005, 2006, 2007, 2008
 * Phillip Lougher <[email protected]>
 *
 * 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_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"
#include "page_actor.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)
{ … }


/*
 * Release cache entry, once usage count is zero it can be reused.
 */
void squashfs_cache_put(struct squashfs_cache_entry *entry)
{ … }

/*
 * Delete cache reclaiming all kmalloced buffers.
 */
void squashfs_cache_delete(struct squashfs_cache *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_SIZE buffers.
 */
struct squashfs_cache *squashfs_cache_init(char *name, int entries,
	int block_size)
{ … }


/*
 * 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)
{ … }


/*
 * 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)
{ … }


/*
 * 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)
{ … }


/*
 * 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)
{ … }


/*
 * Read a filesystem table (uncompressed sequence of bytes) from disk
 */
void *squashfs_read_table(struct super_block *sb, u64 block, int length)
{ … }