mbcache.c 18.3 KB
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679
/*
 * linux/fs/mbcache.c
 * (C) 2001-2002 Andreas Gruenbacher, <a.gruenbacher@computer.org>
 */

/*
 * Filesystem Meta Information Block Cache (mbcache)
 *
 * The mbcache caches blocks of block devices that need to be located
 * by their device/block number, as well as by other criteria (such
 * as the block's contents).
 *
 * There can only be one cache entry in a cache per device and block number.
 * Additional indexes need not be unique in this sense. The number of
 * additional indexes (=other criteria) can be hardwired at compile time
 * or specified at cache create time.
 *
 * Each cache entry is of fixed size. An entry may be `valid' or `invalid'
 * in the cache. A valid entry is in the main hash tables of the cache,
 * and may also be in the lru list. An invalid entry is not in any hashes
 * or lists.
 *
 * A valid cache entry is only in the lru list if no handles refer to it.
 * Invalid cache entries will be freed when the last handle to the cache
 * entry is released. Entries that cannot be freed immediately are put
 * back on the lru list.
 */

#include <linux/kernel.h>
#include <linux/module.h>

#include <linux/hash.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/mbcache.h>


#ifdef MB_CACHE_DEBUG
# define mb_debug(f...) do { \
		printk(KERN_DEBUG f); \
		printk("\n"); \
	} while (0)
#define mb_assert(c) do { if (!(c)) \
		printk(KERN_ERR "assertion " #c " failed\n"); \
	} while(0)
#else
# define mb_debug(f...) do { } while(0)
# define mb_assert(c) do { } while(0)
#endif
#define mb_error(f...) do { \
		printk(KERN_ERR f); \
		printk("\n"); \
	} while(0)

#define MB_CACHE_WRITER ((unsigned short)~0U >> 1)

static DECLARE_WAIT_QUEUE_HEAD(mb_cache_queue);
		
MODULE_AUTHOR("Andreas Gruenbacher <a.gruenbacher@computer.org>");
MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
MODULE_LICENSE("GPL");

EXPORT_SYMBOL(mb_cache_create);
EXPORT_SYMBOL(mb_cache_shrink);
EXPORT_SYMBOL(mb_cache_destroy);
EXPORT_SYMBOL(mb_cache_entry_alloc);
EXPORT_SYMBOL(mb_cache_entry_insert);
EXPORT_SYMBOL(mb_cache_entry_release);
EXPORT_SYMBOL(mb_cache_entry_free);
EXPORT_SYMBOL(mb_cache_entry_get);
#if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)
EXPORT_SYMBOL(mb_cache_entry_find_first);
EXPORT_SYMBOL(mb_cache_entry_find_next);
#endif

struct mb_cache {
	struct list_head		c_cache_list;
	const char			*c_name;
	struct mb_cache_op		c_op;
	atomic_t			c_entry_count;
	int				c_bucket_bits;
#ifndef MB_CACHE_INDEXES_COUNT
	int				c_indexes_count;
#endif
	struct kmem_cache			*c_entry_cache;
	struct list_head		*c_block_hash;
	struct list_head		*c_indexes_hash[0];
};


/*
 * Global data: list of all mbcache's, lru list, and a spinlock for
 * accessing cache data structures on SMP machines. The lru list is
 * global across all mbcaches.
 */

static LIST_HEAD(mb_cache_list);
static LIST_HEAD(mb_cache_lru_list);
static DEFINE_SPINLOCK(mb_cache_spinlock);

static inline int
mb_cache_indexes(struct mb_cache *cache)
{
#ifdef MB_CACHE_INDEXES_COUNT
	return MB_CACHE_INDEXES_COUNT;
#else
	return cache->c_indexes_count;
#endif
}

/*
 * What the mbcache registers as to get shrunk dynamically.
 */

static int mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask);

static struct shrinker mb_cache_shrinker = {
	.shrink = mb_cache_shrink_fn,
	.seeks = DEFAULT_SEEKS,
};

static inline int
__mb_cache_entry_is_hashed(struct mb_cache_entry *ce)
{
	return !list_empty(&ce->e_block_list);
}


static void
__mb_cache_entry_unhash(struct mb_cache_entry *ce)
{
	int n;

	if (__mb_cache_entry_is_hashed(ce)) {
		list_del_init(&ce->e_block_list);
		for (n=0; n<mb_cache_indexes(ce->e_cache); n++)
			list_del(&ce->e_indexes[n].o_list);
	}
}


static void
__mb_cache_entry_forget(struct mb_cache_entry *ce, gfp_t gfp_mask)
{
	struct mb_cache *cache = ce->e_cache;

	mb_assert(!(ce->e_used || ce->e_queued));
	if (cache->c_op.free && cache->c_op.free(ce, gfp_mask)) {
		/* free failed -- put back on the lru list
		   for freeing later. */
		spin_lock(&mb_cache_spinlock);
		list_add(&ce->e_lru_list, &mb_cache_lru_list);
		spin_unlock(&mb_cache_spinlock);
	} else {
		kmem_cache_free(cache->c_entry_cache, ce);
		atomic_dec(&cache->c_entry_count);
	}
}


static void
__mb_cache_entry_release_unlock(struct mb_cache_entry *ce)
	__releases(mb_cache_spinlock)
{
	/* Wake up all processes queuing for this cache entry. */
	if (ce->e_queued)
		wake_up_all(&mb_cache_queue);
	if (ce->e_used >= MB_CACHE_WRITER)
		ce->e_used -= MB_CACHE_WRITER;
	ce->e_used--;
	if (!(ce->e_used || ce->e_queued)) {
		if (!__mb_cache_entry_is_hashed(ce))
			goto forget;
		mb_assert(list_empty(&ce->e_lru_list));
		list_add_tail(&ce->e_lru_list, &mb_cache_lru_list);
	}
	spin_unlock(&mb_cache_spinlock);
	return;
forget:
	spin_unlock(&mb_cache_spinlock);
	__mb_cache_entry_forget(ce, GFP_KERNEL);
}


/*
 * mb_cache_shrink_fn()  memory pressure callback
 *
 * This function is called by the kernel memory management when memory
 * gets low.
 *
 * @nr_to_scan: Number of objects to scan
 * @gfp_mask: (ignored)
 *
 * Returns the number of objects which are present in the cache.
 */
static int
mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
{
	LIST_HEAD(free_list);
	struct list_head *l, *ltmp;
	int count = 0;

	spin_lock(&mb_cache_spinlock);
	list_for_each(l, &mb_cache_list) {
		struct mb_cache *cache =
			list_entry(l, struct mb_cache, c_cache_list);
		mb_debug("cache %s (%d)", cache->c_name,
			  atomic_read(&cache->c_entry_count));
		count += atomic_read(&cache->c_entry_count);
	}
	mb_debug("trying to free %d entries", nr_to_scan);
	if (nr_to_scan == 0) {
		spin_unlock(&mb_cache_spinlock);
		goto out;
	}
	while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
		struct mb_cache_entry *ce =
			list_entry(mb_cache_lru_list.next,
				   struct mb_cache_entry, e_lru_list);
		list_move_tail(&ce->e_lru_list, &free_list);
		__mb_cache_entry_unhash(ce);
	}
	spin_unlock(&mb_cache_spinlock);
	list_for_each_safe(l, ltmp, &free_list) {
		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
						   e_lru_list), gfp_mask);
	}
out:
	return (count / 100) * sysctl_vfs_cache_pressure;
}


/*
 * mb_cache_create()  create a new cache
 *
 * All entries in one cache are equal size. Cache entries may be from
 * multiple devices. If this is the first mbcache created, registers
 * the cache with kernel memory management. Returns NULL if no more
 * memory was available.
 *
 * @name: name of the cache (informal)
 * @cache_op: contains the callback called when freeing a cache entry
 * @entry_size: The size of a cache entry, including
 *              struct mb_cache_entry
 * @indexes_count: number of additional indexes in the cache. Must equal
 *                 MB_CACHE_INDEXES_COUNT if the number of indexes is
 *                 hardwired.
 * @bucket_bits: log2(number of hash buckets)
 */
struct mb_cache *
mb_cache_create(const char *name, struct mb_cache_op *cache_op,
		size_t entry_size, int indexes_count, int bucket_bits)
{
	int m=0, n, bucket_count = 1 << bucket_bits;
	struct mb_cache *cache = NULL;

	if(entry_size < sizeof(struct mb_cache_entry) +
	   indexes_count * sizeof(((struct mb_cache_entry *) 0)->e_indexes[0]))
		return NULL;

	cache = kmalloc(sizeof(struct mb_cache) +
	                indexes_count * sizeof(struct list_head), GFP_KERNEL);
	if (!cache)
		goto fail;
	cache->c_name = name;
	cache->c_op.free = NULL;
	if (cache_op)
		cache->c_op.free = cache_op->free;
	atomic_set(&cache->c_entry_count, 0);
	cache->c_bucket_bits = bucket_bits;
#ifdef MB_CACHE_INDEXES_COUNT
	mb_assert(indexes_count == MB_CACHE_INDEXES_COUNT);
#else
	cache->c_indexes_count = indexes_count;
#endif
	cache->c_block_hash = kmalloc(bucket_count * sizeof(struct list_head),
	                              GFP_KERNEL);
	if (!cache->c_block_hash)
		goto fail;
	for (n=0; n<bucket_count; n++)
		INIT_LIST_HEAD(&cache->c_block_hash[n]);
	for (m=0; m<indexes_count; m++) {
		cache->c_indexes_hash[m] = kmalloc(bucket_count *
		                                 sizeof(struct list_head),
		                                 GFP_KERNEL);
		if (!cache->c_indexes_hash[m])
			goto fail;
		for (n=0; n<bucket_count; n++)
			INIT_LIST_HEAD(&cache->c_indexes_hash[m][n]);
	}
	cache->c_entry_cache = kmem_cache_create(name, entry_size, 0,
		SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
	if (!cache->c_entry_cache)
		goto fail;

	spin_lock(&mb_cache_spinlock);
	list_add(&cache->c_cache_list, &mb_cache_list);
	spin_unlock(&mb_cache_spinlock);
	return cache;

fail:
	if (cache) {
		while (--m >= 0)
			kfree(cache->c_indexes_hash[m]);
		kfree(cache->c_block_hash);
		kfree(cache);
	}
	return NULL;
}


/*
 * mb_cache_shrink()
 *
 * Removes all cache entries of a device from the cache. All cache entries
 * currently in use cannot be freed, and thus remain in the cache. All others
 * are freed.
 *
 * @bdev: which device's cache entries to shrink
 */
void
mb_cache_shrink(struct block_device *bdev)
{
	LIST_HEAD(free_list);
	struct list_head *l, *ltmp;

	spin_lock(&mb_cache_spinlock);
	list_for_each_safe(l, ltmp, &mb_cache_lru_list) {
		struct mb_cache_entry *ce =
			list_entry(l, struct mb_cache_entry, e_lru_list);
		if (ce->e_bdev == bdev) {
			list_move_tail(&ce->e_lru_list, &free_list);
			__mb_cache_entry_unhash(ce);
		}
	}
	spin_unlock(&mb_cache_spinlock);
	list_for_each_safe(l, ltmp, &free_list) {
		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
						   e_lru_list), GFP_KERNEL);
	}
}


/*
 * mb_cache_destroy()
 *
 * Shrinks the cache to its minimum possible size (hopefully 0 entries),
 * and then destroys it. If this was the last mbcache, un-registers the
 * mbcache from kernel memory management.
 */
void
mb_cache_destroy(struct mb_cache *cache)
{
	LIST_HEAD(free_list);
	struct list_head *l, *ltmp;
	int n;

	spin_lock(&mb_cache_spinlock);
	list_for_each_safe(l, ltmp, &mb_cache_lru_list) {
		struct mb_cache_entry *ce =
			list_entry(l, struct mb_cache_entry, e_lru_list);
		if (ce->e_cache == cache) {
			list_move_tail(&ce->e_lru_list, &free_list);
			__mb_cache_entry_unhash(ce);
		}
	}
	list_del(&cache->c_cache_list);
	spin_unlock(&mb_cache_spinlock);

	list_for_each_safe(l, ltmp, &free_list) {
		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
						   e_lru_list), GFP_KERNEL);
	}

	if (atomic_read(&cache->c_entry_count) > 0) {
		mb_error("cache %s: %d orphaned entries",
			  cache->c_name,
			  atomic_read(&cache->c_entry_count));
	}

	kmem_cache_destroy(cache->c_entry_cache);

	for (n=0; n < mb_cache_indexes(cache); n++)
		kfree(cache->c_indexes_hash[n]);
	kfree(cache->c_block_hash);
	kfree(cache);
}


/*
 * mb_cache_entry_alloc()
 *
 * Allocates a new cache entry. The new entry will not be valid initially,
 * and thus cannot be looked up yet. It should be filled with data, and
 * then inserted into the cache using mb_cache_entry_insert(). Returns NULL
 * if no more memory was available.
 */
struct mb_cache_entry *
mb_cache_entry_alloc(struct mb_cache *cache, gfp_t gfp_flags)
{
	struct mb_cache_entry *ce;

	ce = kmem_cache_alloc(cache->c_entry_cache, gfp_flags);
	if (ce) {
		atomic_inc(&cache->c_entry_count);
		INIT_LIST_HEAD(&ce->e_lru_list);
		INIT_LIST_HEAD(&ce->e_block_list);
		ce->e_cache = cache;
		ce->e_used = 1 + MB_CACHE_WRITER;
		ce->e_queued = 0;
	}
	return ce;
}


/*
 * mb_cache_entry_insert()
 *
 * Inserts an entry that was allocated using mb_cache_entry_alloc() into
 * the cache. After this, the cache entry can be looked up, but is not yet
 * in the lru list as the caller still holds a handle to it. Returns 0 on
 * success, or -EBUSY if a cache entry for that device + inode exists
 * already (this may happen after a failed lookup, but when another process
 * has inserted the same cache entry in the meantime).
 *
 * @bdev: device the cache entry belongs to
 * @block: block number
 * @keys: array of additional keys. There must be indexes_count entries
 *        in the array (as specified when creating the cache).
 */
int
mb_cache_entry_insert(struct mb_cache_entry *ce, struct block_device *bdev,
		      sector_t block, unsigned int keys[])
{
	struct mb_cache *cache = ce->e_cache;
	unsigned int bucket;
	struct list_head *l;
	int error = -EBUSY, n;

	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff), 
			   cache->c_bucket_bits);
	spin_lock(&mb_cache_spinlock);
	list_for_each_prev(l, &cache->c_block_hash[bucket]) {
		struct mb_cache_entry *ce =
			list_entry(l, struct mb_cache_entry, e_block_list);
		if (ce->e_bdev == bdev && ce->e_block == block)
			goto out;
	}
	__mb_cache_entry_unhash(ce);
	ce->e_bdev = bdev;
	ce->e_block = block;
	list_add(&ce->e_block_list, &cache->c_block_hash[bucket]);
	for (n=0; n<mb_cache_indexes(cache); n++) {
		ce->e_indexes[n].o_key = keys[n];
		bucket = hash_long(keys[n], cache->c_bucket_bits);
		list_add(&ce->e_indexes[n].o_list,
			 &cache->c_indexes_hash[n][bucket]);
	}
	error = 0;
out:
	spin_unlock(&mb_cache_spinlock);
	return error;
}


/*
 * mb_cache_entry_release()
 *
 * Release a handle to a cache entry. When the last handle to a cache entry
 * is released it is either freed (if it is invalid) or otherwise inserted
 * in to the lru list.
 */
void
mb_cache_entry_release(struct mb_cache_entry *ce)
{
	spin_lock(&mb_cache_spinlock);
	__mb_cache_entry_release_unlock(ce);
}


/*
 * mb_cache_entry_free()
 *
 * This is equivalent to the sequence mb_cache_entry_takeout() --
 * mb_cache_entry_release().
 */
void
mb_cache_entry_free(struct mb_cache_entry *ce)
{
	spin_lock(&mb_cache_spinlock);
	mb_assert(list_empty(&ce->e_lru_list));
	__mb_cache_entry_unhash(ce);
	__mb_cache_entry_release_unlock(ce);
}


/*
 * mb_cache_entry_get()
 *
 * Get a cache entry  by device / block number. (There can only be one entry
 * in the cache per device and block.) Returns NULL if no such cache entry
 * exists. The returned cache entry is locked for exclusive access ("single
 * writer").
 */
struct mb_cache_entry *
mb_cache_entry_get(struct mb_cache *cache, struct block_device *bdev,
		   sector_t block)
{
	unsigned int bucket;
	struct list_head *l;
	struct mb_cache_entry *ce;

	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff),
			   cache->c_bucket_bits);
	spin_lock(&mb_cache_spinlock);
	list_for_each(l, &cache->c_block_hash[bucket]) {
		ce = list_entry(l, struct mb_cache_entry, e_block_list);
		if (ce->e_bdev == bdev && ce->e_block == block) {
			DEFINE_WAIT(wait);

			if (!list_empty(&ce->e_lru_list))
				list_del_init(&ce->e_lru_list);

			while (ce->e_used > 0) {
				ce->e_queued++;
				prepare_to_wait(&mb_cache_queue, &wait,
						TASK_UNINTERRUPTIBLE);
				spin_unlock(&mb_cache_spinlock);
				schedule();
				spin_lock(&mb_cache_spinlock);
				ce->e_queued--;
			}
			finish_wait(&mb_cache_queue, &wait);
			ce->e_used += 1 + MB_CACHE_WRITER;

			if (!__mb_cache_entry_is_hashed(ce)) {
				__mb_cache_entry_release_unlock(ce);
				return NULL;
			}
			goto cleanup;
		}
	}
	ce = NULL;

cleanup:
	spin_unlock(&mb_cache_spinlock);
	return ce;
}

#if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)

static struct mb_cache_entry *
__mb_cache_entry_find(struct list_head *l, struct list_head *head,
		      int index, struct block_device *bdev, unsigned int key)
{
	while (l != head) {
		struct mb_cache_entry *ce =
			list_entry(l, struct mb_cache_entry,
			           e_indexes[index].o_list);
		if (ce->e_bdev == bdev && ce->e_indexes[index].o_key == key) {
			DEFINE_WAIT(wait);

			if (!list_empty(&ce->e_lru_list))
				list_del_init(&ce->e_lru_list);

			/* Incrementing before holding the lock gives readers
			   priority over writers. */
			ce->e_used++;
			while (ce->e_used >= MB_CACHE_WRITER) {
				ce->e_queued++;
				prepare_to_wait(&mb_cache_queue, &wait,
						TASK_UNINTERRUPTIBLE);
				spin_unlock(&mb_cache_spinlock);
				schedule();
				spin_lock(&mb_cache_spinlock);
				ce->e_queued--;
			}
			finish_wait(&mb_cache_queue, &wait);

			if (!__mb_cache_entry_is_hashed(ce)) {
				__mb_cache_entry_release_unlock(ce);
				spin_lock(&mb_cache_spinlock);
				return ERR_PTR(-EAGAIN);
			}
			return ce;
		}
		l = l->next;
	}
	return NULL;
}


/*
 * mb_cache_entry_find_first()
 *
 * Find the first cache entry on a given device with a certain key in
 * an additional index. Additonal matches can be found with
 * mb_cache_entry_find_next(). Returns NULL if no match was found. The
 * returned cache entry is locked for shared access ("multiple readers").
 *
 * @cache: the cache to search
 * @index: the number of the additonal index to search (0<=index<indexes_count)
 * @bdev: the device the cache entry should belong to
 * @key: the key in the index
 */
struct mb_cache_entry *
mb_cache_entry_find_first(struct mb_cache *cache, int index,
			  struct block_device *bdev, unsigned int key)
{
	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
	struct list_head *l;
	struct mb_cache_entry *ce;

	mb_assert(index < mb_cache_indexes(cache));
	spin_lock(&mb_cache_spinlock);
	l = cache->c_indexes_hash[index][bucket].next;
	ce = __mb_cache_entry_find(l, &cache->c_indexes_hash[index][bucket],
	                           index, bdev, key);
	spin_unlock(&mb_cache_spinlock);
	return ce;
}


/*
 * mb_cache_entry_find_next()
 *
 * Find the next cache entry on a given device with a certain key in an
 * additional index. Returns NULL if no match could be found. The previous
 * entry is atomatically released, so that mb_cache_entry_find_next() can
 * be called like this:
 *
 * entry = mb_cache_entry_find_first();
 * while (entry) {
 * 	...
 *	entry = mb_cache_entry_find_next(entry, ...);
 * }
 *
 * @prev: The previous match
 * @index: the number of the additonal index to search (0<=index<indexes_count)
 * @bdev: the device the cache entry should belong to
 * @key: the key in the index
 */
struct mb_cache_entry *
mb_cache_entry_find_next(struct mb_cache_entry *prev, int index,
			 struct block_device *bdev, unsigned int key)
{
	struct mb_cache *cache = prev->e_cache;
	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
	struct list_head *l;
	struct mb_cache_entry *ce;

	mb_assert(index < mb_cache_indexes(cache));
	spin_lock(&mb_cache_spinlock);
	l = prev->e_indexes[index].o_list.next;
	ce = __mb_cache_entry_find(l, &cache->c_indexes_hash[index][bucket],
	                           index, bdev, key);
	__mb_cache_entry_release_unlock(prev);
	return ce;
}

#endif  /* !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0) */

static int __init init_mbcache(void)
{
	register_shrinker(&mb_cache_shrinker);
	return 0;
}

static void __exit exit_mbcache(void)
{
	unregister_shrinker(&mb_cache_shrinker);
}

module_init(init_mbcache)
module_exit(exit_mbcache)