4 * Copyright (C) 2013, Seth Jennings, IBM
6 * Concepts based on zcache internal zbud allocator by Dan Magenheimer.
8 * zbud is an special purpose allocator for storing compressed pages. Contrary
9 * to what its name may suggest, zbud is not a buddy allocator, but rather an
10 * allocator that "buddies" two compressed pages together in a single memory
13 * While this design limits storage density, it has simple and deterministic
14 * reclaim properties that make it preferable to a higher density approach when
15 * reclaim will be used.
17 * zbud works by storing compressed pages, or "zpages", together in pairs in a
18 * single memory page called a "zbud page". The first buddy is "left
19 * justified" at the beginning of the zbud page, and the last buddy is "right
20 * justified" at the end of the zbud page. The benefit is that if either
21 * buddy is freed, the freed buddy space, coalesced with whatever slack space
22 * that existed between the buddies, results in the largest possible free region
23 * within the zbud page.
25 * zbud also provides an attractive lower bound on density. The ratio of zpages
26 * to zbud pages can not be less than 1. This ensures that zbud can never "do
27 * harm" by using more pages to store zpages than the uncompressed zpages would
28 * have used on their own.
30 * zbud pages are divided into "chunks". The size of the chunks is fixed at
31 * compile time and determined by NCHUNKS_ORDER below. Dividing zbud pages
32 * into chunks allows organizing unbuddied zbud pages into a manageable number
33 * of unbuddied lists according to the number of free chunks available in the
36 * The zbud API differs from that of conventional allocators in that the
37 * allocation function, zbud_alloc(), returns an opaque handle to the user,
38 * not a dereferenceable pointer. The user must map the handle using
39 * zbud_map() in order to get a usable pointer by which to access the
40 * allocation data and unmap the handle with zbud_unmap() when operations
41 * on the allocation data are complete.
44 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
46 #include <linux/atomic.h>
47 #include <linux/list.h>
49 #include <linux/module.h>
50 #include <linux/preempt.h>
51 #include <linux/slab.h>
52 #include <linux/spinlock.h>
53 #include <linux/zbud.h>
54 #include <linux/zpool.h>
60 * NCHUNKS_ORDER determines the internal allocation granularity, effectively
61 * adjusting internal fragmentation. It also determines the number of
62 * freelists maintained in each pool. NCHUNKS_ORDER of 6 means that the
63 * allocation granularity will be in chunks of size PAGE_SIZE/64. As one chunk
64 * in allocated page is occupied by zbud header, NCHUNKS will be calculated to
65 * 63 which shows the max number of free chunks in zbud page, also there will be
66 * 63 freelists per pool.
68 #define NCHUNKS_ORDER 6
70 #define CHUNK_SHIFT (PAGE_SHIFT - NCHUNKS_ORDER)
71 #define CHUNK_SIZE (1 << CHUNK_SHIFT)
72 #define ZHDR_SIZE_ALIGNED CHUNK_SIZE
73 #define NCHUNKS ((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT)
76 * struct zbud_pool - stores metadata for each zbud pool
77 * @lock: protects all pool fields and first|last_chunk fields of any
78 * zbud page in the pool
79 * @unbuddied: array of lists tracking zbud pages that only contain one buddy;
80 * the lists each zbud page is added to depends on the size of
82 * @buddied: list tracking the zbud pages that contain two buddies;
83 * these zbud pages are full
84 * @lru: list tracking the zbud pages in LRU order by most recently
86 * @pages_nr: number of zbud pages in the pool.
87 * @ops: pointer to a structure of user defined operations specified at
90 * This structure is allocated at pool creation time and maintains metadata
91 * pertaining to a particular zbud pool.
95 struct list_head unbuddied[NCHUNKS];
96 struct list_head buddied;
99 const struct zbud_ops *ops;
102 const struct zpool_ops *zpool_ops;
107 * struct zbud_header - zbud page metadata occupying the first chunk of each
109 * @buddy: links the zbud page into the unbuddied/buddied lists in the pool
110 * @lru: links the zbud page into the lru list in the pool
111 * @first_chunks: the size of the first buddy in chunks, 0 if free
112 * @last_chunks: the size of the last buddy in chunks, 0 if free
115 struct list_head buddy;
116 struct list_head lru;
117 unsigned int first_chunks;
118 unsigned int last_chunks;
128 static int zbud_zpool_evict(struct zbud_pool *pool, unsigned long handle)
130 if (pool->zpool && pool->zpool_ops && pool->zpool_ops->evict)
131 return pool->zpool_ops->evict(pool->zpool, handle);
136 static const struct zbud_ops zbud_zpool_ops = {
137 .evict = zbud_zpool_evict
140 static void *zbud_zpool_create(const char *name, gfp_t gfp,
141 const struct zpool_ops *zpool_ops,
144 struct zbud_pool *pool;
146 pool = zbud_create_pool(gfp, zpool_ops ? &zbud_zpool_ops : NULL);
149 pool->zpool_ops = zpool_ops;
154 static void zbud_zpool_destroy(void *pool)
156 zbud_destroy_pool(pool);
159 static int zbud_zpool_malloc(void *pool, size_t size, gfp_t gfp,
160 unsigned long *handle)
162 return zbud_alloc(pool, size, gfp, handle);
164 static void zbud_zpool_free(void *pool, unsigned long handle)
166 zbud_free(pool, handle);
169 static int zbud_zpool_shrink(void *pool, unsigned int pages,
170 unsigned int *reclaimed)
172 unsigned int total = 0;
175 while (total < pages) {
176 ret = zbud_reclaim_page(pool, 8);
188 static void *zbud_zpool_map(void *pool, unsigned long handle,
189 enum zpool_mapmode mm)
191 return zbud_map(pool, handle);
193 static void zbud_zpool_unmap(void *pool, unsigned long handle)
195 zbud_unmap(pool, handle);
198 static u64 zbud_zpool_total_size(void *pool)
200 return zbud_get_pool_size(pool) * PAGE_SIZE;
203 static struct zpool_driver zbud_zpool_driver = {
205 .owner = THIS_MODULE,
206 .create = zbud_zpool_create,
207 .destroy = zbud_zpool_destroy,
208 .malloc = zbud_zpool_malloc,
209 .free = zbud_zpool_free,
210 .shrink = zbud_zpool_shrink,
211 .map = zbud_zpool_map,
212 .unmap = zbud_zpool_unmap,
213 .total_size = zbud_zpool_total_size,
216 MODULE_ALIAS("zpool-zbud");
217 #endif /* CONFIG_ZPOOL */
222 /* Just to make the code easier to read */
228 /* Converts an allocation size in bytes to size in zbud chunks */
229 static int size_to_chunks(size_t size)
231 return (size + CHUNK_SIZE - 1) >> CHUNK_SHIFT;
234 #define for_each_unbuddied_list(_iter, _begin) \
235 for ((_iter) = (_begin); (_iter) < NCHUNKS; (_iter)++)
237 /* Initializes the zbud header of a newly allocated zbud page */
238 static struct zbud_header *init_zbud_page(struct page *page)
240 struct zbud_header *zhdr = page_address(page);
241 zhdr->first_chunks = 0;
242 zhdr->last_chunks = 0;
243 INIT_LIST_HEAD(&zhdr->buddy);
244 INIT_LIST_HEAD(&zhdr->lru);
245 zhdr->under_reclaim = 0;
249 /* Resets the struct page fields and frees the page */
250 static void free_zbud_page(struct zbud_header *zhdr)
252 __free_page(virt_to_page(zhdr));
256 * Encodes the handle of a particular buddy within a zbud page
257 * Pool lock should be held as this function accesses first|last_chunks
259 static unsigned long encode_handle(struct zbud_header *zhdr, enum buddy bud)
261 unsigned long handle;
264 * For now, the encoded handle is actually just the pointer to the data
265 * but this might not always be the case. A little information hiding.
266 * Add CHUNK_SIZE to the handle if it is the first allocation to jump
267 * over the zbud header in the first chunk.
269 handle = (unsigned long)zhdr;
271 /* skip over zbud header */
272 handle += ZHDR_SIZE_ALIGNED;
273 else /* bud == LAST */
274 handle += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT);
278 /* Returns the zbud page where a given handle is stored */
279 static struct zbud_header *handle_to_zbud_header(unsigned long handle)
281 return (struct zbud_header *)(handle & PAGE_MASK);
284 /* Returns the number of free chunks in a zbud page */
285 static int num_free_chunks(struct zbud_header *zhdr)
288 * Rather than branch for different situations, just use the fact that
289 * free buddies have a length of zero to simplify everything.
291 return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks;
298 * zbud_create_pool() - create a new zbud pool
299 * @gfp: gfp flags when allocating the zbud pool structure
300 * @ops: user-defined operations for the zbud pool
302 * Return: pointer to the new zbud pool or NULL if the metadata allocation
305 struct zbud_pool *zbud_create_pool(gfp_t gfp, const struct zbud_ops *ops)
307 struct zbud_pool *pool;
310 pool = kzalloc(sizeof(struct zbud_pool), gfp);
313 spin_lock_init(&pool->lock);
314 for_each_unbuddied_list(i, 0)
315 INIT_LIST_HEAD(&pool->unbuddied[i]);
316 INIT_LIST_HEAD(&pool->buddied);
317 INIT_LIST_HEAD(&pool->lru);
324 * zbud_destroy_pool() - destroys an existing zbud pool
325 * @pool: the zbud pool to be destroyed
327 * The pool should be emptied before this function is called.
329 void zbud_destroy_pool(struct zbud_pool *pool)
335 * zbud_alloc() - allocates a region of a given size
336 * @pool: zbud pool from which to allocate
337 * @size: size in bytes of the desired allocation
338 * @gfp: gfp flags used if the pool needs to grow
339 * @handle: handle of the new allocation
341 * This function will attempt to find a free region in the pool large enough to
342 * satisfy the allocation request. A search of the unbuddied lists is
343 * performed first. If no suitable free region is found, then a new page is
344 * allocated and added to the pool to satisfy the request.
346 * gfp should not set __GFP_HIGHMEM as highmem pages cannot be used
347 * as zbud pool pages.
349 * Return: 0 if success and handle is set, otherwise -EINVAL if the size or
350 * gfp arguments are invalid or -ENOMEM if the pool was unable to allocate
353 int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
354 unsigned long *handle)
356 int chunks, i, freechunks;
357 struct zbud_header *zhdr = NULL;
361 if (!size || (gfp & __GFP_HIGHMEM))
363 if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
365 chunks = size_to_chunks(size);
366 spin_lock(&pool->lock);
368 /* First, try to find an unbuddied zbud page. */
370 for_each_unbuddied_list(i, chunks) {
371 if (!list_empty(&pool->unbuddied[i])) {
372 zhdr = list_first_entry(&pool->unbuddied[i],
373 struct zbud_header, buddy);
374 list_del(&zhdr->buddy);
375 if (zhdr->first_chunks == 0)
383 /* Couldn't find unbuddied zbud page, create new one */
384 spin_unlock(&pool->lock);
385 page = alloc_page(gfp);
388 spin_lock(&pool->lock);
390 zhdr = init_zbud_page(page);
395 zhdr->first_chunks = chunks;
397 zhdr->last_chunks = chunks;
399 if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) {
400 /* Add to unbuddied list */
401 freechunks = num_free_chunks(zhdr);
402 list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
404 /* Add to buddied list */
405 list_add(&zhdr->buddy, &pool->buddied);
408 /* Add/move zbud page to beginning of LRU */
409 if (!list_empty(&zhdr->lru))
410 list_del(&zhdr->lru);
411 list_add(&zhdr->lru, &pool->lru);
413 *handle = encode_handle(zhdr, bud);
414 spin_unlock(&pool->lock);
420 * zbud_free() - frees the allocation associated with the given handle
421 * @pool: pool in which the allocation resided
422 * @handle: handle associated with the allocation returned by zbud_alloc()
424 * In the case that the zbud page in which the allocation resides is under
425 * reclaim, as indicated by the PG_reclaim flag being set, this function
426 * only sets the first|last_chunks to 0. The page is actually freed
427 * once both buddies are evicted (see zbud_reclaim_page() below).
429 void zbud_free(struct zbud_pool *pool, unsigned long handle)
431 struct zbud_header *zhdr;
434 spin_lock(&pool->lock);
435 zhdr = handle_to_zbud_header(handle);
437 /* If first buddy, handle will be page aligned */
438 if ((handle - ZHDR_SIZE_ALIGNED) & ~PAGE_MASK)
439 zhdr->last_chunks = 0;
441 zhdr->first_chunks = 0;
443 if (zhdr->under_reclaim) {
444 /* zbud page is under reclaim, reclaim will free */
445 spin_unlock(&pool->lock);
449 /* Remove from existing buddy list */
450 list_del(&zhdr->buddy);
452 if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
453 /* zbud page is empty, free */
454 list_del(&zhdr->lru);
455 free_zbud_page(zhdr);
458 /* Add to unbuddied list */
459 freechunks = num_free_chunks(zhdr);
460 list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
463 spin_unlock(&pool->lock);
466 #define list_tail_entry(ptr, type, member) \
467 list_entry((ptr)->prev, type, member)
470 * zbud_reclaim_page() - evicts allocations from a pool page and frees it
471 * @pool: pool from which a page will attempt to be evicted
472 * @retires: number of pages on the LRU list for which eviction will
473 * be attempted before failing
475 * zbud reclaim is different from normal system reclaim in that the reclaim is
476 * done from the bottom, up. This is because only the bottom layer, zbud, has
477 * information on how the allocations are organized within each zbud page. This
478 * has the potential to create interesting locking situations between zbud and
481 * To avoid these, this is how zbud_reclaim_page() should be called:
483 * The user detects a page should be reclaimed and calls zbud_reclaim_page().
484 * zbud_reclaim_page() will remove a zbud page from the pool LRU list and call
485 * the user-defined eviction handler with the pool and handle as arguments.
487 * If the handle can not be evicted, the eviction handler should return
488 * non-zero. zbud_reclaim_page() will add the zbud page back to the
489 * appropriate list and try the next zbud page on the LRU up to
490 * a user defined number of retries.
492 * If the handle is successfully evicted, the eviction handler should
493 * return 0 _and_ should have called zbud_free() on the handle. zbud_free()
494 * contains logic to delay freeing the page if the page is under reclaim,
495 * as indicated by the setting of the PG_reclaim flag on the underlying page.
497 * If all buddies in the zbud page are successfully evicted, then the
498 * zbud page can be freed.
500 * Returns: 0 if page is successfully freed, otherwise -EINVAL if there are
501 * no pages to evict or an eviction handler is not registered, -EAGAIN if
502 * the retry limit was hit.
504 int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
506 int i, ret, freechunks;
507 struct zbud_header *zhdr;
508 unsigned long first_handle = 0, last_handle = 0;
510 spin_lock(&pool->lock);
511 if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
513 spin_unlock(&pool->lock);
516 for (i = 0; i < retries; i++) {
517 zhdr = list_tail_entry(&pool->lru, struct zbud_header, lru);
518 list_del(&zhdr->lru);
519 list_del(&zhdr->buddy);
520 /* Protect zbud page against free */
521 zhdr->under_reclaim = true;
523 * We need encode the handles before unlocking, since we can
524 * race with free that will set (first|last)_chunks to 0
528 if (zhdr->first_chunks)
529 first_handle = encode_handle(zhdr, FIRST);
530 if (zhdr->last_chunks)
531 last_handle = encode_handle(zhdr, LAST);
532 spin_unlock(&pool->lock);
534 /* Issue the eviction callback(s) */
536 ret = pool->ops->evict(pool, first_handle);
541 ret = pool->ops->evict(pool, last_handle);
546 spin_lock(&pool->lock);
547 zhdr->under_reclaim = false;
548 if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
550 * Both buddies are now free, free the zbud page and
553 free_zbud_page(zhdr);
555 spin_unlock(&pool->lock);
557 } else if (zhdr->first_chunks == 0 ||
558 zhdr->last_chunks == 0) {
559 /* add to unbuddied list */
560 freechunks = num_free_chunks(zhdr);
561 list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
563 /* add to buddied list */
564 list_add(&zhdr->buddy, &pool->buddied);
567 /* add to beginning of LRU */
568 list_add(&zhdr->lru, &pool->lru);
570 spin_unlock(&pool->lock);
575 * zbud_map() - maps the allocation associated with the given handle
576 * @pool: pool in which the allocation resides
577 * @handle: handle associated with the allocation to be mapped
579 * While trivial for zbud, the mapping functions for others allocators
580 * implementing this allocation API could have more complex information encoded
581 * in the handle and could create temporary mappings to make the data
582 * accessible to the user.
584 * Returns: a pointer to the mapped allocation
586 void *zbud_map(struct zbud_pool *pool, unsigned long handle)
588 return (void *)(handle);
592 * zbud_unmap() - maps the allocation associated with the given handle
593 * @pool: pool in which the allocation resides
594 * @handle: handle associated with the allocation to be unmapped
596 void zbud_unmap(struct zbud_pool *pool, unsigned long handle)
601 * zbud_get_pool_size() - gets the zbud pool size in pages
602 * @pool: pool whose size is being queried
604 * Returns: size in pages of the given pool. The pool lock need not be
605 * taken to access pages_nr.
607 u64 zbud_get_pool_size(struct zbud_pool *pool)
609 return pool->pages_nr;
612 static int __init init_zbud(void)
614 /* Make sure the zbud header will fit in one chunk */
615 BUILD_BUG_ON(sizeof(struct zbud_header) > ZHDR_SIZE_ALIGNED);
619 zpool_register_driver(&zbud_zpool_driver);
625 static void __exit exit_zbud(void)
628 zpool_unregister_driver(&zbud_zpool_driver);
631 pr_info("unloaded\n");
634 module_init(init_zbud);
635 module_exit(exit_zbud);
637 MODULE_LICENSE("GPL");
638 MODULE_AUTHOR("Seth Jennings <sjennings@variantweb.net>");
639 MODULE_DESCRIPTION("Buddy Allocator for Compressed Pages");