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Linux/mm/zbud.c

  1 /*
  2  * zbud.c
  3  *
  4  * Copyright (C) 2013, Seth Jennings, IBM
  5  *
  6  * Concepts based on zcache internal zbud allocator by Dan Magenheimer.
  7  *
  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
 11  * page.
 12  *
 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.
 16  *
 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.
 24  *
 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.
 29  *
 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
 34  * zbud page.
 35  *
 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.
 42  */
 43 
 44 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 45 
 46 #include <linux/atomic.h>
 47 #include <linux/list.h>
 48 #include <linux/mm.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>
 55 
 56 /*****************
 57  * Structures
 58 *****************/
 59 /*
 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.
 67  */
 68 #define NCHUNKS_ORDER   6
 69 
 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)
 74 
 75 /**
 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
 81  *              its free region.
 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
 85  *              added buddy.
 86  * @pages_nr:   number of zbud pages in the pool.
 87  * @ops:        pointer to a structure of user defined operations specified at
 88  *              pool creation time.
 89  *
 90  * This structure is allocated at pool creation time and maintains metadata
 91  * pertaining to a particular zbud pool.
 92  */
 93 struct zbud_pool {
 94         spinlock_t lock;
 95         struct list_head unbuddied[NCHUNKS];
 96         struct list_head buddied;
 97         struct list_head lru;
 98         u64 pages_nr;
 99         const struct zbud_ops *ops;
100 #ifdef CONFIG_ZPOOL
101         struct zpool *zpool;
102         const struct zpool_ops *zpool_ops;
103 #endif
104 };
105 
106 /*
107  * struct zbud_header - zbud page metadata occupying the first chunk of each
108  *                      zbud page.
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
113  */
114 struct zbud_header {
115         struct list_head buddy;
116         struct list_head lru;
117         unsigned int first_chunks;
118         unsigned int last_chunks;
119         bool under_reclaim;
120 };
121 
122 /*****************
123  * zpool
124  ****************/
125 
126 #ifdef CONFIG_ZPOOL
127 
128 static int zbud_zpool_evict(struct zbud_pool *pool, unsigned long handle)
129 {
130         if (pool->zpool && pool->zpool_ops && pool->zpool_ops->evict)
131                 return pool->zpool_ops->evict(pool->zpool, handle);
132         else
133                 return -ENOENT;
134 }
135 
136 static const struct zbud_ops zbud_zpool_ops = {
137         .evict =        zbud_zpool_evict
138 };
139 
140 static void *zbud_zpool_create(const char *name, gfp_t gfp,
141                                const struct zpool_ops *zpool_ops,
142                                struct zpool *zpool)
143 {
144         struct zbud_pool *pool;
145 
146         pool = zbud_create_pool(gfp, zpool_ops ? &zbud_zpool_ops : NULL);
147         if (pool) {
148                 pool->zpool = zpool;
149                 pool->zpool_ops = zpool_ops;
150         }
151         return pool;
152 }
153 
154 static void zbud_zpool_destroy(void *pool)
155 {
156         zbud_destroy_pool(pool);
157 }
158 
159 static int zbud_zpool_malloc(void *pool, size_t size, gfp_t gfp,
160                         unsigned long *handle)
161 {
162         return zbud_alloc(pool, size, gfp, handle);
163 }
164 static void zbud_zpool_free(void *pool, unsigned long handle)
165 {
166         zbud_free(pool, handle);
167 }
168 
169 static int zbud_zpool_shrink(void *pool, unsigned int pages,
170                         unsigned int *reclaimed)
171 {
172         unsigned int total = 0;
173         int ret = -EINVAL;
174 
175         while (total < pages) {
176                 ret = zbud_reclaim_page(pool, 8);
177                 if (ret < 0)
178                         break;
179                 total++;
180         }
181 
182         if (reclaimed)
183                 *reclaimed = total;
184 
185         return ret;
186 }
187 
188 static void *zbud_zpool_map(void *pool, unsigned long handle,
189                         enum zpool_mapmode mm)
190 {
191         return zbud_map(pool, handle);
192 }
193 static void zbud_zpool_unmap(void *pool, unsigned long handle)
194 {
195         zbud_unmap(pool, handle);
196 }
197 
198 static u64 zbud_zpool_total_size(void *pool)
199 {
200         return zbud_get_pool_size(pool) * PAGE_SIZE;
201 }
202 
203 static struct zpool_driver zbud_zpool_driver = {
204         .type =         "zbud",
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,
214 };
215 
216 MODULE_ALIAS("zpool-zbud");
217 #endif /* CONFIG_ZPOOL */
218 
219 /*****************
220  * Helpers
221 *****************/
222 /* Just to make the code easier to read */
223 enum buddy {
224         FIRST,
225         LAST
226 };
227 
228 /* Converts an allocation size in bytes to size in zbud chunks */
229 static int size_to_chunks(size_t size)
230 {
231         return (size + CHUNK_SIZE - 1) >> CHUNK_SHIFT;
232 }
233 
234 #define for_each_unbuddied_list(_iter, _begin) \
235         for ((_iter) = (_begin); (_iter) < NCHUNKS; (_iter)++)
236 
237 /* Initializes the zbud header of a newly allocated zbud page */
238 static struct zbud_header *init_zbud_page(struct page *page)
239 {
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;
246         return zhdr;
247 }
248 
249 /* Resets the struct page fields and frees the page */
250 static void free_zbud_page(struct zbud_header *zhdr)
251 {
252         __free_page(virt_to_page(zhdr));
253 }
254 
255 /*
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
258  */
259 static unsigned long encode_handle(struct zbud_header *zhdr, enum buddy bud)
260 {
261         unsigned long handle;
262 
263         /*
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.
268          */
269         handle = (unsigned long)zhdr;
270         if (bud == FIRST)
271                 /* skip over zbud header */
272                 handle += ZHDR_SIZE_ALIGNED;
273         else /* bud == LAST */
274                 handle += PAGE_SIZE - (zhdr->last_chunks  << CHUNK_SHIFT);
275         return handle;
276 }
277 
278 /* Returns the zbud page where a given handle is stored */
279 static struct zbud_header *handle_to_zbud_header(unsigned long handle)
280 {
281         return (struct zbud_header *)(handle & PAGE_MASK);
282 }
283 
284 /* Returns the number of free chunks in a zbud page */
285 static int num_free_chunks(struct zbud_header *zhdr)
286 {
287         /*
288          * Rather than branch for different situations, just use the fact that
289          * free buddies have a length of zero to simplify everything.
290          */
291         return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks;
292 }
293 
294 /*****************
295  * API Functions
296 *****************/
297 /**
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
301  *
302  * Return: pointer to the new zbud pool or NULL if the metadata allocation
303  * failed.
304  */
305 struct zbud_pool *zbud_create_pool(gfp_t gfp, const struct zbud_ops *ops)
306 {
307         struct zbud_pool *pool;
308         int i;
309 
310         pool = kzalloc(sizeof(struct zbud_pool), gfp);
311         if (!pool)
312                 return NULL;
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);
318         pool->pages_nr = 0;
319         pool->ops = ops;
320         return pool;
321 }
322 
323 /**
324  * zbud_destroy_pool() - destroys an existing zbud pool
325  * @pool:       the zbud pool to be destroyed
326  *
327  * The pool should be emptied before this function is called.
328  */
329 void zbud_destroy_pool(struct zbud_pool *pool)
330 {
331         kfree(pool);
332 }
333 
334 /**
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
340  *
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.
345  *
346  * gfp should not set __GFP_HIGHMEM as highmem pages cannot be used
347  * as zbud pool pages.
348  *
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
351  * a new page.
352  */
353 int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
354                         unsigned long *handle)
355 {
356         int chunks, i, freechunks;
357         struct zbud_header *zhdr = NULL;
358         enum buddy bud;
359         struct page *page;
360 
361         if (!size || (gfp & __GFP_HIGHMEM))
362                 return -EINVAL;
363         if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
364                 return -ENOSPC;
365         chunks = size_to_chunks(size);
366         spin_lock(&pool->lock);
367 
368         /* First, try to find an unbuddied zbud page. */
369         zhdr = NULL;
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)
376                                 bud = FIRST;
377                         else
378                                 bud = LAST;
379                         goto found;
380                 }
381         }
382 
383         /* Couldn't find unbuddied zbud page, create new one */
384         spin_unlock(&pool->lock);
385         page = alloc_page(gfp);
386         if (!page)
387                 return -ENOMEM;
388         spin_lock(&pool->lock);
389         pool->pages_nr++;
390         zhdr = init_zbud_page(page);
391         bud = FIRST;
392 
393 found:
394         if (bud == FIRST)
395                 zhdr->first_chunks = chunks;
396         else
397                 zhdr->last_chunks = chunks;
398 
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]);
403         } else {
404                 /* Add to buddied list */
405                 list_add(&zhdr->buddy, &pool->buddied);
406         }
407 
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);
412 
413         *handle = encode_handle(zhdr, bud);
414         spin_unlock(&pool->lock);
415 
416         return 0;
417 }
418 
419 /**
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()
423  *
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).
428  */
429 void zbud_free(struct zbud_pool *pool, unsigned long handle)
430 {
431         struct zbud_header *zhdr;
432         int freechunks;
433 
434         spin_lock(&pool->lock);
435         zhdr = handle_to_zbud_header(handle);
436 
437         /* If first buddy, handle will be page aligned */
438         if ((handle - ZHDR_SIZE_ALIGNED) & ~PAGE_MASK)
439                 zhdr->last_chunks = 0;
440         else
441                 zhdr->first_chunks = 0;
442 
443         if (zhdr->under_reclaim) {
444                 /* zbud page is under reclaim, reclaim will free */
445                 spin_unlock(&pool->lock);
446                 return;
447         }
448 
449         /* Remove from existing buddy list */
450         list_del(&zhdr->buddy);
451 
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);
456                 pool->pages_nr--;
457         } else {
458                 /* Add to unbuddied list */
459                 freechunks = num_free_chunks(zhdr);
460                 list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
461         }
462 
463         spin_unlock(&pool->lock);
464 }
465 
466 /**
467  * zbud_reclaim_page() - evicts allocations from a pool page and frees it
468  * @pool:       pool from which a page will attempt to be evicted
469  * @retires:    number of pages on the LRU list for which eviction will
470  *              be attempted before failing
471  *
472  * zbud reclaim is different from normal system reclaim in that the reclaim is
473  * done from the bottom, up.  This is because only the bottom layer, zbud, has
474  * information on how the allocations are organized within each zbud page. This
475  * has the potential to create interesting locking situations between zbud and
476  * the user, however.
477  *
478  * To avoid these, this is how zbud_reclaim_page() should be called:
479 
480  * The user detects a page should be reclaimed and calls zbud_reclaim_page().
481  * zbud_reclaim_page() will remove a zbud page from the pool LRU list and call
482  * the user-defined eviction handler with the pool and handle as arguments.
483  *
484  * If the handle can not be evicted, the eviction handler should return
485  * non-zero. zbud_reclaim_page() will add the zbud page back to the
486  * appropriate list and try the next zbud page on the LRU up to
487  * a user defined number of retries.
488  *
489  * If the handle is successfully evicted, the eviction handler should
490  * return 0 _and_ should have called zbud_free() on the handle. zbud_free()
491  * contains logic to delay freeing the page if the page is under reclaim,
492  * as indicated by the setting of the PG_reclaim flag on the underlying page.
493  *
494  * If all buddies in the zbud page are successfully evicted, then the
495  * zbud page can be freed.
496  *
497  * Returns: 0 if page is successfully freed, otherwise -EINVAL if there are
498  * no pages to evict or an eviction handler is not registered, -EAGAIN if
499  * the retry limit was hit.
500  */
501 int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
502 {
503         int i, ret, freechunks;
504         struct zbud_header *zhdr;
505         unsigned long first_handle = 0, last_handle = 0;
506 
507         spin_lock(&pool->lock);
508         if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
509                         retries == 0) {
510                 spin_unlock(&pool->lock);
511                 return -EINVAL;
512         }
513         for (i = 0; i < retries; i++) {
514                 zhdr = list_last_entry(&pool->lru, struct zbud_header, lru);
515                 list_del(&zhdr->lru);
516                 list_del(&zhdr->buddy);
517                 /* Protect zbud page against free */
518                 zhdr->under_reclaim = true;
519                 /*
520                  * We need encode the handles before unlocking, since we can
521                  * race with free that will set (first|last)_chunks to 0
522                  */
523                 first_handle = 0;
524                 last_handle = 0;
525                 if (zhdr->first_chunks)
526                         first_handle = encode_handle(zhdr, FIRST);
527                 if (zhdr->last_chunks)
528                         last_handle = encode_handle(zhdr, LAST);
529                 spin_unlock(&pool->lock);
530 
531                 /* Issue the eviction callback(s) */
532                 if (first_handle) {
533                         ret = pool->ops->evict(pool, first_handle);
534                         if (ret)
535                                 goto next;
536                 }
537                 if (last_handle) {
538                         ret = pool->ops->evict(pool, last_handle);
539                         if (ret)
540                                 goto next;
541                 }
542 next:
543                 spin_lock(&pool->lock);
544                 zhdr->under_reclaim = false;
545                 if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
546                         /*
547                          * Both buddies are now free, free the zbud page and
548                          * return success.
549                          */
550                         free_zbud_page(zhdr);
551                         pool->pages_nr--;
552                         spin_unlock(&pool->lock);
553                         return 0;
554                 } else if (zhdr->first_chunks == 0 ||
555                                 zhdr->last_chunks == 0) {
556                         /* add to unbuddied list */
557                         freechunks = num_free_chunks(zhdr);
558                         list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
559                 } else {
560                         /* add to buddied list */
561                         list_add(&zhdr->buddy, &pool->buddied);
562                 }
563 
564                 /* add to beginning of LRU */
565                 list_add(&zhdr->lru, &pool->lru);
566         }
567         spin_unlock(&pool->lock);
568         return -EAGAIN;
569 }
570 
571 /**
572  * zbud_map() - maps the allocation associated with the given handle
573  * @pool:       pool in which the allocation resides
574  * @handle:     handle associated with the allocation to be mapped
575  *
576  * While trivial for zbud, the mapping functions for others allocators
577  * implementing this allocation API could have more complex information encoded
578  * in the handle and could create temporary mappings to make the data
579  * accessible to the user.
580  *
581  * Returns: a pointer to the mapped allocation
582  */
583 void *zbud_map(struct zbud_pool *pool, unsigned long handle)
584 {
585         return (void *)(handle);
586 }
587 
588 /**
589  * zbud_unmap() - maps the allocation associated with the given handle
590  * @pool:       pool in which the allocation resides
591  * @handle:     handle associated with the allocation to be unmapped
592  */
593 void zbud_unmap(struct zbud_pool *pool, unsigned long handle)
594 {
595 }
596 
597 /**
598  * zbud_get_pool_size() - gets the zbud pool size in pages
599  * @pool:       pool whose size is being queried
600  *
601  * Returns: size in pages of the given pool.  The pool lock need not be
602  * taken to access pages_nr.
603  */
604 u64 zbud_get_pool_size(struct zbud_pool *pool)
605 {
606         return pool->pages_nr;
607 }
608 
609 static int __init init_zbud(void)
610 {
611         /* Make sure the zbud header will fit in one chunk */
612         BUILD_BUG_ON(sizeof(struct zbud_header) > ZHDR_SIZE_ALIGNED);
613         pr_info("loaded\n");
614 
615 #ifdef CONFIG_ZPOOL
616         zpool_register_driver(&zbud_zpool_driver);
617 #endif
618 
619         return 0;
620 }
621 
622 static void __exit exit_zbud(void)
623 {
624 #ifdef CONFIG_ZPOOL
625         zpool_unregister_driver(&zbud_zpool_driver);
626 #endif
627 
628         pr_info("unloaded\n");
629 }
630 
631 module_init(init_zbud);
632 module_exit(exit_zbud);
633 
634 MODULE_LICENSE("GPL");
635 MODULE_AUTHOR("Seth Jennings <sjennings@variantweb.net>");
636 MODULE_DESCRIPTION("Buddy Allocator for Compressed Pages");
637 

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