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
 55/*****************
 56 * Structures
 57*****************/
 58/*
 59 * NCHUNKS_ORDER determines the internal allocation granularity, effectively
 60 * adjusting internal fragmentation.  It also determines the number of
 61 * freelists maintained in each pool. NCHUNKS_ORDER of 6 means that the
 62 * allocation granularity will be in chunks of size PAGE_SIZE/64, and there
 63 * will be 64 freelists per pool.
 64 */
 65#define NCHUNKS_ORDER	6
 66
 67#define CHUNK_SHIFT	(PAGE_SHIFT - NCHUNKS_ORDER)
 68#define CHUNK_SIZE	(1 << CHUNK_SHIFT)
 69#define NCHUNKS		(PAGE_SIZE >> CHUNK_SHIFT)
 70#define ZHDR_SIZE_ALIGNED CHUNK_SIZE
 71
 72/**
 73 * struct zbud_pool - stores metadata for each zbud pool
 74 * @lock:	protects all pool fields and first|last_chunk fields of any
 75 *		zbud page in the pool
 76 * @unbuddied:	array of lists tracking zbud pages that only contain one buddy;
 77 *		the lists each zbud page is added to depends on the size of
 78 *		its free region.
 79 * @buddied:	list tracking the zbud pages that contain two buddies;
 80 *		these zbud pages are full
 81 * @lru:	list tracking the zbud pages in LRU order by most recently
 82 *		added buddy.
 83 * @pages_nr:	number of zbud pages in the pool.
 84 * @ops:	pointer to a structure of user defined operations specified at
 85 *		pool creation time.
 86 *
 87 * This structure is allocated at pool creation time and maintains metadata
 88 * pertaining to a particular zbud pool.
 89 */
 90struct zbud_pool {
 91	spinlock_t lock;
 92	struct list_head unbuddied[NCHUNKS];
 93	struct list_head buddied;
 94	struct list_head lru;
 95	u64 pages_nr;
 96	struct zbud_ops *ops;
 97};
 98
 99/*
100 * struct zbud_header - zbud page metadata occupying the first chunk of each
101 *			zbud page.
102 * @buddy:	links the zbud page into the unbuddied/buddied lists in the pool
103 * @lru:	links the zbud page into the lru list in the pool
104 * @first_chunks:	the size of the first buddy in chunks, 0 if free
105 * @last_chunks:	the size of the last buddy in chunks, 0 if free
106 */
107struct zbud_header {
108	struct list_head buddy;
109	struct list_head lru;
110	unsigned int first_chunks;
111	unsigned int last_chunks;
112	bool under_reclaim;
113};
114
115/*****************
116 * Helpers
117*****************/
118/* Just to make the code easier to read */
119enum buddy {
120	FIRST,
121	LAST
122};
123
124/* Converts an allocation size in bytes to size in zbud chunks */
125static int size_to_chunks(int size)
126{
127	return (size + CHUNK_SIZE - 1) >> CHUNK_SHIFT;
128}
129
130#define for_each_unbuddied_list(_iter, _begin) \
131	for ((_iter) = (_begin); (_iter) < NCHUNKS; (_iter)++)
132
133/* Initializes the zbud header of a newly allocated zbud page */
134static struct zbud_header *init_zbud_page(struct page *page)
135{
136	struct zbud_header *zhdr = page_address(page);
137	zhdr->first_chunks = 0;
138	zhdr->last_chunks = 0;
139	INIT_LIST_HEAD(&zhdr->buddy);
140	INIT_LIST_HEAD(&zhdr->lru);
141	zhdr->under_reclaim = 0;
142	return zhdr;
143}
144
145/* Resets the struct page fields and frees the page */
146static void free_zbud_page(struct zbud_header *zhdr)
147{
148	__free_page(virt_to_page(zhdr));
149}
150
151/*
152 * Encodes the handle of a particular buddy within a zbud page
153 * Pool lock should be held as this function accesses first|last_chunks
154 */
155static unsigned long encode_handle(struct zbud_header *zhdr, enum buddy bud)
156{
157	unsigned long handle;
158
159	/*
160	 * For now, the encoded handle is actually just the pointer to the data
161	 * but this might not always be the case.  A little information hiding.
162	 * Add CHUNK_SIZE to the handle if it is the first allocation to jump
163	 * over the zbud header in the first chunk.
164	 */
165	handle = (unsigned long)zhdr;
166	if (bud == FIRST)
167		/* skip over zbud header */
168		handle += ZHDR_SIZE_ALIGNED;
169	else /* bud == LAST */
170		handle += PAGE_SIZE - (zhdr->last_chunks  << CHUNK_SHIFT);
171	return handle;
172}
173
174/* Returns the zbud page where a given handle is stored */
175static struct zbud_header *handle_to_zbud_header(unsigned long handle)
176{
177	return (struct zbud_header *)(handle & PAGE_MASK);
178}
179
180/* Returns the number of free chunks in a zbud page */
181static int num_free_chunks(struct zbud_header *zhdr)
182{
183	/*
184	 * Rather than branch for different situations, just use the fact that
185	 * free buddies have a length of zero to simplify everything. -1 at the
186	 * end for the zbud header.
187	 */
188	return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks - 1;
189}
190
191/*****************
192 * API Functions
193*****************/
194/**
195 * zbud_create_pool() - create a new zbud pool
196 * @gfp:	gfp flags when allocating the zbud pool structure
197 * @ops:	user-defined operations for the zbud pool
198 *
199 * Return: pointer to the new zbud pool or NULL if the metadata allocation
200 * failed.
201 */
202struct zbud_pool *zbud_create_pool(gfp_t gfp, struct zbud_ops *ops)
203{
204	struct zbud_pool *pool;
205	int i;
206
207	pool = kmalloc(sizeof(struct zbud_pool), gfp);
208	if (!pool)
209		return NULL;
210	spin_lock_init(&pool->lock);
211	for_each_unbuddied_list(i, 0)
212		INIT_LIST_HEAD(&pool->unbuddied[i]);
213	INIT_LIST_HEAD(&pool->buddied);
214	INIT_LIST_HEAD(&pool->lru);
215	pool->pages_nr = 0;
216	pool->ops = ops;
217	return pool;
218}
219
220/**
221 * zbud_destroy_pool() - destroys an existing zbud pool
222 * @pool:	the zbud pool to be destroyed
223 *
224 * The pool should be emptied before this function is called.
225 */
226void zbud_destroy_pool(struct zbud_pool *pool)
227{
228	kfree(pool);
229}
230
231/**
232 * zbud_alloc() - allocates a region of a given size
233 * @pool:	zbud pool from which to allocate
234 * @size:	size in bytes of the desired allocation
235 * @gfp:	gfp flags used if the pool needs to grow
236 * @handle:	handle of the new allocation
237 *
238 * This function will attempt to find a free region in the pool large enough to
239 * satisfy the allocation request.  A search of the unbuddied lists is
240 * performed first. If no suitable free region is found, then a new page is
241 * allocated and added to the pool to satisfy the request.
242 *
243 * gfp should not set __GFP_HIGHMEM as highmem pages cannot be used
244 * as zbud pool pages.
245 *
246 * Return: 0 if success and handle is set, otherwise -EINVAL if the size or
247 * gfp arguments are invalid or -ENOMEM if the pool was unable to allocate
248 * a new page.
249 */
250int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
251			unsigned long *handle)
252{
253	int chunks, i, freechunks;
254	struct zbud_header *zhdr = NULL;
255	enum buddy bud;
256	struct page *page;
257
258	if (size <= 0 || gfp & __GFP_HIGHMEM)
259		return -EINVAL;
260	if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
261		return -ENOSPC;
262	chunks = size_to_chunks(size);
263	spin_lock(&pool->lock);
264
265	/* First, try to find an unbuddied zbud page. */
266	zhdr = NULL;
267	for_each_unbuddied_list(i, chunks) {
268		if (!list_empty(&pool->unbuddied[i])) {
269			zhdr = list_first_entry(&pool->unbuddied[i],
270					struct zbud_header, buddy);
271			list_del(&zhdr->buddy);
272			if (zhdr->first_chunks == 0)
273				bud = FIRST;
274			else
275				bud = LAST;
276			goto found;
277		}
278	}
279
280	/* Couldn't find unbuddied zbud page, create new one */
281	spin_unlock(&pool->lock);
282	page = alloc_page(gfp);
283	if (!page)
284		return -ENOMEM;
285	spin_lock(&pool->lock);
286	pool->pages_nr++;
287	zhdr = init_zbud_page(page);
288	bud = FIRST;
289
290found:
291	if (bud == FIRST)
292		zhdr->first_chunks = chunks;
293	else
294		zhdr->last_chunks = chunks;
295
296	if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) {
297		/* Add to unbuddied list */
298		freechunks = num_free_chunks(zhdr);
299		list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
300	} else {
301		/* Add to buddied list */
302		list_add(&zhdr->buddy, &pool->buddied);
303	}
304
305	/* Add/move zbud page to beginning of LRU */
306	if (!list_empty(&zhdr->lru))
307		list_del(&zhdr->lru);
308	list_add(&zhdr->lru, &pool->lru);
309
310	*handle = encode_handle(zhdr, bud);
311	spin_unlock(&pool->lock);
312
313	return 0;
314}
315
316/**
317 * zbud_free() - frees the allocation associated with the given handle
318 * @pool:	pool in which the allocation resided
319 * @handle:	handle associated with the allocation returned by zbud_alloc()
320 *
321 * In the case that the zbud page in which the allocation resides is under
322 * reclaim, as indicated by the PG_reclaim flag being set, this function
323 * only sets the first|last_chunks to 0.  The page is actually freed
324 * once both buddies are evicted (see zbud_reclaim_page() below).
325 */
326void zbud_free(struct zbud_pool *pool, unsigned long handle)
327{
328	struct zbud_header *zhdr;
329	int freechunks;
330
331	spin_lock(&pool->lock);
332	zhdr = handle_to_zbud_header(handle);
333
334	/* If first buddy, handle will be page aligned */
335	if ((handle - ZHDR_SIZE_ALIGNED) & ~PAGE_MASK)
336		zhdr->last_chunks = 0;
337	else
338		zhdr->first_chunks = 0;
339
340	if (zhdr->under_reclaim) {
341		/* zbud page is under reclaim, reclaim will free */
342		spin_unlock(&pool->lock);
343		return;
344	}
345
346	/* Remove from existing buddy list */
347	list_del(&zhdr->buddy);
348
349	if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
350		/* zbud page is empty, free */
351		list_del(&zhdr->lru);
352		free_zbud_page(zhdr);
353		pool->pages_nr--;
354	} else {
355		/* Add to unbuddied list */
356		freechunks = num_free_chunks(zhdr);
357		list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
358	}
359
360	spin_unlock(&pool->lock);
361}
362
363#define list_tail_entry(ptr, type, member) \
364	list_entry((ptr)->prev, type, member)
365
366/**
367 * zbud_reclaim_page() - evicts allocations from a pool page and frees it
368 * @pool:	pool from which a page will attempt to be evicted
369 * @retires:	number of pages on the LRU list for which eviction will
370 *		be attempted before failing
371 *
372 * zbud reclaim is different from normal system reclaim in that the reclaim is
373 * done from the bottom, up.  This is because only the bottom layer, zbud, has
374 * information on how the allocations are organized within each zbud page. This
375 * has the potential to create interesting locking situations between zbud and
376 * the user, however.
377 *
378 * To avoid these, this is how zbud_reclaim_page() should be called:
379
380 * The user detects a page should be reclaimed and calls zbud_reclaim_page().
381 * zbud_reclaim_page() will remove a zbud page from the pool LRU list and call
382 * the user-defined eviction handler with the pool and handle as arguments.
383 *
384 * If the handle can not be evicted, the eviction handler should return
385 * non-zero. zbud_reclaim_page() will add the zbud page back to the
386 * appropriate list and try the next zbud page on the LRU up to
387 * a user defined number of retries.
388 *
389 * If the handle is successfully evicted, the eviction handler should
390 * return 0 _and_ should have called zbud_free() on the handle. zbud_free()
391 * contains logic to delay freeing the page if the page is under reclaim,
392 * as indicated by the setting of the PG_reclaim flag on the underlying page.
393 *
394 * If all buddies in the zbud page are successfully evicted, then the
395 * zbud page can be freed.
396 *
397 * Returns: 0 if page is successfully freed, otherwise -EINVAL if there are
398 * no pages to evict or an eviction handler is not registered, -EAGAIN if
399 * the retry limit was hit.
400 */
401int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
402{
403	int i, ret, freechunks;
404	struct zbud_header *zhdr;
405	unsigned long first_handle = 0, last_handle = 0;
406
407	spin_lock(&pool->lock);
408	if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
409			retries == 0) {
410		spin_unlock(&pool->lock);
411		return -EINVAL;
412	}
413	for (i = 0; i < retries; i++) {
414		zhdr = list_tail_entry(&pool->lru, struct zbud_header, lru);
415		list_del(&zhdr->lru);
416		list_del(&zhdr->buddy);
417		/* Protect zbud page against free */
418		zhdr->under_reclaim = true;
419		/*
420		 * We need encode the handles before unlocking, since we can
421		 * race with free that will set (first|last)_chunks to 0
422		 */
423		first_handle = 0;
424		last_handle = 0;
425		if (zhdr->first_chunks)
426			first_handle = encode_handle(zhdr, FIRST);
427		if (zhdr->last_chunks)
428			last_handle = encode_handle(zhdr, LAST);
429		spin_unlock(&pool->lock);
430
431		/* Issue the eviction callback(s) */
432		if (first_handle) {
433			ret = pool->ops->evict(pool, first_handle);
434			if (ret)
435				goto next;
436		}
437		if (last_handle) {
438			ret = pool->ops->evict(pool, last_handle);
439			if (ret)
440				goto next;
441		}
442next:
443		spin_lock(&pool->lock);
444		zhdr->under_reclaim = false;
445		if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
446			/*
447			 * Both buddies are now free, free the zbud page and
448			 * return success.
449			 */
450			free_zbud_page(zhdr);
451			pool->pages_nr--;
452			spin_unlock(&pool->lock);
453			return 0;
454		} else if (zhdr->first_chunks == 0 ||
455				zhdr->last_chunks == 0) {
456			/* add to unbuddied list */
457			freechunks = num_free_chunks(zhdr);
458			list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
459		} else {
460			/* add to buddied list */
461			list_add(&zhdr->buddy, &pool->buddied);
462		}
463
464		/* add to beginning of LRU */
465		list_add(&zhdr->lru, &pool->lru);
466	}
467	spin_unlock(&pool->lock);
468	return -EAGAIN;
469}
470
471/**
472 * zbud_map() - maps the allocation associated with the given handle
473 * @pool:	pool in which the allocation resides
474 * @handle:	handle associated with the allocation to be mapped
475 *
476 * While trivial for zbud, the mapping functions for others allocators
477 * implementing this allocation API could have more complex information encoded
478 * in the handle and could create temporary mappings to make the data
479 * accessible to the user.
480 *
481 * Returns: a pointer to the mapped allocation
482 */
483void *zbud_map(struct zbud_pool *pool, unsigned long handle)
484{
485	return (void *)(handle);
486}
487
488/**
489 * zbud_unmap() - maps the allocation associated with the given handle
490 * @pool:	pool in which the allocation resides
491 * @handle:	handle associated with the allocation to be unmapped
492 */
493void zbud_unmap(struct zbud_pool *pool, unsigned long handle)
494{
495}
496
497/**
498 * zbud_get_pool_size() - gets the zbud pool size in pages
499 * @pool:	pool whose size is being queried
500 *
501 * Returns: size in pages of the given pool.  The pool lock need not be
502 * taken to access pages_nr.
503 */
504u64 zbud_get_pool_size(struct zbud_pool *pool)
505{
506	return pool->pages_nr;
507}
508
509static int __init init_zbud(void)
510{
511	/* Make sure the zbud header will fit in one chunk */
512	BUILD_BUG_ON(sizeof(struct zbud_header) > ZHDR_SIZE_ALIGNED);
513	pr_info("loaded\n");
514	return 0;
515}
516
517static void __exit exit_zbud(void)
518{
519	pr_info("unloaded\n");
520}
521
522module_init(init_zbud);
523module_exit(exit_zbud);
524
525MODULE_LICENSE("GPL");
526MODULE_AUTHOR("Seth Jennings <sjenning@linux.vnet.ibm.com>");
527MODULE_DESCRIPTION("Buddy Allocator for Compressed Pages");