/* * zbud.c * * Copyright (C) 2013, Seth Jennings, IBM * * Concepts based on zcache internal zbud allocator by Dan Magenheimer. * * zbud is an special purpose allocator for storing compressed pages. Contrary * to what its name may suggest, zbud is not a buddy allocator, but rather an * allocator that "buddies" two compressed pages together in a single memory * page. * * While this design limits storage density, it has simple and deterministic * reclaim properties that make it preferable to a higher density approach when * reclaim will be used. * * zbud works by storing compressed pages, or "zpages", together in pairs in a * single memory page called a "zbud page". The first buddy is "left * justified" at the beginning of the zbud page, and the last buddy is "right * justified" at the end of the zbud page. The benefit is that if either * buddy is freed, the freed buddy space, coalesced with whatever slack space * that existed between the buddies, results in the largest possible free region * within the zbud page. * * zbud also provides an attractive lower bound on density. The ratio of zpages * to zbud pages can not be less than 1. This ensures that zbud can never "do * harm" by using more pages to store zpages than the uncompressed zpages would * have used on their own. * * zbud pages are divided into "chunks". The size of the chunks is fixed at * compile time and determined by NCHUNKS_ORDER below. Dividing zbud pages * into chunks allows organizing unbuddied zbud pages into a manageable number * of unbuddied lists according to the number of free chunks available in the * zbud page. * * The zbud API differs from that of conventional allocators in that the * allocation function, zbud_alloc(), returns an opaque handle to the user, * not a dereferenceable pointer. The user must map the handle using * zbud_map() in order to get a usable pointer by which to access the * allocation data and unmap the handle with zbud_unmap() when operations * on the allocation data are complete. */ #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt #include <linux/atomic.h> #include <linux/list.h> #include <linux/mm.h> #include <linux/module.h> #include <linux/preempt.h> #include <linux/slab.h> #include <linux/spinlock.h> #include <linux/zbud.h> #include <linux/zpool.h> /***************** * Structures *****************/ /* * NCHUNKS_ORDER determines the internal allocation granularity, effectively * adjusting internal fragmentation. It also determines the number of * freelists maintained in each pool. NCHUNKS_ORDER of 6 means that the * allocation granularity will be in chunks of size PAGE_SIZE/64. As one chunk * in allocated page is occupied by zbud header, NCHUNKS will be calculated to * 63 which shows the max number of free chunks in zbud page, also there will be * 63 freelists per pool. */ #define NCHUNKS_ORDER 6 #define CHUNK_SHIFT (PAGE_SHIFT - NCHUNKS_ORDER) #define CHUNK_SIZE (1 << CHUNK_SHIFT) #define ZHDR_SIZE_ALIGNED CHUNK_SIZE #define NCHUNKS ((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT) /** * struct zbud_pool - stores metadata for each zbud pool * @lock: protects all pool fields and first|last_chunk fields of any * zbud page in the pool * @unbuddied: array of lists tracking zbud pages that only contain one buddy; * the lists each zbud page is added to depends on the size of * its free region. * @buddied: list tracking the zbud pages that contain two buddies; * these zbud pages are full * @lru: list tracking the zbud pages in LRU order by most recently * added buddy. * @pages_nr: number of zbud pages in the pool. * @ops: pointer to a structure of user defined operations specified at * pool creation time. * * This structure is allocated at pool creation time and maintains metadata * pertaining to a particular zbud pool. */ struct zbud_pool { spinlock_t lock; struct list_head unbuddied[NCHUNKS]; struct list_head buddied; struct list_head lru; u64 pages_nr; const struct zbud_ops *ops; #ifdef CONFIG_ZPOOL struct zpool *zpool; const struct zpool_ops *zpool_ops; #endif }; /* * struct zbud_header - zbud page metadata occupying the first chunk of each * zbud page. * @buddy: links the zbud page into the unbuddied/buddied lists in the pool * @lru: links the zbud page into the lru list in the pool * @first_chunks: the size of the first buddy in chunks, 0 if free * @last_chunks: the size of the last buddy in chunks, 0 if free */ struct zbud_header { struct list_head buddy; struct list_head lru; unsigned int first_chunks; unsigned int last_chunks; bool under_reclaim; }; /***************** * zpool ****************/ #ifdef CONFIG_ZPOOL static int zbud_zpool_evict(struct zbud_pool *pool, unsigned long handle) { if (pool->zpool && pool->zpool_ops && pool->zpool_ops->evict) return pool->zpool_ops->evict(pool->zpool, handle); else return -ENOENT; } static const struct zbud_ops zbud_zpool_ops = { .evict = zbud_zpool_evict }; static void *zbud_zpool_create(const char *name, gfp_t gfp, const struct zpool_ops *zpool_ops, struct zpool *zpool) { struct zbud_pool *pool; pool = zbud_create_pool(gfp, zpool_ops ? &zbud_zpool_ops : NULL); if (pool) { pool->zpool = zpool; pool->zpool_ops = zpool_ops; } return pool; } static void zbud_zpool_destroy(void *pool) { zbud_destroy_pool(pool); } static int zbud_zpool_malloc(void *pool, size_t size, gfp_t gfp, unsigned long *handle) { return zbud_alloc(pool, size, gfp, handle); } static void zbud_zpool_free(void *pool, unsigned long handle) { zbud_free(pool, handle); } static int zbud_zpool_shrink(void *pool, unsigned int pages, unsigned int *reclaimed) { unsigned int total = 0; int ret = -EINVAL; while (total < pages) { ret = zbud_reclaim_page(pool, 8); if (ret < 0) break; total++; } if (reclaimed) *reclaimed = total; return ret; } static void *zbud_zpool_map(void *pool, unsigned long handle, enum zpool_mapmode mm) { return zbud_map(pool, handle); } static void zbud_zpool_unmap(void *pool, unsigned long handle) { zbud_unmap(pool, handle); } static u64 zbud_zpool_total_size(void *pool) { return zbud_get_pool_size(pool) * PAGE_SIZE; } static struct zpool_driver zbud_zpool_driver = { .type = "zbud", .owner = THIS_MODULE, .create = zbud_zpool_create, .destroy = zbud_zpool_destroy, .malloc = zbud_zpool_malloc, .free = zbud_zpool_free, .shrink = zbud_zpool_shrink, .map = zbud_zpool_map, .unmap = zbud_zpool_unmap, .total_size = zbud_zpool_total_size, }; MODULE_ALIAS("zpool-zbud"); #endif /* CONFIG_ZPOOL */ /***************** * Helpers *****************/ /* Just to make the code easier to read */ enum buddy { FIRST, LAST }; /* Converts an allocation size in bytes to size in zbud chunks */ static int size_to_chunks(size_t size) { return (size + CHUNK_SIZE - 1) >> CHUNK_SHIFT; } #define for_each_unbuddied_list(_iter, _begin) \ for ((_iter) = (_begin); (_iter) < NCHUNKS; (_iter)++) /* Initializes the zbud header of a newly allocated zbud page */ static struct zbud_header *init_zbud_page(struct page *page) { struct zbud_header *zhdr = page_address(page); zhdr->first_chunks = 0; zhdr->last_chunks = 0; INIT_LIST_HEAD(&zhdr->buddy); INIT_LIST_HEAD(&zhdr->lru); zhdr->under_reclaim = 0; return zhdr; } /* Resets the struct page fields and frees the page */ static void free_zbud_page(struct zbud_header *zhdr) { __free_page(virt_to_page(zhdr)); } /* * Encodes the handle of a particular buddy within a zbud page * Pool lock should be held as this function accesses first|last_chunks */ static unsigned long encode_handle(struct zbud_header *zhdr, enum buddy bud) { unsigned long handle; /* * For now, the encoded handle is actually just the pointer to the data * but this might not always be the case. A little information hiding. * Add CHUNK_SIZE to the handle if it is the first allocation to jump * over the zbud header in the first chunk. */ handle = (unsigned long)zhdr; if (bud == FIRST) /* skip over zbud header */ handle += ZHDR_SIZE_ALIGNED; else /* bud == LAST */ handle += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT); return handle; } /* Returns the zbud page where a given handle is stored */ static struct zbud_header *handle_to_zbud_header(unsigned long handle) { return (struct zbud_header *)(handle & PAGE_MASK); } /* Returns the number of free chunks in a zbud page */ static int num_free_chunks(struct zbud_header *zhdr) { /* * Rather than branch for different situations, just use the fact that * free buddies have a length of zero to simplify everything. */ return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks; } /***************** * API Functions *****************/ /** * zbud_create_pool() - create a new zbud pool * @gfp: gfp flags when allocating the zbud pool structure * @ops: user-defined operations for the zbud pool * * Return: pointer to the new zbud pool or NULL if the metadata allocation * failed. */ struct zbud_pool *zbud_create_pool(gfp_t gfp, const struct zbud_ops *ops) { struct zbud_pool *pool; int i; pool = kzalloc(sizeof(struct zbud_pool), gfp); if (!pool) return NULL; spin_lock_init(&pool->lock); for_each_unbuddied_list(i, 0) INIT_LIST_HEAD(&pool->unbuddied[i]); INIT_LIST_HEAD(&pool->buddied); INIT_LIST_HEAD(&pool->lru); pool->pages_nr = 0; pool->ops = ops; return pool; } /** * zbud_destroy_pool() - destroys an existing zbud pool * @pool: the zbud pool to be destroyed * * The pool should be emptied before this function is called. */ void zbud_destroy_pool(struct zbud_pool *pool) { kfree(pool); } /** * zbud_alloc() - allocates a region of a given size * @pool: zbud pool from which to allocate * @size: size in bytes of the desired allocation * @gfp: gfp flags used if the pool needs to grow * @handle: handle of the new allocation * * This function will attempt to find a free region in the pool large enough to * satisfy the allocation request. A search of the unbuddied lists is * performed first. If no suitable free region is found, then a new page is * allocated and added to the pool to satisfy the request. * * gfp should not set __GFP_HIGHMEM as highmem pages cannot be used * as zbud pool pages. * * Return: 0 if success and handle is set, otherwise -EINVAL if the size or * gfp arguments are invalid or -ENOMEM if the pool was unable to allocate * a new page. */ int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp, unsigned long *handle) { int chunks, i, freechunks; struct zbud_header *zhdr = NULL; enum buddy bud; struct page *page; if (!size || (gfp & __GFP_HIGHMEM)) return -EINVAL; if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE) return -ENOSPC; chunks = size_to_chunks(size); spin_lock(&pool->lock); /* First, try to find an unbuddied zbud page. */ zhdr = NULL; for_each_unbuddied_list(i, chunks) { if (!list_empty(&pool->unbuddied[i])) { zhdr = list_first_entry(&pool->unbuddied[i], struct zbud_header, buddy); list_del(&zhdr->buddy); if (zhdr->first_chunks == 0) bud = FIRST; else bud = LAST; goto found; } } /* Couldn't find unbuddied zbud page, create new one */ spin_unlock(&pool->lock); page = alloc_page(gfp); if (!page) return -ENOMEM; spin_lock(&pool->lock); pool->pages_nr++; zhdr = init_zbud_page(page); bud = FIRST; found: if (bud == FIRST) zhdr->first_chunks = chunks; else zhdr->last_chunks = chunks; if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) { /* Add to unbuddied list */ freechunks = num_free_chunks(zhdr); list_add(&zhdr->buddy, &pool->unbuddied[freechunks]); } else { /* Add to buddied list */ list_add(&zhdr->buddy, &pool->buddied); } /* Add/move zbud page to beginning of LRU */ if (!list_empty(&zhdr->lru)) list_del(&zhdr->lru); list_add(&zhdr->lru, &pool->lru); *handle = encode_handle(zhdr, bud); spin_unlock(&pool->lock); return 0; } /** * zbud_free() - frees the allocation associated with the given handle * @pool: pool in which the allocation resided * @handle: handle associated with the allocation returned by zbud_alloc() * * In the case that the zbud page in which the allocation resides is under * reclaim, as indicated by the PG_reclaim flag being set, this function * only sets the first|last_chunks to 0. The page is actually freed * once both buddies are evicted (see zbud_reclaim_page() below). */ void zbud_free(struct zbud_pool *pool, unsigned long handle) { struct zbud_header *zhdr; int freechunks; spin_lock(&pool->lock); zhdr = handle_to_zbud_header(handle); /* If first buddy, handle will be page aligned */ if ((handle - ZHDR_SIZE_ALIGNED) & ~PAGE_MASK) zhdr->last_chunks = 0; else zhdr->first_chunks = 0; if (zhdr->under_reclaim) { /* zbud page is under reclaim, reclaim will free */ spin_unlock(&pool->lock); return; } /* Remove from existing buddy list */ list_del(&zhdr->buddy); if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) { /* zbud page is empty, free */ list_del(&zhdr->lru); free_zbud_page(zhdr); pool->pages_nr--; } else { /* Add to unbuddied list */ freechunks = num_free_chunks(zhdr); list_add(&zhdr->buddy, &pool->unbuddied[freechunks]); } spin_unlock(&pool->lock); } /** * zbud_reclaim_page() - evicts allocations from a pool page and frees it * @pool: pool from which a page will attempt to be evicted * @retries: number of pages on the LRU list for which eviction will * be attempted before failing * * zbud reclaim is different from normal system reclaim in that the reclaim is * done from the bottom, up. This is because only the bottom layer, zbud, has * information on how the allocations are organized within each zbud page. This * has the potential to create interesting locking situations between zbud and * the user, however. * * To avoid these, this is how zbud_reclaim_page() should be called: * * The user detects a page should be reclaimed and calls zbud_reclaim_page(). * zbud_reclaim_page() will remove a zbud page from the pool LRU list and call * the user-defined eviction handler with the pool and handle as arguments. * * If the handle can not be evicted, the eviction handler should return * non-zero. zbud_reclaim_page() will add the zbud page back to the * appropriate list and try the next zbud page on the LRU up to * a user defined number of retries. * * If the handle is successfully evicted, the eviction handler should * return 0 _and_ should have called zbud_free() on the handle. zbud_free() * contains logic to delay freeing the page if the page is under reclaim, * as indicated by the setting of the PG_reclaim flag on the underlying page. * * If all buddies in the zbud page are successfully evicted, then the * zbud page can be freed. * * Returns: 0 if page is successfully freed, otherwise -EINVAL if there are * no pages to evict or an eviction handler is not registered, -EAGAIN if * the retry limit was hit. */ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries) { int i, ret, freechunks; struct zbud_header *zhdr; unsigned long first_handle = 0, last_handle = 0; spin_lock(&pool->lock); if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) || retries == 0) { spin_unlock(&pool->lock); return -EINVAL; } for (i = 0; i < retries; i++) { zhdr = list_last_entry(&pool->lru, struct zbud_header, lru); list_del(&zhdr->lru); list_del(&zhdr->buddy); /* Protect zbud page against free */ zhdr->under_reclaim = true; /* * We need encode the handles before unlocking, since we can * race with free that will set (first|last)_chunks to 0 */ first_handle = 0; last_handle = 0; if (zhdr->first_chunks) first_handle = encode_handle(zhdr, FIRST); if (zhdr->last_chunks) last_handle = encode_handle(zhdr, LAST); spin_unlock(&pool->lock); /* Issue the eviction callback(s) */ if (first_handle) { ret = pool->ops->evict(pool, first_handle); if (ret) goto next; } if (last_handle) { ret = pool->ops->evict(pool, last_handle); if (ret) goto next; } next: spin_lock(&pool->lock); zhdr->under_reclaim = false; if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) { /* * Both buddies are now free, free the zbud page and * return success. */ free_zbud_page(zhdr); pool->pages_nr--; spin_unlock(&pool->lock); return 0; } else if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) { /* add to unbuddied list */ freechunks = num_free_chunks(zhdr); list_add(&zhdr->buddy, &pool->unbuddied[freechunks]); } else { /* add to buddied list */ list_add(&zhdr->buddy, &pool->buddied); } /* add to beginning of LRU */ list_add(&zhdr->lru, &pool->lru); } spin_unlock(&pool->lock); return -EAGAIN; } /** * zbud_map() - maps the allocation associated with the given handle * @pool: pool in which the allocation resides * @handle: handle associated with the allocation to be mapped * * While trivial for zbud, the mapping functions for others allocators * implementing this allocation API could have more complex information encoded * in the handle and could create temporary mappings to make the data * accessible to the user. * * Returns: a pointer to the mapped allocation */ void *zbud_map(struct zbud_pool *pool, unsigned long handle) { return (void *)(handle); } /** * zbud_unmap() - maps the allocation associated with the given handle * @pool: pool in which the allocation resides * @handle: handle associated with the allocation to be unmapped */ void zbud_unmap(struct zbud_pool *pool, unsigned long handle) { } /** * zbud_get_pool_size() - gets the zbud pool size in pages * @pool: pool whose size is being queried * * Returns: size in pages of the given pool. The pool lock need not be * taken to access pages_nr. */ u64 zbud_get_pool_size(struct zbud_pool *pool) { return pool->pages_nr; } static int __init init_zbud(void) { /* Make sure the zbud header will fit in one chunk */ BUILD_BUG_ON(sizeof(struct zbud_header) > ZHDR_SIZE_ALIGNED); pr_info("loaded\n"); #ifdef CONFIG_ZPOOL zpool_register_driver(&zbud_zpool_driver); #endif return 0; } static void __exit exit_zbud(void) { #ifdef CONFIG_ZPOOL zpool_unregister_driver(&zbud_zpool_driver); #endif pr_info("unloaded\n"); } module_init(init_zbud); module_exit(exit_zbud); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Seth Jennings <sjennings@variantweb.net>"); MODULE_DESCRIPTION("Buddy Allocator for Compressed Pages");