// SPDX-License-Identifier: GPL-2.0-only /* * 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) … #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/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 … #define CHUNK_SHIFT … #define CHUNK_SIZE … #define ZHDR_SIZE_ALIGNED … #define NCHUNKS … struct zbud_pool; /** * 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 * @pages_nr: number of zbud pages in the pool. * * This structure is allocated at pool creation time and maintains metadata * pertaining to a particular zbud pool. */ struct zbud_pool { … }; /* * 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 * @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 { … }; /***************** * Helpers *****************/ /* Just to make the code easier to read */ enum buddy { … }; /* Converts an allocation size in bytes to size in zbud chunks */ static int size_to_chunks(size_t size) { … } #define for_each_unbuddied_list(_iter, _begin) … /* Initializes the zbud header of a newly allocated zbud page */ static struct zbud_header *init_zbud_page(struct page *page) { … } /* Resets the struct page fields and frees the page */ static void free_zbud_page(struct zbud_header *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) { … } /* Returns the zbud page where a given handle is stored */ static struct zbud_header *handle_to_zbud_header(unsigned long handle) { … } /* Returns the number of free chunks in a zbud page */ static int num_free_chunks(struct zbud_header *zhdr) { … } /***************** * API Functions *****************/ /** * zbud_create_pool() - create a new zbud pool * @gfp: gfp flags when allocating the zbud pool structure * * Return: pointer to the new zbud pool or NULL if the metadata allocation * failed. */ static struct zbud_pool *zbud_create_pool(gfp_t gfp) { … } /** * 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. */ static void zbud_destroy_pool(struct zbud_pool *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. */ static int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp, unsigned long *handle) { … } /** * 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() */ static void zbud_free(struct zbud_pool *pool, unsigned long handle) { … } /** * 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 */ static void *zbud_map(struct zbud_pool *pool, unsigned long 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 */ static void zbud_unmap(struct zbud_pool *pool, unsigned long handle) { … } /** * zbud_get_pool_pages() - 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. */ static u64 zbud_get_pool_pages(struct zbud_pool *pool) { … } /***************** * zpool ****************/ static void *zbud_zpool_create(const char *name, gfp_t gfp) { … } static void zbud_zpool_destroy(void *pool) { … } static int zbud_zpool_malloc(void *pool, size_t size, gfp_t gfp, unsigned long *handle) { … } static void zbud_zpool_free(void *pool, unsigned long handle) { … } static void *zbud_zpool_map(void *pool, unsigned long handle, enum zpool_mapmode mm) { … } static void zbud_zpool_unmap(void *pool, unsigned long handle) { … } static u64 zbud_zpool_total_pages(void *pool) { … } static struct zpool_driver zbud_zpool_driver = …; MODULE_ALIAS(…) …; static int __init init_zbud(void) { … } static void __exit exit_zbud(void) { … } module_init(…) …; module_exit(exit_zbud); MODULE_LICENSE(…) …; MODULE_AUTHOR(…) …; MODULE_DESCRIPTION(…) …;
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