--- /dev/null
+/*
+ * 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
+ * justifed" 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>
+
+/*****************
+ * 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, and there
+ * will be 64 freelists per pool.
+ */
+#define NCHUNKS_ORDER 6
+
+#define CHUNK_SHIFT (PAGE_SHIFT - NCHUNKS_ORDER)
+#define CHUNK_SIZE (1 << CHUNK_SHIFT)
+#define NCHUNKS (PAGE_SIZE >> CHUNK_SHIFT)
+#define ZHDR_SIZE_ALIGNED CHUNK_SIZE
+
+/**
+ * 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;
+ struct zbud_ops *ops;
+};
+
+/*
+ * 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;
+};
+
+/*****************
+ * 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(int 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);
+ 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. -1 at the
+ * end for the zbud header.
+ */
+ return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks - 1;
+}
+
+/*****************
+ * 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, struct zbud_ops *ops)
+{
+ struct zbud_pool *pool;
+ int i;
+
+ pool = kmalloc(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 is 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, int size, gfp_t gfp,
+ unsigned long *handle)
+{
+ int chunks, i, freechunks;
+ struct zbud_header *zhdr = NULL;
+ enum buddy bud;
+ struct page *page;
+
+ if (size <= 0 || gfp & __GFP_HIGHMEM)
+ return -EINVAL;
+ if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED)
+ 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);
+}
+
+#define list_tail_entry(ptr, type, member) \
+ list_entry((ptr)->prev, type, member)
+
+/**
+ * zbud_reclaim_page() - evicts allocations from a pool page and frees it
+ * @pool: pool from which a page will attempt to be evicted
+ * @retires: 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_tail_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");
+ return 0;
+}
+
+static void __exit exit_zbud(void)
+{
+ pr_info("unloaded\n");
+}
+
+module_init(init_zbud);
+module_exit(exit_zbud);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Seth Jennings <sjenning@linux.vnet.ibm.com>");
+MODULE_DESCRIPTION("Buddy Allocator for Compressed Pages");
projects / karo-tx-linux.git / commitdiff