/*
* 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);
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. -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 - 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);
}
#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");
