Hi Seth,
On 06/04/2013 04:33 AM, Seth Jennings wrote:
> zbud is an special purpose allocator for storing compressed pages. It is
> designed to store up to two compressed pages per physical 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.
>
> This implementation is a rewrite of the zbud allocator internally used
> by zcache in the driver/staging tree. The rewrite was necessary to
> remove some of the zcache specific elements that were ingrained throughout
> and provide a generic allocation interface that can later be used by
> zsmalloc and others.
>
> This patch adds zbud to mm/ for later use by zswap.
>
> Signed-off-by: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
> include/linux/zbud.h | 22 +++
> mm/Kconfig | 10 +
> mm/Makefile | 1 +
> mm/zbud.c | 526 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 559 insertions(+)
> create mode 100644 include/linux/zbud.h
> create mode 100644 mm/zbud.c
>
> diff --git a/include/linux/zbud.h b/include/linux/zbud.h
> new file mode 100644
> index 0000000..2571a5c
> --- /dev/null
> +++ b/include/linux/zbud.h
> @@ -0,0 +1,22 @@
> +#ifndef _ZBUD_H_
> +#define _ZBUD_H_
> +
> +#include <linux/types.h>
> +
> +struct zbud_pool;
> +
> +struct zbud_ops {
> + int (*evict)(struct zbud_pool *pool, unsigned long handle);
> +};
> +
> +struct zbud_pool *zbud_create_pool(gfp_t gfp, struct zbud_ops *ops);
> +void zbud_destroy_pool(struct zbud_pool *pool);
> +int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> + unsigned long *handle);
> +void zbud_free(struct zbud_pool *pool, unsigned long handle);
> +int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries);
> +void *zbud_map(struct zbud_pool *pool, unsigned long handle);
> +void zbud_unmap(struct zbud_pool *pool, unsigned long handle);
> +u64 zbud_get_pool_size(struct zbud_pool *pool);
> +
> +#endif /* _ZBUD_H_ */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e742d06..3367ac3 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -477,3 +477,13 @@ config FRONTSWAP
> and swap data is stored as normal on the matching swap device.
>
> If unsure, say Y to enable frontswap.
> +
> +config ZBUD
> + tristate
> + default n
> + help
> + A special purpose allocator for storing compressed pages.
> + It is designed to store up to two compressed pages per physical
> + 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.
> diff --git a/mm/Makefile b/mm/Makefile
> index 72c5acb..95f0197 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -58,3 +58,4 @@ obj-$(CONFIG_DEBUG_KMEMLEAK) += kmemleak.o
> obj-$(CONFIG_DEBUG_KMEMLEAK_TEST) += kmemleak-test.o
> obj-$(CONFIG_CLEANCACHE) += cleancache.o
> obj-$(CONFIG_MEMORY_ISOLATION) += page_isolation.o
> +obj-$(CONFIG_ZBUD) += zbud.o
> diff --git a/mm/zbud.c b/mm/zbud.c
> new file mode 100644
> index 0000000..d63ae6e
> --- /dev/null
> +++ b/mm/zbud.c
> @@ -0,0 +1,526 @@
> +/*
> + * 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
Missing under_reclaim.
> + */
> +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 */
How about moving zswap_is_full() to here.
if (zswap_is_full()) {
/* Don't alloc any new page, try to reclaim and direct use the
reclaimed page instead */
}
> + 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;
> +}
> +
It looks good for me except two things.
One is about what the performance might be after the zswap pool is full.
The other is still about the 20% limit of zswap pool size.
I need more time to test it.
--
Regards,
-Bob
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