Date:	Tue, 14 May 2013 16:47:24 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...1.net>, Joe Perches <joe@...ches.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Hugh Dickens <hughd@...gle.com>,
	Paul Mackerras <paulus@...ba.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv11 2/4] zbud: add to mm/

Hi Seth,

On 05/13/2013 08:40 PM, 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 patch adds zbud to mm/ for later use by zswap.
> 
> Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>
> ---

Good job! And I'm testing it!

>  include/linux/zbud.h |   22 ++
>  mm/Kconfig           |   10 +
>  mm/Makefile          |    1 +
>  mm/zbud.c            |  564 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 597 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..954252b
> --- /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);
> +int zbud_get_pool_size(struct zbud_pool *pool);
> +
> +#endif /* _ZBUD_H_ */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e742d06..908f41b 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 "Buddy allocator for compressed pages"
> +	default n
> +	help
> +	  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.  
> 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..e5bd0e6
> --- /dev/null
> +++ b/mm/zbud.c
> @@ -0,0 +1,564 @@
> +/*
> + * zbud.c - Buddy Allocator for Compressed Pages
> + *
> + * 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. 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.
> + *
> + * 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
> +*****************/
> +/**
> + * struct zbud_page - zbud page metadata overlay
> + * @page:	typed reference to the underlying struct page
> + * @donotuse:	this overlays the page flags and should not be used
> + * @first_chunks:	the size of the first buddy in chunks, 0 if free

Nitpick, the name here seems not directly to me.
But I don't have a better idea yet.
Maybe first_buddy_size/first_buddy_nrchunks or buddy0_size.

> + * @last_chunks:	the size of the last buddy in chunks, 0 if free
> + * @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
> + *
> + * This structure overlays the struct page to store metadata needed for a
> + * single storage page in for zbud.  There is a BUILD_BUG_ON in zbud_init()
> + * that ensures this structure is not larger that struct page.
> + *
> + * The PG_reclaim flag of the underlying page is used for indicating
> + * that this zbud page is under reclaim (see zbud_reclaim_page())
> + */
> +struct zbud_page {
> +	union {
> +		struct page page;
> +		struct {
> +			unsigned long donotuse;
> +			u16 first_chunks;
> +			u16 last_chunks;
> +			struct list_head buddy;
> +			struct list_head lru;
> +		};
> +	};
> +};
> +
> +/*
> + * 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)
> +
> +/**
> + * struct zbud_pool - stores metadata for each zbud pool
> + * @lock:	protects all pool lists 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

Lack of list_head lru.

> + * @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;
> +	atomic_t pages_nr;
> +	struct zbud_ops *ops;
> +};
> +
> +/*****************
> + * 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 inline 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 a zbud page from a newly allocated page */
> +static inline struct zbud_page *init_zbud_page(struct page *page)
> +{
> +	struct zbud_page *zbpage = (struct zbud_page *)page;
> +	zbpage->first_chunks = 0;
> +	zbpage->last_chunks = 0;
> +	INIT_LIST_HEAD(&zbpage->buddy);
> +	INIT_LIST_HEAD(&zbpage->lru);
> +	return zbpage;
> +}
> +
> +/* Resets a zbud page so that it can be properly freed  */

Better with comment: the caller must hold the pool->lock?

> +static inline struct page *reset_zbud_page(struct zbud_page *zbpage)
> +{
> +	struct page *page = &zbpage->page;
> +	set_page_private(page, 0);
> +	page->mapping = NULL;
> +	page->index = 0;
> +	page_mapcount_reset(page);
> +	init_page_count(page);
> +	INIT_LIST_HEAD(&page->lru);
> +	return page;
> +}
> +
> +/*
> + * Encodes the handle of a particular buddy within a zbud page
> + * Pool lock should be held as this function accesses first|last_chunks
> + */
> +static inline unsigned long encode_handle(struct zbud_page *zbpage,
> +					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.
> +	 */
> +	handle = (unsigned long)page_address(&zbpage->page);
> +	if (bud == FIRST)
> +		return handle;
> +	handle += PAGE_SIZE - (zbpage->last_chunks  << CHUNK_SHIFT);
> +	return handle;
> +}
> +
> +/* Returns the zbud page where a given handle is stored */
> +static inline struct zbud_page *handle_to_zbud_page(unsigned long handle)
> +{
> +	return (struct zbud_page *)(virt_to_page(handle));
> +}
> +
> +/* Returns the number of free chunks in a zbud page */
> +static inline int num_free_chunks(struct zbud_page *zbpage)
> +{
> +	/*
> +	 * Rather than branch for different situations, just use the fact that
> +	 * free buddies have a length of zero to simplify everything.
> +	 */
> +	return NCHUNKS - zbpage->first_chunks - zbpage->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, 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);
> +	atomic_set(&pool->pages_nr, 0);
> +	pool->ops = ops;
> +	return pool;
> +}
> +EXPORT_SYMBOL_GPL(zbud_create_pool);
> +
> +/**
> + * zbud_destroy_pool() - destroys an existing zbud pool
> + * @pool:	the zbud pool to be destroyed
> + */
> +void zbud_destroy_pool(struct zbud_pool *pool)
> +{
> +	kfree(pool);

Pages in zbud pool should also be freed here? or if they are freed
before call this function some check may be needed.
But there isn't a problem currently since no actual user.

> +}
> +EXPORT_SYMBOL_GPL(zbud_destroy_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.  First, it tries to use
> + * free space in the most recently used zbud page, at the beginning of
> + * the pool LRU list.  If that zbud page is full or doesn't have the
> + * required free space, a best fit search of the unbuddied lists is
> + * performed. 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_page *zbpage = NULL;
> +	enum buddy bud;
> +	struct page *page;
> +
> +	if (size <= 0 || size > PAGE_SIZE || gfp & __GFP_HIGHMEM)
> +		return -EINVAL;
> +	chunks = size_to_chunks(size);
> +	spin_lock(&pool->lock);
> +
> +	/*
> +	 * First, try to use the zbpage we last used (at the head of the
> +	 * LRU) to increase LRU locality of the buddies. This is first fit.
> +	 */
> +	if (!list_empty(&pool->lru)) {
> +		zbpage = list_first_entry(&pool->lru, struct zbud_page, lru);
> +		if (num_free_chunks(zbpage) >= chunks) {
> +			if (zbpage->first_chunks == 0) {
> +				list_del(&zbpage->buddy);
> +				bud = FIRST;
> +				goto found;
> +			}
> +			if (zbpage->last_chunks == 0) {
> +				list_del(&zbpage->buddy);
> +				bud = LAST;
> +				goto found;
> +			}
> +		}
> +	}

I'd prefer to drop above lines to keep things simple since no way to
prove the benefit of it.

> +
> +	/* Second, try to find an unbuddied zbpage. This is best fit. */
> +	zbpage = NULL;
> +	for_each_unbuddied_list(i, chunks) {
> +		if (!list_empty(&pool->unbuddied[i])) {
> +			zbpage = list_first_entry(&pool->unbuddied[i],
> +					struct zbud_page, buddy);
> +			list_del(&zbpage->buddy);
> +			if (zbpage->first_chunks == 0)
> +				bud = FIRST;
> +			else
> +				bud = LAST;
> +			goto found;
> +		}
> +	}
> +
> +	/* Lastly, couldn't find unbuddied zbpage, create new one */
> +	spin_unlock(&pool->lock);
> +	page = alloc_page(gfp);
> +	if (!page)
> +		return -ENOMEM;
> +	spin_lock(&pool->lock);
> +	atomic_inc(&pool->pages_nr);
> +	zbpage = init_zbud_page(page);
> +	bud = FIRST;
> +
> +found:
> +	if (bud == FIRST)
> +		zbpage->first_chunks = chunks;
> +	else
> +		zbpage->last_chunks = chunks;
> +
> +	if (zbpage->first_chunks == 0 || zbpage->last_chunks == 0) {
> +		/* Add to unbuddied list */
> +		freechunks = num_free_chunks(zbpage);
> +		list_add(&zbpage->buddy, &pool->unbuddied[freechunks]);
> +	} else {
> +		/* Add to buddied list */
> +		list_add(&zbpage->buddy, &pool->buddied);
> +	}
> +
> +	/* Add/move zbpage to beginning of LRU */
> +	if (!list_empty(&zbpage->lru))
> +		list_del(&zbpage->lru);
> +	list_add(&zbpage->lru, &pool->lru);
> +
> +	*handle = encode_handle(zbpage, bud);
> +	spin_unlock(&pool->lock);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(zbud_alloc);
> +
> +/**
> + * 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_page *zbpage;
> +	int freechunks;
> +
> +	spin_lock(&pool->lock);
> +	zbpage = handle_to_zbud_page(handle);
> +
> +	/* If first buddy, handle will be page aligned */
> +	if (handle & ~PAGE_MASK)
> +		zbpage->last_chunks = 0;
> +	else
> +		zbpage->first_chunks = 0;
> +
> +	if (PageReclaim(&zbpage->page)) {
> +		/* zbpage is under reclaim, reclaim will free */
> +		spin_unlock(&pool->lock);
> +		return;
> +	}
> +
> +	/* Remove from existing buddy list */
> +	list_del(&zbpage->buddy);
> +
> +	if (zbpage->first_chunks == 0 && zbpage->last_chunks == 0) {
> +		/* zbpage is empty, free */
> +		list_del(&zbpage->lru);
> +		__free_page(reset_zbud_page(zbpage));
> +		atomic_dec(&pool->pages_nr);
> +	} else {
> +		/* Add to unbuddied list */
> +		freechunks = num_free_chunks(zbpage);
> +		list_add(&zbpage->buddy, &pool->unbuddied[freechunks]);
> +	}
> +
> +	spin_unlock(&pool->lock);
> +}
> +EXPORT_SYMBOL_GPL(zbud_free);
> +
> +#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_page *zbpage;
> +	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++) {
> +		zbpage = list_tail_entry(&pool->lru, struct zbud_page, lru);
> +		list_del(&zbpage->lru);
> +		list_del(&zbpage->buddy);
> +		/* Protect zbpage against free */
> +		SetPageReclaim(&zbpage->page);
> +		/*
> +		 * 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 (zbpage->first_chunks)
> +			first_handle = encode_handle(zbpage, FIRST);
> +		if (zbpage->last_chunks)
> +			last_handle = encode_handle(zbpage, 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;

Will go to next anyway!

> +		}
> +next:
> +		spin_lock(&pool->lock);
> +		ClearPageReclaim(&zbpage->page);
> +		if (zbpage->first_chunks == 0 && zbpage->last_chunks == 0) {
> +			/*
> +			 * Both buddies are now free, free the zbpage and
> +			 * return success.
> +			 */
> +			__free_page(reset_zbud_page(zbpage));
> +			atomic_dec(&pool->pages_nr);
> +			spin_unlock(&pool->lock);
> +			return 0;
> +		} else if (zbpage->first_chunks == 0 ||
> +				zbpage->last_chunks == 0) {
> +			/* add to unbuddied list */
> +			freechunks = num_free_chunks(zbpage);
> +			list_add(&zbpage->buddy, &pool->unbuddied[freechunks]);
> +		} else {
> +			/* add to buddied list */
> +			list_add(&zbpage->buddy, &pool->buddied);
> +		}
> +
> +		/* add to beginning of LRU */
> +		list_add(&zbpage->lru, &pool->lru);
> +	}
> +	spin_unlock(&pool->lock);
> +	return -EAGAIN;
> +}
> +EXPORT_SYMBOL_GPL(zbud_reclaim_page);
> +
> +/**
> + * 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);
> +}
> +EXPORT_SYMBOL_GPL(zbud_map);
> +
> +/**
> + * 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)
> +{
> +}
> +EXPORT_SYMBOL_GPL(zbud_unmap);
> +
> +/**
> + * 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
> + */
> +int zbud_get_pool_size(struct zbud_pool *pool)
> +{
> +	return atomic_read(&pool->pages_nr);

Should hold pool->lock?
I saw some other place dec/inc pool->pages_nr with holding pool->lock.

> +}
> +EXPORT_SYMBOL_GPL(zbud_get_pool_size);
> +
> +static int __init init_zbud(void)
> +{
> +	/* Make sure we aren't overflowing the underlying struct page */
> +	BUILD_BUG_ON(sizeof(struct zbud_page) != sizeof(struct page));
> +	/* Make sure we can represent any chunk offset with a u16 */
> +	BUILD_BUG_ON(sizeof(u16) * BITS_PER_BYTE < PAGE_SHIFT - CHUNK_SHIFT);
> +	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@...ux.vnet.ibm.com>");
> +MODULE_DESCRIPTION("Buddy Allocator for Compressed Pages");
> 

-- 
Regards,
-Bob
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