Message-ID: <20130514170311.GD4024@medulla>
Date:	Tue, 14 May 2013 12:03:11 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Bob Liu <bob.liu@...cle.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/

On Tue, May 14, 2013 at 04:47:24PM +0800, Bob Liu wrote:
> 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!

Thanks for the review!

> 
> >  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.

Yeah, I went through a few different names here.  I abandoned the buddy0|1
naming in favor of "first" and "last". Adding _buddy might make it clearer
but it adds length and the fields are documented.  So I'm inclined to
keep them as they are for now just to reduce churn.

> 
> > + * @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.

Ah yes, will add.

> 
> > + * @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?

You don't have to hold the pool lock for reset_zbud_page(). At the point
you call it, the page is already removed from all the zbud pool
structures.

> 
> > +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.

Calling zbud_destroy_pool() on a pool with outstanding allocations is
a usage error.  I can add that to the documentation.  Not sure if we
want to go as far as adding checks to make sure that all the lists
are empty.

> 
> > +}
> > +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.

Both you and Dan have pointed this out.  While I think there is benefit, I
can't prove it.  In an effort to not be hypocritical, I'll pull it out
until such time as I can demonstrate and measure the benefit.

> 
> > +
> > +	/* 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!

Yes, yes it will :)

> 
> > +		}
> > +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.

It is atomic specifically to avoid having to hold the lock.  However,
I think I do hold the lock everywhere else, so it is confusing.  I'll
document this and possibly move the other references outside the lock
so it is obvious.

Pending these changes, can I get your ack?

Thanks,
Seth

> 
> > +}
> > +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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