Date:	Tue, 21 May 2013 11:31:40 +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>,
	Heesub Shin <heesub.shin@...sung.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv12 3/4] zswap: add to mm/


On 05/21/2013 12:26 AM, Seth Jennings wrote:
> zswap is a thin backend for frontswap that takes pages that are in the process
> of being swapped out and attempts to compress them and store them in a
> RAM-based memory pool.  This can result in a significant I/O reduction on the
> swap device and, in the case where decompressing from RAM is faster than
> reading from the swap device, can also improve workload performance.
> 
> It also has support for evicting swap pages that are currently compressed in
> zswap to the swap device on an LRU(ish) basis. This functionality makes zswap a
> true cache in that, once the cache is full, the oldest pages can be moved out
> of zswap to the swap device so newer pages can be compressed and stored in
> zswap.
> 
> This patch adds the zswap driver to mm/
> 
> Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>
> Acked-by: Rik van Riel <riel@...hat.com>
> ---
>  mm/Kconfig  |  22 +-
>  mm/Makefile |   1 +
>  mm/zswap.c  | 947 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 969 insertions(+), 1 deletion(-)
>  create mode 100644 mm/zswap.c
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 45ec90d..eec97f2 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -486,4 +486,24 @@ config ZBUD
>  	  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.  
> +	  density approach when reclaim will be used.
> +
> +config ZSWAP
> +	bool "Compressed cache for swap pages (EXPERIMENTAL)"
> +	depends on FRONTSWAP && CRYPTO
> +	select CRYPTO_LZO
> +	select ZBUD
> +	default n
> +	help
> +	  A lightweight compressed cache for swap pages.  It takes
> +	  pages that are in the process of being swapped out and attempts to
> +	  compress them into a dynamically allocated RAM-based memory pool.
> +	  This can result in a significant I/O reduction on swap device and,
> +	  in the case where decompressing from RAM is faster that swap device
> +	  reads, can also improve workload performance.
> +
> +	  This is marked experimental because it is a new feature (as of
> +	  v3.11) that interacts heavily with memory reclaim.  While these
> +	  interactions don't cause any known issues on simple memory setups,
> +	  they have not be fully explored on the large set of potential
> +	  configurations and workloads that exist.
> diff --git a/mm/Makefile b/mm/Makefile
> index 95f0197..f008033 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -32,6 +32,7 @@ obj-$(CONFIG_HAVE_MEMBLOCK) += memblock.o
>  obj-$(CONFIG_BOUNCE)	+= bounce.o
>  obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o
>  obj-$(CONFIG_FRONTSWAP)	+= frontswap.o
> +obj-$(CONFIG_ZSWAP)	+= zswap.o
>  obj-$(CONFIG_HAS_DMA)	+= dmapool.o
>  obj-$(CONFIG_HUGETLBFS)	+= hugetlb.o
>  obj-$(CONFIG_NUMA) 	+= mempolicy.o
> diff --git a/mm/zswap.c b/mm/zswap.c
> new file mode 100644
> index 0000000..22cc034
> --- /dev/null
> +++ b/mm/zswap.c
> @@ -0,0 +1,947 @@
> +/*
> + * zswap.c - zswap driver file
> + *
> + * zswap is a backend for frontswap that takes pages that are in the process
> + * of being swapped out and attempts to compress and store them in a
> + * RAM-based memory pool.  This can result in a significant I/O reduction on
> + * the swap device and, in the case where decompressing from RAM is faster
> + * than reading from the swap device, can also improve workload performance.
> + *
> + * Copyright (C) 2012  Seth Jennings <sjenning@...ux.vnet.ibm.com>
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; either version 2
> + * of the License, or (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> +*/
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/module.h>
> +#include <linux/cpu.h>
> +#include <linux/highmem.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/types.h>
> +#include <linux/atomic.h>
> +#include <linux/frontswap.h>
> +#include <linux/rbtree.h>
> +#include <linux/swap.h>
> +#include <linux/crypto.h>
> +#include <linux/mempool.h>
> +#include <linux/zbud.h>
> +
> +#include <linux/mm_types.h>
> +#include <linux/page-flags.h>
> +#include <linux/swapops.h>
> +#include <linux/writeback.h>
> +#include <linux/pagemap.h>
> +
> +/*********************************
> +* statistics
> +**********************************/
> +/* Number of memory pages used by the compressed pool */
> +static u64 zswap_pool_pages;
> +/* The number of compressed pages currently stored in zswap */
> +static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
> +
> +/*
> + * The statistics below are not protected from concurrent access for
> + * performance reasons so they may not be a 100% accurate.  However,
> + * they do provide useful information on roughly how many times a
> + * certain event is occurring.
> +*/
> +
> +/* Pool limit was hit (see zswap_max_pool_percent) */
> +static u64 zswap_pool_limit_hit;
> +/* Pages written back when pool limit was reached */
> +static u64 zswap_written_back_pages;
> +/* Store failed due to a reclaim failure after pool limit was reached */
> +static u64 zswap_reject_reclaim_fail;
> +/* Compressed page was too big for the allocator to (optimally) store */
> +static u64 zswap_reject_compress_poor;
> +/* Store failed because underlying allocator could not get memory */
> +static u64 zswap_reject_alloc_fail;
> +/* Store failed because the entry metadata could not be allocated (rare) */
> +static u64 zswap_reject_kmemcache_fail;
> +/* Duplicate store was encountered (rare) */
> +static u64 zswap_duplicate_entry;
> +
> +/*********************************
> +* tunables
> +**********************************/
> +/* Enable/disable zswap (disabled by default, fixed at boot for now) */
> +static bool zswap_enabled __read_mostly;
> +module_param_named(enabled, zswap_enabled, bool, 0);
> +
> +/* Compressor to be used by zswap (fixed at boot for now) */
> +#define ZSWAP_COMPRESSOR_DEFAULT "lzo"
> +static char *zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;
> +module_param_named(compressor, zswap_compressor, charp, 0);
> +
> +/* The maximum percentage of memory that the compressed pool can occupy */
> +static unsigned int zswap_max_pool_percent = 20;
> +module_param_named(max_pool_percent,
> +			zswap_max_pool_percent, uint, 0644);
> +
> +/*********************************
> +* compression functions
> +**********************************/
> +/* per-cpu compression transforms */
> +static struct crypto_comp * __percpu *zswap_comp_pcpu_tfms;
> +
> +enum comp_op {
> +	ZSWAP_COMPOP_COMPRESS,
> +	ZSWAP_COMPOP_DECOMPRESS
> +};
> +
> +static int zswap_comp_op(enum comp_op op, const u8 *src, unsigned int slen,
> +				u8 *dst, unsigned int *dlen)
> +{
> +	struct crypto_comp *tfm;
> +	int ret;
> +
> +	tfm = *per_cpu_ptr(zswap_comp_pcpu_tfms, get_cpu());
> +	switch (op) {
> +	case ZSWAP_COMPOP_COMPRESS:
> +		ret = crypto_comp_compress(tfm, src, slen, dst, dlen);
> +		break;
> +	case ZSWAP_COMPOP_DECOMPRESS:
> +		ret = crypto_comp_decompress(tfm, src, slen, dst, dlen);
> +		break;
> +	default:
> +		ret = -EINVAL;
> +	}
> +
> +	put_cpu();
> +	return ret;
> +}
> +
> +static int __init zswap_comp_init(void)
> +{
> +	if (!crypto_has_comp(zswap_compressor, 0, 0)) {
> +		pr_info("%s compressor not available\n", zswap_compressor);
> +		/* fall back to default compressor */
> +		zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;
> +		if (!crypto_has_comp(zswap_compressor, 0, 0))
> +			/* can't even load the default compressor */
> +			return -ENODEV;
> +	}
> +	pr_info("using %s compressor\n", zswap_compressor);
> +
> +	/* alloc percpu transforms */
> +	zswap_comp_pcpu_tfms = alloc_percpu(struct crypto_comp *);
> +	if (!zswap_comp_pcpu_tfms)
> +		return -ENOMEM;
> +	return 0;
> +}
> +
> +static void zswap_comp_exit(void)
> +{
> +	/* free percpu transforms */
> +	if (zswap_comp_pcpu_tfms)
> +		free_percpu(zswap_comp_pcpu_tfms);
> +}
> +
> +/*********************************
> +* data structures
> +**********************************/
> +/*
> + * struct zswap_entry
> + *
> + * This structure contains the metadata for tracking a single compressed
> + * page within zswap.
> + *
> + * rbnode - links the entry into red-black tree for the appropriate swap type
> + * refcount - the number of outstanding reference to the entry. This is needed
> + *            to protect against premature freeing of the entry by code
> + *            concurent calls to load, invalidate, and writeback.  The lock
> + *            for the zswap_tree structure that contains the entry must
> + *            be held while changing the refcount.  Since the lock must
> + *            be held, there is no reason to also make refcount atomic.
> + * offset - the swap offset for the entry.  Index into the red-black tree.
> + * handle - zsmalloc allocation handle that stores the compressed page data
> + * length - the length in bytes of the compressed page data.  Needed during
> + *           decompression
> + */
> +struct zswap_entry {
> +	struct rb_node rbnode;
> +	pgoff_t offset;
> +	int refcount;
> +	unsigned int length;
> +	unsigned long handle;
> +};
> +
> +struct zswap_header {
> +	swp_entry_t swpentry;
> +};
> +
> +/*
> + * The tree lock in the zswap_tree struct protects a few things:
> + * - the rbtree
> + * - the refcount field of each entry in the tree
> + */
> +struct zswap_tree {
> +	struct rb_root rbroot;
> +	spinlock_t lock;
> +	struct zbud_pool *pool;
> +};
> +
> +static struct zswap_tree *zswap_trees[MAX_SWAPFILES];
> +
> +/*********************************
> +* zswap entry functions
> +**********************************/
> +#define ZSWAP_KMEM_CACHE_NAME "zswap_entry_cache"
> +static struct kmem_cache *zswap_entry_cache;
> +
> +static inline int zswap_entry_cache_create(void)
> +{
> +	zswap_entry_cache =
> +		kmem_cache_create(ZSWAP_KMEM_CACHE_NAME,
> +			sizeof(struct zswap_entry), 0, 0, NULL);
> +	return (zswap_entry_cache == NULL);
> +}
> +
> +static inline void zswap_entry_cache_destory(void)
> +{
> +	kmem_cache_destroy(zswap_entry_cache);
> +}
> +
> +static inline struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
> +{
> +	struct zswap_entry *entry;
> +	entry = kmem_cache_alloc(zswap_entry_cache, gfp);
> +	if (!entry)
> +		return NULL;
> +	entry->refcount = 1;
> +	return entry;
> +}
> +
> +static inline void zswap_entry_cache_free(struct zswap_entry *entry)
> +{
> +	kmem_cache_free(zswap_entry_cache, entry);
> +}
> +
> +/* caller must hold the tree lock */
> +static inline void zswap_entry_get(struct zswap_entry *entry)
> +{
> +	entry->refcount++;
> +}
> +
> +/* caller must hold the tree lock */
> +static inline int zswap_entry_put(struct zswap_entry *entry)
> +{
> +	entry->refcount--;
> +	return entry->refcount;
> +}
> +
> +/*********************************
> +* rbtree functions
> +**********************************/
> +static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
> +{
> +	struct rb_node *node = root->rb_node;
> +	struct zswap_entry *entry;
> +
> +	while (node) {
> +		entry = rb_entry(node, struct zswap_entry, rbnode);
> +		if (entry->offset > offset)
> +			node = node->rb_left;
> +		else if (entry->offset < offset)
> +			node = node->rb_right;
> +		else
> +			return entry;
> +	}
> +	return NULL;
> +}
> +
> +/*
> + * In the case that a entry with the same offset is found, it a pointer to
> + * the existing entry is stored in dupentry and the function returns -EEXIST
> +*/
> +static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
> +			struct zswap_entry **dupentry)
> +{
> +	struct rb_node **link = &root->rb_node, *parent = NULL;
> +	struct zswap_entry *myentry;
> +
> +	while (*link) {
> +		parent = *link;
> +		myentry = rb_entry(parent, struct zswap_entry, rbnode);
> +		if (myentry->offset > entry->offset)
> +			link = &(*link)->rb_left;
> +		else if (myentry->offset < entry->offset)
> +			link = &(*link)->rb_right;
> +		else {
> +			*dupentry = myentry;
> +			return -EEXIST;
> +		}
> +	}
> +	rb_link_node(&entry->rbnode, parent, link);
> +	rb_insert_color(&entry->rbnode, root);
> +	return 0;
> +}
> +
> +/*********************************
> +* per-cpu code
> +**********************************/
> +static DEFINE_PER_CPU(u8 *, zswap_dstmem);
> +
> +static int __zswap_cpu_notifier(unsigned long action, unsigned long cpu)
> +{
> +	struct crypto_comp *tfm;
> +	u8 *dst;
> +
> +	switch (action) {
> +	case CPU_UP_PREPARE:
> +		tfm = crypto_alloc_comp(zswap_compressor, 0, 0);
> +		if (IS_ERR(tfm)) {
> +			pr_err("can't allocate compressor transform\n");
> +			return NOTIFY_BAD;
> +		}
> +		*per_cpu_ptr(zswap_comp_pcpu_tfms, cpu) = tfm;
> +		dst = kmalloc(PAGE_SIZE * 2, GFP_KERNEL);
> +		if (!dst) {
> +			pr_err("can't allocate compressor buffer\n");
> +			crypto_free_comp(tfm);
> +			*per_cpu_ptr(zswap_comp_pcpu_tfms, cpu) = NULL;
> +			return NOTIFY_BAD;
> +		}
> +		per_cpu(zswap_dstmem, cpu) = dst;
> +		break;
> +	case CPU_DEAD:
> +	case CPU_UP_CANCELED:
> +		tfm = *per_cpu_ptr(zswap_comp_pcpu_tfms, cpu);
> +		if (tfm) {
> +			crypto_free_comp(tfm);
> +			*per_cpu_ptr(zswap_comp_pcpu_tfms, cpu) = NULL;
> +		}
> +		dst = per_cpu(zswap_dstmem, cpu);
> +		kfree(dst);
> +		per_cpu(zswap_dstmem, cpu) = NULL;
> +		break;
> +	default:
> +		break;
> +	}
> +	return NOTIFY_OK;
> +}
> +
> +static int zswap_cpu_notifier(struct notifier_block *nb,
> +				unsigned long action, void *pcpu)
> +{
> +	unsigned long cpu = (unsigned long)pcpu;
> +	return __zswap_cpu_notifier(action, cpu);
> +}
> +
> +static struct notifier_block zswap_cpu_notifier_block = {
> +	.notifier_call = zswap_cpu_notifier
> +};
> +
> +static int zswap_cpu_init(void)
> +{
> +	unsigned long cpu;
> +
> +	get_online_cpus();
> +	for_each_online_cpu(cpu)
> +		if (__zswap_cpu_notifier(CPU_UP_PREPARE, cpu) != NOTIFY_OK)
> +			goto cleanup;
> +	register_cpu_notifier(&zswap_cpu_notifier_block);
> +	put_online_cpus();
> +	return 0;
> +
> +cleanup:
> +	for_each_online_cpu(cpu)
> +		__zswap_cpu_notifier(CPU_UP_CANCELED, cpu);
> +	put_online_cpus();
> +	return -ENOMEM;
> +}
> +
> +/*********************************
> +* helpers
> +**********************************/
> +static inline bool zswap_is_full(void)
> +{
> +	return (totalram_pages * zswap_max_pool_percent / 100 <
> +		zswap_pool_pages);
> +}
> +
> +/*
> + * Carries out the common pattern of freeing and entry's zsmalloc allocation,
> + * freeing the entry itself, and decrementing the number of stored pages.
> + */
> +static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
> +{
> +	zbud_free(tree->pool, entry->handle);
> +	zswap_entry_cache_free(entry);
> +	atomic_dec(&zswap_stored_pages);
> +	zswap_pool_pages = zbud_get_pool_size(tree->pool);
> +}
> +
> +/*********************************
> +* writeback code
> +**********************************/
> +/* return enum for zswap_get_swap_cache_page */
> +enum zswap_get_swap_ret {
> +	ZSWAP_SWAPCACHE_NEW,
> +	ZSWAP_SWAPCACHE_EXIST,
> +	ZSWAP_SWAPCACHE_NOMEM
> +};
> +
> +/*
> + * zswap_get_swap_cache_page
> + *
> + * This is an adaption of read_swap_cache_async()
> + *
> + * This function tries to find a page with the given swap entry
> + * in the swapper_space address space (the swap cache).  If the page
> + * is found, it is returned in retpage.  Otherwise, a page is allocated,
> + * added to the swap cache, and returned in retpage.
> + *
> + * If success, the swap cache page is returned in retpage
> + * Returns 0 if page was already in the swap cache, page is not locked
> + * Returns 1 if the new page needs to be populated, page is locked
> + * Returns <0 on error
> + */
> +static int zswap_get_swap_cache_page(swp_entry_t entry,
> +				struct page **retpage)
> +{
> +	struct page *found_page, *new_page = NULL;
> +	struct address_space *swapper_space = &swapper_spaces[swp_type(entry)];
> +	int err;
> +
> +	*retpage = NULL;
> +	do {
> +		/*
> +		 * First check the swap cache.  Since this is normally
> +		 * called after lookup_swap_cache() failed, re-calling
> +		 * that would confuse statistics.
> +		 */
> +		found_page = find_get_page(swapper_space, entry.val);
> +		if (found_page)
> +			break;
> +
> +		/*
> +		 * Get a new page to read into from swap.
> +		 */
> +		if (!new_page) {
> +			new_page = alloc_page(GFP_KERNEL);
> +			if (!new_page)
> +				break; /* Out of memory */
> +		}
> +
> +		/*
> +		 * call radix_tree_preload() while we can wait.
> +		 */
> +		err = radix_tree_preload(GFP_KERNEL);
> +		if (err)
> +			break;
> +
> +		/*
> +		 * Swap entry may have been freed since our caller observed it.
> +		 */
> +		err = swapcache_prepare(entry);
> +		if (err == -EEXIST) { /* seems racy */
> +			radix_tree_preload_end();
> +			continue;
> +		}
> +		if (err) { /* swp entry is obsolete ? */
> +			radix_tree_preload_end();
> +			break;
> +		}
> +
> +		/* May fail (-ENOMEM) if radix-tree node allocation failed. */
> +		__set_page_locked(new_page);
> +		SetPageSwapBacked(new_page);
> +		err = __add_to_swap_cache(new_page, entry);
> +		if (likely(!err)) {
> +			radix_tree_preload_end();
> +			lru_cache_add_anon(new_page);
> +			*retpage = new_page;
> +			return ZSWAP_SWAPCACHE_NEW;
> +		}
> +		radix_tree_preload_end();
> +		ClearPageSwapBacked(new_page);
> +		__clear_page_locked(new_page);
> +		/*
> +		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
> +		 * clear SWAP_HAS_CACHE flag.
> +		 */
> +		swapcache_free(entry, NULL);
> +	} while (err != -ENOMEM);
> +
> +	if (new_page)
> +		page_cache_release(new_page);
> +	if (!found_page)
> +		return ZSWAP_SWAPCACHE_NOMEM;
> +	*retpage = found_page;
> +	return ZSWAP_SWAPCACHE_EXIST;
> +}
> +
> +/*
> + * Attempts to free and entry by adding a page to the swap cache,
> + * decompressing the entry data into the page, and issuing a
> + * bio write to write the page back to the swap device.
> + *
> + * This can be thought of as a "resumed writeback" of the page
> + * to the swap device.  We are basically resuming the same swap
> + * writeback path that was intercepted with the frontswap_store()
> + * in the first place.  After the page has been decompressed into
> + * the swap cache, the compressed version stored by zswap can be
> + * freed.
> + */
> +static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
> +{
> +	struct zswap_header *zhdr;
> +	swp_entry_t swpentry;
> +	struct zswap_tree *tree;
> +	pgoff_t offset;
> +	struct zswap_entry *entry;
> +	struct page *page;
> +	u8 *src, *dst;
> +	unsigned int dlen;
> +	int ret, refcount;
> +	struct writeback_control wbc = {
> +		.sync_mode = WB_SYNC_NONE,
> +	};
> +
> +	/* extract swpentry from data */
> +	zhdr = zbud_map(pool, handle);
> +	swpentry = zhdr->swpentry; /* here */
> +	zbud_unmap(pool, handle);
> +	tree = zswap_trees[swp_type(swpentry)];
> +	offset = swp_offset(swpentry);
> +	BUG_ON(pool != tree->pool);
> +
> +	/* find and ref zswap entry */
> +	spin_lock(&tree->lock);
> +	entry = zswap_rb_search(&tree->rbroot, offset);
> +	if (!entry) {
> +		/* entry was invalidated */
> +		spin_unlock(&tree->lock);
> +		return 0;
> +	}
> +	zswap_entry_get(entry);
> +	spin_unlock(&tree->lock);
> +	BUG_ON(offset != entry->offset);
> +
> +	/* try to allocate swap cache page */
> +	switch (zswap_get_swap_cache_page(swpentry, &page)) {
> +	case ZSWAP_SWAPCACHE_NOMEM: /* no memory */
> +		ret = -ENOMEM;
> +		goto fail;
> +
> +	case ZSWAP_SWAPCACHE_EXIST: /* page is unlocked */
> +		/* page is already in the swap cache, ignore for now */
> +		page_cache_release(page);
> +		ret = -EEXIST;
> +		goto fail;
> +
> +	case ZSWAP_SWAPCACHE_NEW: /* page is locked */
> +		/* decompress */
> +		dlen = PAGE_SIZE;
> +		src = (u8 *)zbud_map(tree->pool, entry->handle) +
> +			sizeof(struct zswap_header);
> +		dst = kmap_atomic(page);
> +		ret = zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src,
> +				entry->length, dst, &dlen);
> +		kunmap_atomic(dst);
> +		zbud_unmap(tree->pool, entry->handle);
> +		BUG_ON(ret);
> +		BUG_ON(dlen != PAGE_SIZE);
> +
> +		/* page is up to date */
> +		SetPageUptodate(page);
> +	}
> +
> +	/* start writeback */
> +	__swap_writepage(page, &wbc, end_swap_bio_write);
> +	page_cache_release(page);
> +	zswap_written_back_pages++;
> +
> +	spin_lock(&tree->lock);
> +
> +	/* drop local reference */
> +	zswap_entry_put(entry);
> +	/* drop the initial reference from entry creation */
> +	refcount = zswap_entry_put(entry);
> +
> +	/*
> +	 * There are three possible values for refcount here:
> +	 * (1) refcount is 1, load is in progress, unlink from rbtree,
> +	 *     load will free
> +	 * (2) refcount is 0, (normal case) entry is valid,
> +	 *     remove from rbtree and free entry
> +	 * (3) refcount is -1, invalidate happened during writeback,
> +	 *     free entry
> +	 */
> +	if (refcount >= 0) {
> +		/* no invalidate yet, remove from rbtree */
> +		rb_erase(&entry->rbnode, &tree->rbroot);
> +	}
> +	spin_unlock(&tree->lock);
> +	if (refcount <= 0) {
> +		/* free the entry */
> +		zswap_free_entry(tree, entry);
> +		return 0;
> +	}
> +	return -EAGAIN;
> +
> +fail:
> +	spin_lock(&tree->lock);
> +	zswap_entry_put(entry);
> +	spin_unlock(&tree->lock);
> +	return ret;
> +}
> +
> +/*********************************
> +* frontswap hooks
> +**********************************/
> +/* attempts to compress and store an single page */
> +static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> +				struct page *page)
> +{
> +	struct zswap_tree *tree = zswap_trees[type];
> +	struct zswap_entry *entry, *dupentry;
> +	int ret;
> +	unsigned int dlen = PAGE_SIZE, len;
> +	unsigned long handle;
> +	char *buf;
> +	u8 *src, *dst;
> +	struct zswap_header *zhdr;
> +
> +	if (!tree) {
> +		ret = -ENODEV;
> +		goto reject;
> +	}
> +
> +	/* reclaim space if needed */
> +	if (zswap_is_full()) {
> +		zswap_pool_limit_hit++;
> +		if (zbud_reclaim_page(tree->pool, 8)) {

So once the zswap is full, the performance will drop worse?
There maybe two writeback disk-IO instead of one compared with disable
zswap.
Every time frontswap_store() entered there will be two pages need to be
written out to disk.
In this case, the performance of zswap is worse than disable it?

> +			zswap_reject_reclaim_fail++;
> +			ret = -ENOMEM;
> +			goto reject;
> +		}
> +	}

-- 
Regards,
-Bob
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