Re: [PATCHv3 5/6] zswap: add to mm/

Seth Jennings
2013-01-29T17:29:33Z

On 01/29/2013 12:27 AM, Minchan Kim wrote:
> First feeling is it's simple and nice approach.
> Although we have some problems to decide policy, it could solve by later patch
> so I hope we make basic infrasture more solid by lots of comment.

Thanks very much for the review!

>
> There are two things to review hard.
>
> 1. data structure life - when any data structure is died by whom?
> Please write down it in changelog or header of zswap.c
>
> 2. Flush routine - I hope it would be nice to separate it as another
> incremental patches if it is possible. If it's impossible, let's add
> lots of words.

It seems like it would be difficult to break the flushing
functionality into it's own patch, but I can start the process and
see. As long as people keep in mind that some of the design rationale
in the version without the flush code is dictated by the addition of
the flush code.

But yes, more comments.

> On Mon, Jan 28, 2013 at 03:49:26PM -0600, Seth Jennings wrote:
>> zswap is a thin compression backend for frontswap. It receives
>> pages from frontswap and attempts to store them in a compressed
>> memory pool, resulting in an effective partial memory reclaim and
>> dramatically reduced swap device I/O.
>>
>> Additional, in most cases, pages can be retrieved from this
>> compressed store much more quickly than reading from tradition
>> swap devices resulting in faster performance for many workloads.
>>
>> This patch adds the zswap driver to mm/
>>
>> Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
>> ---
>> mm/Kconfig | 15 +
>> mm/Makefile | 1 +
>> mm/zswap.c | 1073 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 1089 insertions(+)
>> create mode 100644 mm/zswap.c
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index 278e3ab..14b9acb 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -446,3 +446,18 @@ config FRONTSWAP
>> and swap data is stored as normal on the matching swap device.
>>
>> If unsure, say Y to enable frontswap.
>> +
>> +config ZSWAP
>> + bool "In-kernel swap page compression"
>> + depends on FRONTSWAP && CRYPTO
>
> Couldn't we support CRYPTO optionally?

No. zswap depends on the cryptographic API for access to the
compression modules.

>
>> + select CRYPTO_LZO
>> + select ZSMALLOC
>> + default n
>> + help
>> + Zswap is a backend for the frontswap mechanism in the VMM.
>> + It receives pages from frontswap and attempts to store them
>> + in a compressed memory pool, resulting in an effective
>> + partial memory reclaim. In addition, pages and be retrieved
>> + from this compressed store much faster than most tradition
>> + swap devices resulting in reduced I/O and faster performance
>> + for many workloads.
>> diff --git a/mm/Makefile b/mm/Makefile
>> index 3a46287..1b1ed5c 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..050b6db
>> --- /dev/null
>> +++ b/mm/zswap.c
>> @@ -0,0 +1,1073 @@
>> +/*
>> + * zswap-drv.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 them and store
>> + * them in a RAM-based memory pool. This results in a significant I/O
>> + * reduction on the real swap device and, in the case of a slow swap
>> + * device, can also improve workload performance.
>> + *
>> + * Copyright (C) 2012 Seth Jennings <sjenning@linux.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.
>> +*/
>> +
>> +#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/zsmalloc.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 atomic_t zswap_pool_pages = ATOMIC_INIT(0);
>> +/* The number of compressed pages currently stored in zswap */
>> +static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
>> +/* The number of outstanding pages awaiting writeback */
>> +static atomic_t zswap_outstanding_flushes = ATOMIC_INIT(0);
>> +
>> +/*
>> + * The statistics below are not protected from concurrent access for
>> + * performance reasons so they may not be a 100% accurate. However,
>> + * the do provide useful information on roughly how many times a
>> + * certain event is occurring.
>> +*/
>> +static u64 zswap_flushed_pages;
>> +static u64 zswap_reject_compress_poor;
>> +static u64 zswap_flush_attempted;
>> +static u64 zswap_reject_tmppage_fail;
>> +static u64 zswap_reject_flush_fail;
>> +static u64 zswap_reject_zsmalloc_fail;
>> +static u64 zswap_reject_kmemcache_fail;
>> +static u64 zswap_saved_by_flush;
>> +static u64 zswap_duplicate_entry;
>> +
>> +/*********************************
>> +* tunables
>> +**********************************/
>> +/* Enable/disable zswap (enabled by default, fixed at boot for now) */
>> +static bool zswap_enabled;
>> +module_param_named(enabled, zswap_enabled, bool, 0);
>
> It seems default is disable at the moment?

Yes, that's right. While zswap should be always-on safe (i.e. no
impact if swap isn't being used), I thought this would be the safe way
for now.

>
>> +
>> +/* 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);
>> +
>> +/*
>> + * Maximum compression ratio, as as percentage, for an acceptable
>> + * compressed page. Any pages that do not compress by at least
>> + * this ratio will be rejected.
>> +*/
>> +static unsigned int zswap_max_compression_ratio = 80;
>> +module_param_named(max_compression_ratio,
>> + zswap_max_compression_ratio, uint, 0644);
>> +
>> +/*
>> + * Maximum number of outstanding flushes allowed at any given time.
>> + * This is to prevent decompressing an unbounded number of compressed
>> + * pages into the swap cache all at once, and to help with writeback
>> + * congestion.
>> +*/
>> +#define ZSWAP_MAX_OUTSTANDING_FLUSHES 64
>> +
>> +/*********************************
>> +* 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("zswap: %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("zswap: 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 {
>> + struct rb_node rbnode;
>> + struct list_head lru;
>> + int refcount;
>> + unsigned type;
>> + pgoff_t offset;
>> + unsigned long handle;
>> + unsigned int length;
>> +};
>> +
>> +/*
>> + * The tree lock in the zswap_tree struct protects a few things:
>> + * - the rbtree
>> + * - the lru list
>> + * - the refcount field of each entry in the tree
>> + */
>> +struct zswap_tree {
>> + struct rb_root rbroot;
>> + struct list_head lru;
>> + spinlock_t lock;
>> + struct zs_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;
>> + INIT_LIST_HEAD(&entry->lru);
>> + entry->refcount = 1;
>> + return entry;
>> +}
>> +
>> +static inline void zswap_entry_cache_free(struct zswap_entry *entry)
>> +{
>> + kmem_cache_free(zswap_entry_cache, entry);
>> +}
>> +
>> +static inline void zswap_entry_get(struct zswap_entry *entry)
>> +{
>> + entry->refcount++;
>> +}
>> +
>> +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("zswap: can't allocate compressor transform\n");
>> + return NOTIFY_BAD;
>> + }
>> + *per_cpu_ptr(zswap_comp_pcpu_tfms, cpu) = tfm;
>> + dst = (u8 *)__get_free_pages(GFP_KERNEL, 1);
>> + if (!dst) {
>> + pr_err("zswap: 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);
>> + if (dst) {
>> + free_pages((unsigned long)dst, 1);
>> + 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;
>> +}
>> +
>> +/*********************************
>> +* zsmalloc callbacks
>> +**********************************/
>> +static mempool_t *zswap_page_pool;
>> +
>> +static u64 zswap_pool_limit_hit;
>> +
>> +static inline unsigned int zswap_max_pool_pages(void)
>> +{
>> + return zswap_max_pool_percent * totalram_pages / 100;
>> +}
>> +
>> +static inline int zswap_page_pool_create(void)
>> +{
>> + zswap_page_pool = mempool_create_page_pool(256, 0);
>
> Could you write down why you select pool to 256?
> If you have a plan, please write down it as TODO.
> I think it could be a function of zswap_max_pool_pages with min/max.

Yes, that would probably be better.

>
> Another question.
>
> What's the benefit of using mempool for zsmalloc?
> As you know, zsmalloc doesn't use mempool as default.
> I guess you see some benefit. if so, zram could be changed.
> If we can change zsmalloc's default scheme to use mempool,
> all of customer of zsmalloc could be enhanced, too.

In the case of zswap, through experimentation, I found that adding a
mempool behind the zsmalloc pool added some elasticity to the pool.
Fewer stores failed if we kept a small reserve of pages around instead
of having to go back to the buddy allocator who, under memory
pressure, is more likely to reject our request.

I don't see this situation being applicable to all zsmalloc users
however. I don't think we want incorporate it directly into zsmalloc
for now. The ability to register custom page alloc/free functions at
pool creation time allows users to do something special, like back
with a mempool, if they want to do that.

>
>> + if (!zswap_page_pool)
>> + return -ENOMEM;
>> + return 0;
>> +}
>> +
>> +static inline void zswap_page_pool_destroy(void)
>> +{
>> + mempool_destroy(zswap_page_pool);
>> +}
>> +
>> +static struct page *zswap_alloc_page(gfp_t flags)
>> +{
>> + struct page *page;
>> +
>> + if (atomic_read(&zswap_pool_pages) >= zswap_max_pool_pages()) {
>> + zswap_pool_limit_hit++;
>> + return NULL;
>> + }
>> + page = mempool_alloc(zswap_page_pool, flags);
>> + if (page)
>> + atomic_inc(&zswap_pool_pages);
>> + return page;
>> +}
>> +
>> +static void zswap_free_page(struct page *page)
>> +{
>> + mempool_free(page, zswap_page_pool);
>> + atomic_dec(&zswap_pool_pages);
>
> Nit.
>
> By semantic, free function can receive NULL argument so
> atomic_dec would be nice to check it before decreasing count.

Are you saying I should check the page != NULL before doing anything
here? If so, I can add that.

>
>> +}
>> +
>> +static struct zs_ops zswap_zs_ops = {
>> + .alloc = zswap_alloc_page,
>> + .free = zswap_free_page
>> +};
>> +
>> +/*********************************
>> +* flush code
>> +**********************************/
>> +static void zswap_end_swap_write(struct bio *bio, int err)
>> +{
>> + end_swap_bio_write(bio, err);
>> + atomic_dec(&zswap_outstanding_flushes);
>> + zswap_flushed_pages++;
>> +}
>> +
>> +/*
>> + * zswap_get_swap_cache_page
>> + *
>> + * This is an adaption of read_swap_cache_async()
>> + *
>> + * If success, 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
>> + */
>> +static int zswap_get_swap_cache_page(swp_entry_t entry,
>> + struct page **retpage)
>> +{
>> + struct page *found_page, *new_page = NULL;
>> + 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 1;
>> + }
>> + 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 -ENOMEM;
>> + *retpage = found_page;
>> + return 0;
>> +}
>> +
>> +static int zswap_flush_entry(struct zswap_entry *entry)
>> +{
>> + unsigned long type = entry->type;
>> + struct zswap_tree *tree = zswap_trees[type];
>> + struct page *page;
>> + swp_entry_t swpentry;
>> + u8 *src, *dst;
>> + unsigned int dlen;
>> + int ret, refcount;
>> + struct writeback_control wbc = {
>> + .sync_mode = WB_SYNC_NONE,
>> + };
>> +
>> + /* get/allocate page in the swap cache */
>> + swpentry = swp_entry(type, entry->offset);
>> + ret = zswap_get_swap_cache_page(swpentry, &page);
>> + if (ret < 0)
>> + return ret;
>> + else if (ret) {
>> + /* decompress */
>> + dlen = PAGE_SIZE;
>> + src = zs_map_object(tree->pool, entry->handle, ZS_MM_RO);
>> + dst = kmap_atomic(page);
>> + ret = zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src, entry->length,
>> + dst, &dlen);
>> + kunmap_atomic(dst);
>> + zs_unmap_object(tree->pool, entry->handle);
>> + BUG_ON(ret);
>> + BUG_ON(dlen != PAGE_SIZE);
>> + SetPageUptodate(page);
>> + } else {
>> + /* page is already in the swap cache, ignore for now */
>> + spin_lock(&tree->lock);
>> + refcount = zswap_entry_put(entry);
>> + spin_unlock(&tree->lock);
>> +
>> + if (likely(refcount))
>> + return 0;
>> +
>> + /* if the refcount is zero, invalidate must have come in */
>> + /* free */
>> + zs_free(tree->pool, entry->handle);
>> + zswap_entry_cache_free(entry);
>> + atomic_dec(&zswap_stored_pages);
>> +
>> + return 0;
>> + }
>> +
>> + /* start writeback */
>> + SetPageReclaim(page);
>> + /*
>> + * Return value is ignored here because it doesn't change anything
>> + * for us. Page is returned unlocked.
>> + */
>> + (void)__swap_writepage(page, &wbc, zswap_end_swap_write);
>> + page_cache_release(page);
>> + atomic_inc(&zswap_outstanding_flushes);
>> +
>> + /* remove */
>> + spin_lock(&tree->lock);
>> + refcount = zswap_entry_put(entry);
>> + if (refcount > 1) {
>> + /* load in progress, load will free */
>> + spin_unlock(&tree->lock);
>> + return 0;
>> + }
>> + if (refcount == 1)
>> + /* no invalidate yet, remove from rbtree */
>> + rb_erase(&entry->rbnode, &tree->rbroot);
>> + spin_unlock(&tree->lock);
>> +
>> + /* free */
>> + zs_free(tree->pool, entry->handle);
>> + zswap_entry_cache_free(entry);
>> + atomic_dec(&zswap_stored_pages);
>> +
>> + return 0;
>> +}
>> +
>> +static void zswap_flush_entries(unsigned type, int nr)
>> +{
>> + struct zswap_tree *tree = zswap_trees[type];
>> + struct zswap_entry *entry;
>> + int i, ret;
>> +
>> +/*
>> + * This limits is arbitrary for now until a better
>> + * policy can be implemented. This is so we don't
>> + * eat all of RAM decompressing pages for writeback.
>> + */
>> + if (atomic_read(&zswap_outstanding_flushes) >
>> + ZSWAP_MAX_OUTSTANDING_FLUSHES)
>> + return;
>> +
>> + for (i = 0; i < nr; i++) {
>> + /* dequeue from lru */
>> + spin_lock(&tree->lock);
>> + if (list_empty(&tree->lru)) {
>> + spin_unlock(&tree->lock);
>> + break;
>> + }
>> + entry = list_first_entry(&tree->lru,
>> + struct zswap_entry, lru);
>> + list_del(&entry->lru);
>> + zswap_entry_get(entry);
>> + spin_unlock(&tree->lock);
>> + ret = zswap_flush_entry(entry);
>> + if (ret) {
>> + /* put back on the lru */
>> + spin_lock(&tree->lock);
>> + list_add(&entry->lru, &tree->lru);
>> + spin_unlock(&tree->lock);
>> + } else {
>> + if (atomic_read(&zswap_outstanding_flushes) >
>> + ZSWAP_MAX_OUTSTANDING_FLUSHES)
>> + break;
>> + }
>> + }
>> +}
>
> The flush part complexity is very high so we needs careful review.
> I hope you can make the patch step by step from ugly approach to smart one.
> It helps reviewers much.
> If it isn't easy, I will review as it is.

(see comment at top)

>
>> +
>> +/*******************************************
>> +* page pool for temporary compression result
>> +********************************************/
>> +#define ZSWAP_TMPPAGE_POOL_PAGES 16
>> +static LIST_HEAD(zswap_tmppage_list);
>> +static DEFINE_SPINLOCK(zswap_tmppage_lock);
>> +
>> +static void zswap_tmppage_pool_destroy(void)
>> +{
>> + struct page *page, *tmppage;
>> +
>> + spin_lock(&zswap_tmppage_lock);
>> + list_for_each_entry_safe(page, tmppage, &zswap_tmppage_list, lru) {
>> + list_del(&page->lru);
>> + __free_pages(page, 1);
>> + }
>> + spin_unlock(&zswap_tmppage_lock);
>> +}
>> +
>> +static int zswap_tmppage_pool_create(void)
>> +{
>> + int i;
>> + struct page *page;
>> +
>> + for (i = 0; i < ZSWAP_TMPPAGE_POOL_PAGES; i++) {
>> + page = alloc_pages(GFP_KERNEL, 1);
>> + if (!page) {
>> + zswap_tmppage_pool_destroy();
>> + return -ENOMEM;
>> + }
>> + spin_lock(&zswap_tmppage_lock);
>> + list_add(&page->lru, &zswap_tmppage_list);
>> + spin_unlock(&zswap_tmppage_lock);
>> + }
>> + return 0;
>> +}
>> +
>> +static inline struct page *zswap_tmppage_alloc(void)
>> +{
>> + struct page *page;
>> +
>> + spin_lock(&zswap_tmppage_lock);
>> + if (list_empty(&zswap_tmppage_list)) {
>> + spin_unlock(&zswap_tmppage_lock);
>> + return NULL;
>> + }
>> + page = list_first_entry(&zswap_tmppage_list, struct page, lru);
>> + list_del(&page->lru);
>> + spin_unlock(&zswap_tmppage_lock);
>> + return page;
>> +}
>> +
>> +static inline void zswap_tmppage_free(struct page *page)
>> +{
>> + spin_lock(&zswap_tmppage_lock);
>> + list_add(&page->lru, &zswap_tmppage_list);
>> + spin_unlock(&zswap_tmppage_lock);
>> +}
>> +
>> +/*********************************
>> +* 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;
>> + unsigned long handle;
>> + char *buf;
>> + u8 *src, *dst, *tmpdst;
>> + struct page *tmppage;
>> + bool flush_attempted = 0;
>> +
>> + if (!tree) {
>> + ret = -ENODEV;
>> + goto reject;
>> + }
>> +
>> + /* compress */
>> + dst = get_cpu_var(zswap_dstmem);
>> + src = kmap_atomic(page);
>> + ret = zswap_comp_op(ZSWAP_COMPOP_COMPRESS, src, PAGE_SIZE, dst, &dlen);
>> + kunmap_atomic(src);
>> + if (ret) {
>> + ret = -EINVAL;
>> + goto freepage;
>> + }
>> + if ((dlen * 100 / PAGE_SIZE) > zswap_max_compression_ratio) {
>> + zswap_reject_compress_poor++;
>> + ret = -E2BIG;
>> + goto freepage;
>> + }
>> +
>> + /* store */
>> + handle = zs_malloc(tree->pool, dlen,
>> + __GFP_NORETRY | __GFP_HIGHMEM | __GFP_NOMEMALLOC |
>> + __GFP_NOWARN);
>> + if (!handle) {
>> + zswap_flush_attempted++;
>> + /*
>> + * Copy compressed buffer out of per-cpu storage so
>> + * we can re-enable preemption.
>> + */
>> + tmppage = zswap_tmppage_alloc();
>> + if (!tmppage) {
>> + zswap_reject_tmppage_fail++;
>> + ret = -ENOMEM;
>> + goto freepage;
>> + }
>> + flush_attempted = 1;
>> + tmpdst = page_address(tmppage);
>> + memcpy(tmpdst, dst, dlen);
>> + dst = tmpdst;
>> + put_cpu_var(zswap_dstmem);
>> +
>> + /* try to free up some space */
>> + /* TODO: replace with more targeted policy */
>> + zswap_flush_entries(type, 16);
>> + /* try again, allowing wait */
>> + handle = zs_malloc(tree->pool, dlen,
>> + __GFP_NORETRY | __GFP_HIGHMEM | __GFP_NOMEMALLOC |
>> + __GFP_NOWARN);
>> + if (!handle) {
>> + /* still no space, fail */
>> + zswap_reject_zsmalloc_fail++;
>> + ret = -ENOMEM;
>> + goto freepage;
>> + }
>> + zswap_saved_by_flush++;
>> + }
>> +
>> + buf = zs_map_object(tree->pool, handle, ZS_MM_WO);
>> + memcpy(buf, dst, dlen);
>> + zs_unmap_object(tree->pool, handle);
>> + if (flush_attempted)
>> + zswap_tmppage_free(tmppage);
>> + else
>> + put_cpu_var(zswap_dstmem);
>> +
>> + /* allocate entry */
>> + entry = zswap_entry_cache_alloc(GFP_KERNEL);
>> + if (!entry) {
>
> zs_free

Good catch!

>
>> + zswap_reject_kmemcache_fail++;
>> + ret = -ENOMEM;
>> + goto reject;
>> + }
>> +
>> + /* populate entry */
>> + entry->type = type;
>> + entry->offset = offset;
>> + entry->handle = handle;
>> + entry->length = dlen;
>> +
>> + /* map */
>> + spin_lock(&tree->lock);
>> + do {
>> + ret = zswap_rb_insert(&tree->rbroot, entry, &dupentry);
>
> Could you describe why existing entry could be there?
> IIUC, it is caused by delayed swap slot free of VM. Right?

Honestly, I don't know. A comment in the frontswap code said I need
to handle this case, but I'm not sure under what conditions it
happens. I know that it very rarely _does_ happen because I had a bug
during development related to this path.

I'll see if I can get an answer for you.

> Anyway, it would be nice to add comment about it.
>
>
>> + if (ret == -EEXIST) {
>> + zswap_duplicate_entry++;
>> + /* remove from rbtree and lru */
>> + rb_erase(&dupentry->rbnode, &tree->rbroot);
>> + if (dupentry->lru.next != LIST_POISON1)
>
> Why do we need this check? Is there any case entry isn't linked at LRU list?
> Because you already removed it in load part. Right?

Yes, it could be in the process of being loaded or flushed, either one
will have detached the entry from the list.

> Let's add comment about that.

Will do.

> And let's not dependent on LIST_POISON1 of
> list util but use explicit zswap's one.

Good suggestion.

>
>> + list_del(&dupentry->lru);
>> + if (!zswap_entry_put(dupentry)) {
>> + /* free */
>> + zs_free(tree->pool, dupentry->handle);
>> + zswap_entry_cache_free(dupentry);
>> + atomic_dec(&zswap_stored_pages);
>> + }
>> + }
>> + } while (ret == -EEXIST);
>> + list_add_tail(&entry->lru, &tree->lru);
>> + spin_unlock(&tree->lock);
>> +
>> + /* update stats */
>> + atomic_inc(&zswap_stored_pages);
>> +
>> + return 0;
>> +
>> +freepage:
>> + if (flush_attempted)
>> + zswap_tmppage_free(tmppage);
>> + else
>> + put_cpu_var(zswap_dstmem);
>> +reject:
>> + return ret;
>> +}
>> +
>> +/*
>> + * returns 0 if the page was successfully decompressed
>> + * return -1 on entry not found or error
>> +*/
>> +static int zswap_frontswap_load(unsigned type, pgoff_t offset, struct page *page)
>> +{
>> + struct zswap_tree *tree = zswap_trees[type];
>> + struct zswap_entry *entry;
>> + u8 *src, *dst;
>> + unsigned int dlen;
>> + int refcount;
>> +
>> + /* find */
>> + spin_lock(&tree->lock);
>> + entry = zswap_rb_search(&tree->rbroot, offset);
>> + if (!entry) {
>> + /* entry was flushed */
>> + spin_unlock(&tree->lock);
>> + return -1;
>> + }
>> + zswap_entry_get(entry);
>> +
>> + /* remove from lru */
>> + if (entry->lru.next != LIST_POISON1)
>
> When do we encounter this case?
> By racing with flush?

Yes.

> Need comment, too.

Yes.

>
>> + list_del(&entry->lru);
>> + spin_unlock(&tree->lock);
>> +
>> + /* decompress */
>> + dlen = PAGE_SIZE;
>> + src = zs_map_object(tree->pool, entry->handle, ZS_MM_RO);
>> + dst = kmap_atomic(page);
>> + zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src, entry->length,
>> + dst, &dlen);
>> + kunmap_atomic(dst);
>> + zs_unmap_object(tree->pool, entry->handle);
>> +
>> + spin_lock(&tree->lock);
>> + refcount = zswap_entry_put(entry);
>> + if (likely(refcount)) {
>> + list_add_tail(&entry->lru, &tree->lru);
>> + spin_unlock(&tree->lock);
>> + return 0;
>> + }
>> + spin_unlock(&tree->lock);
>> +
>> + /*
>> + * We don't have to unlink from the rbtree because zswap_flush_entry()
>> + * or zswap_frontswap_invalidate page() has already done this for us if we
>> + * are the last reference.
>> + */
>> + /* free */
>> + zs_free(tree->pool, entry->handle);
>> + zswap_entry_cache_free(entry);
>> + atomic_dec(&zswap_stored_pages);
>> +
>> + return 0;
>> +}
>> +
>> +/* invalidates a single page */
>> +static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
>> +{
>> + struct zswap_tree *tree = zswap_trees[type];
>> + struct zswap_entry *entry;
>> + int refcount;
>> +
>> + if (!tree)
>
> When do we encounter this case?

Upon second look, I'm not sure that we do. The check in
zswap_frontswap_store() prevents pages from being stored for a
particular swap type if the tree allocation for that type failed
during zswap_frontswap_init().

In that case checking the frontswap_map should prevent any call to
load or invalidate that entry.

I'll make sure. If that is the case, I'll remove this check.

Thanks again for the review!

Seth

>
>> + return;
>> +
>> + /* find */
>> + spin_lock(&tree->lock);
>> + entry = zswap_rb_search(&tree->rbroot, offset);
>> + if (!entry) {
>> + /* entry was flushed */
>> + spin_unlock(&tree->lock);
>> + return;
>> + }
>> +
>> + /* remove from rbtree and lru */
>> + rb_erase(&entry->rbnode, &tree->rbroot);
>> + if (entry->lru.next != LIST_POISON1)
>> + list_del(&entry->lru);
>> + refcount = zswap_entry_put(entry);
>> + spin_unlock(&tree->lock);
>> + if (refcount) {
>> + /* must be flushing */
>> + return;
>> + }
>> +
>> + /* free */
>> + zs_free(tree->pool, entry->handle);
>> + zswap_entry_cache_free(entry);
>> + atomic_dec(&zswap_stored_pages);
>> +}
>> +
>> +/* invalidates all pages for the given swap type */
>> +static void zswap_frontswap_invalidate_area(unsigned type)
>> +{
>> + struct zswap_tree *tree = zswap_trees[type];
>> + struct rb_node *node, *next;
>> + struct zswap_entry *entry;
>> +
>> + if (!tree)
>
> When do we encounter this case?

We can encouter this case.

Since the zswap_frontswap_init() has a void return type, the upper
layer can't know that we failed the tree allocation during swapon.
During a swapoff, it will call zswap_frontswap_invalidate_area(). So
checking !tree is the way we determine if zswap_frontswap_init()
failed at swapon time. If so, there is nothing to do.

>
>> + return;
>> +
>> + /* walk the tree and free everything */
>> + spin_lock(&tree->lock);
>> + node = rb_first(&tree->rbroot);
>> + while (node) {
>> + entry = rb_entry(node, struct zswap_entry, rbnode);
>> + zs_free(tree->pool, entry->handle);
>> + next = rb_next(node);
>> + zswap_entry_cache_free(entry);
>> + node = next;
>> + }
>> + tree->rbroot = RB_ROOT;
>> + INIT_LIST_HEAD(&tree->lru);
>> + spin_unlock(&tree->lock);
>> +}
>> +
>> +/* NOTE: this is called in atomic context from swapon and must not sleep */
>> +static void zswap_frontswap_init(unsigned type)
>> +{
>> + struct zswap_tree *tree;
>> +
>> + tree = kzalloc(sizeof(struct zswap_tree), GFP_NOWAIT);
>> + if (!tree)
>> + goto err;
>> + tree->pool = zs_create_pool(GFP_NOWAIT, &zswap_zs_ops);
>> + if (!tree->pool)
>> + goto freetree;
>> + tree->rbroot = RB_ROOT;
>> + INIT_LIST_HEAD(&tree->lru);
>> + spin_lock_init(&tree->lock);
>> + zswap_trees[type] = tree;
>> + return;
>> +
>> +freetree:
>> + kfree(tree);
>> +err:
>> + pr_err("zswap: alloc failed, zswap disabled for swap type %d\n", type);
>> +}
>> +
>> +static struct frontswap_ops zswap_frontswap_ops = {
>> + .store = zswap_frontswap_store,
>> + .load = zswap_frontswap_load,
>> + .invalidate_page = zswap_frontswap_invalidate_page,
>> + .invalidate_area = zswap_frontswap_invalidate_area,
>> + .init = zswap_frontswap_init
>> +};
>> +
>> +/*********************************
>> +* debugfs functions
>> +**********************************/
>> +#ifdef CONFIG_DEBUG_FS
>> +#include <linux/debugfs.h>
>> +
>> +static struct dentry *zswap_debugfs_root;
>> +
>> +static int __init zswap_debugfs_init(void)
>> +{
>> + if (!debugfs_initialized())
>> + return -ENODEV;
>> +
>> + zswap_debugfs_root = debugfs_create_dir("zswap", NULL);
>> + if (!zswap_debugfs_root)
>> + return -ENOMEM;
>> +
>> + debugfs_create_u64("saved_by_flush", S_IRUGO,
>> + zswap_debugfs_root, &zswap_saved_by_flush);
>> + debugfs_create_u64("pool_limit_hit", S_IRUGO,
>> + zswap_debugfs_root, &zswap_pool_limit_hit);
>> + debugfs_create_u64("reject_flush_attempted", S_IRUGO,
>> + zswap_debugfs_root, &zswap_flush_attempted);
>> + debugfs_create_u64("reject_tmppage_fail", S_IRUGO,
>> + zswap_debugfs_root, &zswap_reject_tmppage_fail);
>> + debugfs_create_u64("reject_flush_fail", S_IRUGO,
>> + zswap_debugfs_root, &zswap_reject_flush_fail);
>> + debugfs_create_u64("reject_zsmalloc_fail", S_IRUGO,
>> + zswap_debugfs_root, &zswap_reject_zsmalloc_fail);
>> + debugfs_create_u64("reject_kmemcache_fail", S_IRUGO,
>> + zswap_debugfs_root, &zswap_reject_kmemcache_fail);
>> + debugfs_create_u64("reject_compress_poor", S_IRUGO,
>> + zswap_debugfs_root, &zswap_reject_compress_poor);
>> + debugfs_create_u64("flushed_pages", S_IRUGO,
>> + zswap_debugfs_root, &zswap_flushed_pages);
>> + debugfs_create_u64("duplicate_entry", S_IRUGO,
>> + zswap_debugfs_root, &zswap_duplicate_entry);
>> + debugfs_create_atomic_t("pool_pages", S_IRUGO,
>> + zswap_debugfs_root, &zswap_pool_pages);
>> + debugfs_create_atomic_t("stored_pages", S_IRUGO,
>> + zswap_debugfs_root, &zswap_stored_pages);
>> + debugfs_create_atomic_t("outstanding_flushes", S_IRUGO,
>> + zswap_debugfs_root, &zswap_outstanding_flushes);
>> +
>> + return 0;
>> +}
>> +
>> +static void __exit zswap_debugfs_exit(void)
>> +{
>> + if (zswap_debugfs_root)
>> + debugfs_remove_recursive(zswap_debugfs_root);
>> +}
>> +#else
>> +static inline int __init zswap_debugfs_init(void)
>> +{
>> + return 0;
>> +}
>> +
>> +static inline void __exit zswap_debugfs_exit(void) { }
>> +#endif
>> +
>> +/*********************************
>> +* module init and exit
>> +**********************************/
>> +static int __init init_zswap(void)
>> +{
>> + if (!zswap_enabled)
>> + return 0;
>> +
>> + pr_info("loading zswap\n");
>> + if (zswap_entry_cache_create()) {
>> + pr_err("zswap: entry cache creation failed\n");
>> + goto error;
>> + }
>> + if (zswap_page_pool_create()) {
>> + pr_err("zswap: page pool initialization failed\n");
>> + goto pagepoolfail;
>> + }
>> + if (zswap_tmppage_pool_create()) {
>> + pr_err("zswap: workmem pool initialization failed\n");
>> + goto tmppoolfail;
>> + }
>> + if (zswap_comp_init()) {
>> + pr_err("zswap: compressor initialization failed\n");
>> + goto compfail;
>> + }
>> + if (zswap_cpu_init()) {
>> + pr_err("zswap: per-cpu initialization failed\n");
>> + goto pcpufail;
>> + }
>> + frontswap_register_ops(&zswap_frontswap_ops);
>> + if (zswap_debugfs_init())
>> + pr_warn("zswap: debugfs initialization failed\n");
>> + return 0;
>> +pcpufail:
>> + zswap_comp_exit();
>> +compfail:
>> + zswap_tmppage_pool_destroy();
>> +tmppoolfail:
>> + zswap_page_pool_destroy();
>> +pagepoolfail:
>> + zswap_entry_cache_destory();
>> +error:
>> + return -ENOMEM;
>> +}
>> +/* must be late so crypto has time to come up */
>> +late_initcall(init_zswap);
>> +
>> +MODULE_LICENSE("GPL");
>> +MODULE_AUTHOR("Seth Jennings <sjenning@linux.vnet.ibm.com>");
>> +MODULE_DESCRIPTION("Compression backend for frontswap pages");
>> --
>> 1.7.9.5
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org. For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: email@kvack.org
>