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Date: Fri, 17 Feb 2017 15:06:38 -0500
From: Dan Streetman <ddstreet@...e.org>
To: srividya.dr@...sung.com
Cc: Seth Jennings <sjenning@...hat.com>, Linux-MM <linux-mm@...ck.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Dinakar Reddy Pathireddy <dinakar.p@...sung.com>,
샤란 <sharan.allur@...sung.com>,
SUNEEL KUMAR SURIMANI <suneel@...sung.com>,
김주훈 <juhunkim@...sung.com>
Subject: Re: [PATCH 3/4] zswap: Zero-filled pages handling
On Wed, Aug 17, 2016 at 6:18 AM, Srividya Desireddy
<srividya.dr@...sung.com> wrote:
> From: Srividya Desireddy <srividya.dr@...sung.com>
> Date: Wed, 17 Aug 2016 14:34:14 +0530
> Subject: [PATCH 3/4] zswap: Zero-filled pages handling
>
> This patch adds a check in zswap_frontswap_store() to identify zero-filled
> page before compression of the page. If the page is a zero-filled page, set
> zswap_entry.zeroflag and skip the compression of the page and alloction
> of memory in zpool. In zswap_frontswap_load(), check if the zeroflag is
> set for the page in zswap_entry. If the flag is set, memset the page with
> zero. This saves the decompression time during load.
>
> The overall overhead caused due to zero-filled page check is very minimal
> when compared to the time saved by avoiding compression and allocation in
> case of zero-filled pages. The load time of a zero-filled page is reduced
> by 80% when compared to baseline.
this is unrelated to the same-page patches. send this patch by itself.
>
> Signed-off-by: Srividya Desireddy <srividya.dr@...sung.com>
> ---
> mm/zswap.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 50 insertions(+), 8 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index ae39c77..d0c3f96 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -58,6 +58,9 @@ static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
> */
> static atomic_t zswap_duplicate_pages = ATOMIC_INIT(0);
>
> +/* The number of zero filled pages swapped out to zswap */
> +static atomic_t zswap_zero_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,
> @@ -172,6 +175,8 @@ struct zswap_handle {
> * be held, there is no reason to also make refcount atomic.
> * pool - the zswap_pool the entry's data is in
> * zhandle - pointer to struct zswap_handle
> + * zeroflag - the flag is set if the content of the page is filled with
> + * zeros
> */
> struct zswap_entry {
> struct rb_node rbnode;
> @@ -179,6 +184,7 @@ struct zswap_entry {
> int refcount;
> struct zswap_pool *pool;
> struct zswap_handle *zhandle;
> + unsigned char zeroflag;
> };
>
> struct zswap_header {
> @@ -269,6 +275,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
> if (!entry)
> return NULL;
> entry->refcount = 1;
> + entry->zeroflag = 0;
> entry->zhandle = NULL;
> RB_CLEAR_NODE(&entry->rbnode);
> return entry;
> @@ -477,13 +484,17 @@ static bool zswap_handle_is_unique(struct zswap_handle *zhandle)
> */
> static void zswap_free_entry(struct zswap_entry *entry)
> {
> - if (zswap_handle_is_unique(entry->zhandle)) {
> - zpool_free(entry->pool->zpool, entry->zhandle->handle);
> - zswap_handle_cache_free(entry->zhandle);
> - zswap_pool_put(entry->pool);
> - } else {
> - entry->zhandle->ref_count--;
> - atomic_dec(&zswap_duplicate_pages);
> + if (entry->zeroflag)
> + atomic_dec(&zswap_zero_pages);
> + else {
> + if (zswap_handle_is_unique(entry->zhandle)) {
> + zpool_free(entry->pool->zpool, entry->zhandle->handle);
> + zswap_handle_cache_free(entry->zhandle);
> + zswap_pool_put(entry->pool);
> + } else {
> + entry->zhandle->ref_count--;
> + atomic_dec(&zswap_duplicate_pages);
> + }
> }
> zswap_entry_cache_free(entry);
> atomic_dec(&zswap_stored_pages);
> @@ -1140,6 +1151,21 @@ static int zswap_shrink(void)
> return ret;
> }
>
> +static int zswap_is_page_zero_filled(void *ptr)
> +{
> + unsigned int pos;
> + unsigned long *page;
> +
> + page = (unsigned long *)ptr;
> +
> + for (pos = 0; pos != PAGE_SIZE / sizeof(*page); pos++) {
> + if (page[pos])
> + return 0;
> + }
> +
> + return 1;
> +}
> +
> /*********************************
> * frontswap hooks
> **********************************/
> @@ -1183,6 +1209,13 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> }
>
> src = kmap_atomic(page);
> + if (zswap_is_page_zero_filled(src)) {
> + kunmap_atomic(src);
> + entry->offset = offset;
> + entry->zeroflag = 1;
> + atomic_inc(&zswap_zero_pages);
> + goto insert_entry;
> + }
>
> if (zswap_same_page_sharing) {
> checksum = jhash2((const u32 *)src, PAGE_SIZE / 4, 17);
> @@ -1314,6 +1347,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> }
> spin_unlock(&tree->lock);
>
> + if (entry->zeroflag) {
> + dst = kmap_atomic(page);
> + memset(dst, 0, PAGE_SIZE);
> + kunmap_atomic(dst);
> + goto freeentry;
> + }
> +
> /* decompress */
> dlen = PAGE_SIZE;
> src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->zhandle->handle,
> @@ -1327,6 +1367,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> zpool_unmap_handle(entry->pool->zpool, entry->zhandle->handle);
> BUG_ON(ret);
>
> +freeentry:
> spin_lock(&tree->lock);
> zswap_entry_put(tree, entry);
> spin_unlock(&tree->lock);
> @@ -1446,7 +1487,8 @@ static int __init zswap_debugfs_init(void)
> zswap_debugfs_root, &zswap_stored_pages);
> debugfs_create_atomic_t("duplicate_pages", S_IRUGO,
> zswap_debugfs_root, &zswap_duplicate_pages);
> -
> + debugfs_create_atomic_t("zero_pages", S_IRUGO,
> + zswap_debugfs_root, &zswap_zero_pages);
>
> return 0;
> }
> --
> 1.7.9.5
>
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