From: Srividya Desireddy <srividya...@samsung.com>
Date: Thu, 23 Feb 2017 15:04:06 +0530
Subject: [PATCH] zswap: Zero-filled pages handling
Zswap is a cache which compresses the pages that are being swapped out
and stores them into a dynamically allocated RAM-based memory pool.
Experiments have shown that around 10-20% of pages stored in zswap
are zero-filled pages (i.e. contents of the page are all zeros), but
these pages are handled as normal pages by compressing and allocating
memory in the pool.
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 to check for a zero-filled page is very minimal
when compared to the time saved by avoiding compression and allocation in
case of zero-filled pages. Although, compressed size of a zero-filled page
is very less, with this patch load time of a zero-filled page is reduced by
80% when compared to baseline.
Signed-off-by: Srividya Desireddy <srividya...@samsung.com>
---
mm/zswap.c | 48 +++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 45 insertions(+), 3 deletions(-)
diff --git a/mm/zswap.c b/mm/zswap.c
index 067a0d6..a574008 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -49,6 +49,8 @@
static u64 zswap_pool_total_size;
/* The number of compressed pages currently stored in zswap */
static atomic_t zswap_stored_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
@@ -140,6 +142,8 @@ struct zswap_pool {
* decompression
* pool - the zswap_pool the entry's data is in
* handle - zpool allocation handle that stores the compressed page data
+ * zeroflag - the flag is set if the content of the page is filled with
+ * zeros
*/
struct zswap_entry {
struct rb_node rbnode;
@@ -148,6 +152,7 @@ struct zswap_entry {
unsigned int length;
struct zswap_pool *pool;
unsigned long handle;
+ unsigned char zeroflag;
};
struct zswap_header {
@@ -236,6 +241,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t
gfp)
if (!entry)
return NULL;
entry->refcount = 1;
+ entry->zeroflag = 0;
RB_CLEAR_NODE(&entry->rbnode);
return entry;
}
@@ -306,8 +312,12 @@ static void zswap_rb_erase(struct rb_root *root, struct
zswap_entry *entry)
*/
static void zswap_free_entry(struct zswap_entry *entry)
{
- zpool_free(entry->pool->zpool, entry->handle);
- zswap_pool_put(entry->pool);
+ if (entry->zeroflag)
+ atomic_dec(&zswap_zero_pages);
+ else {
+ zpool_free(entry->pool->zpool, entry->handle);
+ zswap_pool_put(entry->pool);
+ }
zswap_entry_cache_free(entry);
atomic_dec(&zswap_stored_pages);
zswap_update_total_size();
@@ -877,6 +887,19 @@ 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
**********************************/
@@ -917,6 +940,15 @@ static int zswap_frontswap_store(unsigned type, pgoff_t
offset,
goto reject;
}
+ 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 entry is successfully added, it keeps the reference */
entry->pool = zswap_pool_current_get();
if (!entry->pool) {
@@ -927,7 +959,6 @@ static int zswap_frontswap_store(unsigned type, pgoff_t
offset,
/* compress */
dst = get_cpu_var(zswap_dstmem);
tfm = *get_cpu_ptr(entry->pool->tfm);
- src = kmap_atomic(page);
ret = crypto_comp_compress(tfm, src, PAGE_SIZE, dst, &dlen);
kunmap_atomic(src);
put_cpu_ptr(entry->pool->tfm);
@@ -961,6 +992,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t
offset,
entry->handle = handle;
entry->length = dlen;
+insert_entry:
/* map */
spin_lock(&tree->lock);
do {
@@ -1013,6 +1045,13 @@ 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->handle,
@@ -1025,6 +1064,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t
offset,
zpool_unmap_handle(entry->pool->zpool, entry->handle);
BUG_ON(ret);
+freeentry:
spin_lock(&tree->lock);
zswap_entry_put(tree, entry);
spin_unlock(&tree->lock);
@@ -1133,6 +1173,8 @@ static int __init zswap_debugfs_init(void)
zswap_debugfs_root, &zswap_pool_total_size);
debugfs_create_atomic_t("stored_pages", S_IRUGO,
zswap_debugfs_root, &zswap_stored_pages);
+ debugfs_create_atomic_t("zero_pages", 0444,
+ zswap_debugfs_root, &zswap_zero_pages);
return 0;
}
--
1.7.9.5
