From: Srividya Desireddy <srividya.dr@xxxxxxxxxxx>
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.
Signed-off-by: Srividya Desireddy <srividya.dr@xxxxxxxxxxx>
---
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
