[PATCH] zswap: Zero-filled pages handling

[Srividya Desireddy]

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