[PATCH] zswap: Same-filled pages handling

[Srividya Desireddy]

From: Srividya Desireddy <srividya.dr@xxxxxxxxxxx>
Date: Wed, 18 Oct 2017 15:39:02 +0530
Subject: [PATCH] zswap: Same-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 same-filled pages (i.e. contents of the page are all same), 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 same-filled
page before compression of the page. If the page is a same-filled page, set
zswap_entry.length to zero, save the same-filled value and skip the
compression of the page and alloction of memory in zpool.
In zswap_frontswap_load(), check if value of zswap_entry.length is zero
corresponding to the page to be loaded. If zswap_entry.length is zero,
fill the page with same-filled value. This saves the decompression time
during load.

On a ARM Quad Core 32-bit device with 1.5GB RAM by launching and
relaunching different applications, out of ~64000 pages stored in
zswap, ~11000 pages were same-value filled pages (including zero-filled
pages) and ~9000 pages were zero-filled pages.

An average of 17% of pages(including zero-filled pages) in zswap are
same-value filled pages and 14% pages are zero-filled pages.
An average of 3% of pages are same-filled non-zero pages.

The below table shows the execution time profiling with the patch.

                          Baseline    With patch  % Improvement
-----------------------------------------------------------------
*Zswap Store Time           26.5ms       18ms          32%
 (of same value pages)
*Zswap Load Time
 (of same value pages)      25.5ms       13ms          49%
-----------------------------------------------------------------

On Ubuntu PC with 2GB RAM, while executing kernel build and other test
scripts and running multimedia applications, out of 360000 pages
stored in zswap 78000(~22%) of pages were found to be same-value filled
pages (including zero-filled pages) and 64000(~17%) are zero-filled
pages. So an average of %5 of pages are same-filled non-zero pages.

The below table shows the execution time profiling with the patch.

                          Baseline    With patch  % Improvement
-----------------------------------------------------------------
*Zswap Store Time           91ms        74ms           19%
 (of same value pages)
*Zswap Load Time            50ms        7.5ms          85%
 (of same value pages)
-----------------------------------------------------------------

*The execution times may vary with test device used.

Signed-off-by: Srividya Desireddy <srividya.dr@xxxxxxxxxxx>
---
 mm/zswap.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 72 insertions(+), 5 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index d39581a..4dd8b89 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 same-value filled pages currently stored in zswap */
+static atomic_t zswap_same_filled_pages = ATOMIC_INIT(0);
 
 /*
  * The statistics below are not protected from concurrent access for
@@ -116,6 +118,11 @@ static int zswap_compressor_param_set(const char *,
 static unsigned int zswap_max_pool_percent = 20;
 module_param_named(max_pool_percent, zswap_max_pool_percent, uint, 0644);
 
+/* Enable/disable handling same-value filled pages (enabled by default) */
+static bool zswap_same_filled_pages_enabled = true;
+module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
+		   bool, 0644);
+
 /*********************************
 * data structures
 **********************************/
@@ -145,9 +152,10 @@ struct zswap_pool {
  *            be held while changing the refcount.  Since the lock must
  *            be held, there is no reason to also make refcount atomic.
  * length - the length in bytes of the compressed page data.  Needed during
- *          decompression
+ *          decompression. For a same value filled page length is 0.
  * pool - the zswap_pool the entry's data is in
  * handle - zpool allocation handle that stores the compressed page data
+ * value - value of the same-value filled pages which have same content
  */
 struct zswap_entry {
 	struct rb_node rbnode;
@@ -155,7 +163,10 @@ struct zswap_entry {
 	int refcount;
 	unsigned int length;
 	struct zswap_pool *pool;
-	unsigned long handle;
+	union {
+		unsigned long handle;
+		unsigned long value;
+	};
 };
 
 struct zswap_header {
@@ -320,8 +331,12 @@ 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->length)
+		atomic_dec(&zswap_same_filled_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();
@@ -953,6 +968,34 @@ static int zswap_shrink(void)
 	return ret;
 }
 
+static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
+{
+	unsigned int pos;
+	unsigned long *page;
+
+	page = (unsigned long *)ptr;
+	for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) {
+		if (page[pos] != page[0])
+			return 0;
+	}
+	*value = page[0];
+	return 1;
+}
+
+static void zswap_fill_page(void *ptr, unsigned long value)
+{
+	unsigned int pos;
+	unsigned long *page;
+
+	page = (unsigned long *)ptr;
+	if (value == 0)
+		memset(page, 0, PAGE_SIZE);
+	else {
+		for (pos = 0; pos < PAGE_SIZE / sizeof(*page); pos++)
+			page[pos] = value;
+	}
+}
+
 /*********************************
 * frontswap hooks
 **********************************/
@@ -965,7 +1008,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	struct crypto_comp *tfm;
 	int ret;
 	unsigned int dlen = PAGE_SIZE, len;
-	unsigned long handle;
+	unsigned long handle, value;
 	char *buf;
 	u8 *src, *dst;
 	struct zswap_header *zhdr;
@@ -993,6 +1036,19 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 		goto reject;
 	}
 
+	if (zswap_same_filled_pages_enabled) {
+		src = kmap_atomic(page);
+		if (zswap_is_page_same_filled(src, &value)) {
+			kunmap_atomic(src);
+			entry->offset = offset;
+			entry->length = 0;
+			entry->value = value;
+			atomic_inc(&zswap_same_filled_pages);
+			goto insert_entry;
+		}
+		kunmap_atomic(src);
+	}
+
 	/* if entry is successfully added, it keeps the reference */
 	entry->pool = zswap_pool_current_get();
 	if (!entry->pool) {
@@ -1037,6 +1093,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	entry->handle = handle;
 	entry->length = dlen;
 
+insert_entry:
 	/* map */
 	spin_lock(&tree->lock);
 	do {
@@ -1089,6 +1146,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 	}
 	spin_unlock(&tree->lock);
 
+	if (!entry->length) {
+		dst = kmap_atomic(page);
+		zswap_fill_page(dst, entry->value);
+		kunmap_atomic(dst);
+		goto freeentry;
+	}
+
 	/* decompress */
 	dlen = PAGE_SIZE;
 	src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
@@ -1101,6 +1165,7 @@ 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);
@@ -1209,6 +1274,8 @@ 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("same_filled_pages", 0444,
+			zswap_debugfs_root, &zswap_same_filled_pages);
 
 	return 0;
 }
-- 
1.9.1