Message-Id: <1354258489-2703-2-git-send-email-ngupta@vflare.org>
Date:	Thu, 29 Nov 2012 22:54:49 -0800
From:	Nitin Gupta <ngupta@...are.org>
To:	Greg KH <greg@...ah.com>
Cc:	Jerome Marchand <jmarchan@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Sam Hansen <solid.se7en@...il.com>,
	Linux Driver Project <devel@...uxdriverproject.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 2/2] zram: reduce metadata overhead

Changelog v2 vs v1:
 - Use is_zero_page() instead of direct handle comparison
 - Use 1 as invalid handle value instead of -1 since handle
is unsigned and thus -1 may refer to a valid object. While 1
is guaranteed to be invalid since <pfn:0,offset:1> can never
refer to (end of) a valid object.
 - Remove references to 'table' in comments and messages since
we just have a plain array of handles now.

For every allocated object, zram maintains the the handle, size,
flags and count fields. Of these, only the handle is required
since zsmalloc now provides the object size given the handle.
The flags field was needed only to mark a given page as zero-filled.
Instead of this field, we now use an invalid value (-1) to mark such
pages. Lastly, the count field was unused, so was simply removed.

Signed-off-by: Nitin Gupta <ngupta@...are.org>
Reviewed-by: Jerome Marchand <jmarchan@...hat.com>
---
 drivers/staging/zram/zram_drv.c |   97 ++++++++++++++++-----------------------
 drivers/staging/zram/zram_drv.h |   20 ++------
 2 files changed, 43 insertions(+), 74 deletions(-)

diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
index f2a73bd..e6c9bec 100644
--- a/drivers/staging/zram/zram_drv.c
+++ b/drivers/staging/zram/zram_drv.c
@@ -71,24 +71,6 @@ static void zram_stat64_inc(struct zram *zram, u64 *v)
 	zram_stat64_add(zram, v, 1);
 }
 
-static int zram_test_flag(struct zram *zram, u32 index,
-			enum zram_pageflags flag)
-{
-	return zram->table[index].flags & BIT(flag);
-}
-
-static void zram_set_flag(struct zram *zram, u32 index,
-			enum zram_pageflags flag)
-{
-	zram->table[index].flags |= BIT(flag);
-}
-
-static void zram_clear_flag(struct zram *zram, u32 index,
-			enum zram_pageflags flag)
-{
-	zram->table[index].flags &= ~BIT(flag);
-}
-
 static int page_zero_filled(void *ptr)
 {
 	unsigned int pos;
@@ -104,6 +86,11 @@ static int page_zero_filled(void *ptr)
 	return 1;
 }
 
+static inline int is_zero_page(unsigned long handle)
+{
+	return handle == zero_page_handle;
+}
+
 static void zram_set_disksize(struct zram *zram, size_t totalram_bytes)
 {
 	if (!zram->disksize) {
@@ -135,21 +122,20 @@ static void zram_set_disksize(struct zram *zram, size_t totalram_bytes)
 
 static void zram_free_page(struct zram *zram, size_t index)
 {
-	unsigned long handle = zram->table[index].handle;
-	u16 size = zram->table[index].size;
+	unsigned long handle = zram->handle[index];
+	size_t size;
 
-	if (unlikely(!handle)) {
-		/*
-		 * No memory is allocated for zero filled pages.
-		 * Simply clear zero page flag.
-		 */
-		if (zram_test_flag(zram, index, ZRAM_ZERO)) {
-			zram_clear_flag(zram, index, ZRAM_ZERO);
-			zram_stat_dec(&zram->stats.pages_zero);
-		}
+	if (unlikely(!handle))
+		return;
+
+	if (is_zero_page(handle)) {
+		/* No memory is allocated for zero filled pages */
+		zram->handle[index] = 0;
+		zram_stat_dec(&zram->stats.pages_zero);
 		return;
 	}
 
+	size = zs_get_object_size(zram->mem_pool, handle);
 	if (unlikely(size > max_zpage_size))
 		zram_stat_dec(&zram->stats.bad_compress);
 
@@ -158,12 +144,10 @@ static void zram_free_page(struct zram *zram, size_t index)
 	if (size <= PAGE_SIZE / 2)
 		zram_stat_dec(&zram->stats.good_compress);
 
-	zram_stat64_sub(zram, &zram->stats.compr_size,
-			zram->table[index].size);
+	zram_stat64_sub(zram, &zram->stats.compr_size, size);
 	zram_stat_dec(&zram->stats.pages_stored);
 
-	zram->table[index].handle = 0;
-	zram->table[index].size = 0;
+	zram->handle[index] = 0;
 }
 
 static void handle_zero_page(struct bio_vec *bvec)
@@ -188,19 +172,20 @@ static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
 	int ret = LZO_E_OK;
 	size_t clen = PAGE_SIZE;
 	unsigned char *cmem;
-	unsigned long handle = zram->table[index].handle;
+	unsigned long handle = zram->handle[index];
+	size_t objsize;
 
-	if (!handle || zram_test_flag(zram, index, ZRAM_ZERO)) {
+	if (!handle || is_zero_page(handle)) {
 		memset(mem, 0, PAGE_SIZE);
 		return 0;
 	}
 
+	objsize = zs_get_object_size(zram->mem_pool, handle);
 	cmem = zs_map_object(zram->mem_pool, handle, ZS_MM_RO);
-	if (zram->table[index].size == PAGE_SIZE)
+	if (objsize == PAGE_SIZE)
 		memcpy(mem, cmem, PAGE_SIZE);
 	else
-		ret = lzo1x_decompress_safe(cmem, zram->table[index].size,
-						mem, &clen);
+		ret = lzo1x_decompress_safe(cmem, objsize, mem, &clen);
 	zs_unmap_object(zram->mem_pool, handle);
 
 	/* Should NEVER happen. Return bio error if it does. */
@@ -222,8 +207,8 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
 
 	page = bvec->bv_page;
 
-	if (unlikely(!zram->table[index].handle) ||
-			zram_test_flag(zram, index, ZRAM_ZERO)) {
+	if (unlikely(!zram->handle[index]) ||
+			is_zero_page(zram->handle[index])) {
 		handle_zero_page(bvec);
 		return 0;
 	}
@@ -294,8 +279,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 	 * System overwrites unused sectors. Free memory associated
 	 * with this sector now.
 	 */
-	if (zram->table[index].handle ||
-	    zram_test_flag(zram, index, ZRAM_ZERO))
+	if (zram->handle[index])
 		zram_free_page(zram, index);
 
 	user_mem = kmap_atomic(page);
@@ -313,7 +297,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 		if (!is_partial_io(bvec))
 			kunmap_atomic(user_mem);
 		zram_stat_inc(&zram->stats.pages_zero);
-		zram_set_flag(zram, index, ZRAM_ZERO);
+		zram->handle[index] = zero_page_handle;
 		ret = 0;
 		goto out;
 	}
@@ -357,8 +341,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 
 	zs_unmap_object(zram->mem_pool, handle);
 
-	zram->table[index].handle = handle;
-	zram->table[index].size = clen;
+	zram->handle[index] = handle;
 
 	/* Update stats */
 	zram_stat64_add(zram, &zram->stats.compr_size, clen);
@@ -517,15 +500,15 @@ void __zram_reset_device(struct zram *zram)
 
 	/* Free all pages that are still in this zram device */
 	for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
-		unsigned long handle = zram->table[index].handle;
-		if (!handle)
+		unsigned long handle = zram->handle[index];
+		if (!handle || is_zero_page(handle))
 			continue;
 
 		zs_free(zram->mem_pool, handle);
 	}
 
-	vfree(zram->table);
-	zram->table = NULL;
+	vfree(zram->handle);
+	zram->handle = NULL;
 
 	zs_destroy_pool(zram->mem_pool);
 	zram->mem_pool = NULL;
@@ -561,7 +544,7 @@ int zram_init_device(struct zram *zram)
 	if (!zram->compress_workmem) {
 		pr_err("Error allocating compressor working memory!\n");
 		ret = -ENOMEM;
-		goto fail_no_table;
+		goto fail_no_handles;
 	}
 
 	zram->compress_buffer =
@@ -569,15 +552,15 @@ int zram_init_device(struct zram *zram)
 	if (!zram->compress_buffer) {
 		pr_err("Error allocating compressor buffer space\n");
 		ret = -ENOMEM;
-		goto fail_no_table;
+		goto fail_no_handles;
 	}
 
 	num_pages = zram->disksize >> PAGE_SHIFT;
-	zram->table = vzalloc(num_pages * sizeof(*zram->table));
-	if (!zram->table) {
-		pr_err("Error allocating zram address table\n");
+	zram->handle = vzalloc(num_pages * sizeof(*zram->handle));
+	if (!zram->handle) {
+		pr_err("Error allocating object handle space\n");
 		ret = -ENOMEM;
-		goto fail_no_table;
+		goto fail_no_handles;
 	}
 
 	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
@@ -598,8 +581,8 @@ int zram_init_device(struct zram *zram)
 	pr_debug("Initialization done!\n");
 	return 0;
 
-fail_no_table:
-	/* To prevent accessing table entries during cleanup */
+fail_no_handles:
+	/* To prevent accessing handle values during cleanup */
 	zram->disksize = 0;
 fail:
 	__zram_reset_device(zram);
diff --git a/drivers/staging/zram/zram_drv.h b/drivers/staging/zram/zram_drv.h
index df2eec4..837068f 100644
--- a/drivers/staging/zram/zram_drv.h
+++ b/drivers/staging/zram/zram_drv.h
@@ -54,24 +54,10 @@ static const size_t max_zpage_size = PAGE_SIZE / 4 * 3;
 #define ZRAM_SECTOR_PER_LOGICAL_BLOCK	\
 	(1 << (ZRAM_LOGICAL_BLOCK_SHIFT - SECTOR_SHIFT))
 
-/* Flags for zram pages (table[page_no].flags) */
-enum zram_pageflags {
-	/* Page consists entirely of zeros */
-	ZRAM_ZERO,
-
-	__NR_ZRAM_PAGEFLAGS,
-};
+static const unsigned long zero_page_handle = 1;
 
 /*-- Data structures */
 
-/* Allocated for each disk page */
-struct table {
-	unsigned long handle;
-	u16 size;	/* object size (excluding header) */
-	u8 count;	/* object ref count (not yet used) */
-	u8 flags;
-} __aligned(4);
-
 struct zram_stats {
 	u64 compr_size;		/* compressed size of pages stored */
 	u64 num_reads;		/* failed + successful */
@@ -90,9 +76,9 @@ struct zram {
 	struct zs_pool *mem_pool;
 	void *compress_workmem;
 	void *compress_buffer;
-	struct table *table;
+	unsigned long *handle;	/* memory handle for each disk page */
 	spinlock_t stat64_lock;	/* protect 64-bit stats */
-	struct rw_semaphore lock; /* protect compression buffers and table
+	struct rw_semaphore lock; /* protect compression buffers and handles
 				   * against concurrent read and writes */
 	struct request_queue *queue;
 	struct gendisk *disk;
-- 
1.7.10.4

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