[PATCH 3.12 171/182] dm cache: prevent corruption caused by discard_block_size > cache_block_size

[Jiri Slaby]

From: Mike Snitzer <snitzer@xxxxxxxxxx>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit d132cc6d9e92424bb9d4fd35f5bd0e55d583f4be upstream.

If the discard block size is larger than the cache block size we will
not properly quiesce IO to a region that is about to be discarded.  This
results in a race between a cache migration where no copy is needed, and
a write to an adjacent cache block that's within the same large discard
block.

Workaround this by limiting the discard_block_size to cache_block_size.
Also limit the max_discard_sectors to cache_block_size.

A more comprehensive fix that introduces range locking support in the
bio_prison and proper quiescing of a discard range that spans multiple
cache blocks is already in development.

Reported-by: Morgan Mears <Morgan.Mears@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Acked-by: Joe Thornber <ejt@xxxxxxxxxx>
Acked-by: Heinz Mauelshagen <heinzm@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 drivers/md/dm-cache-target.c | 37 +++----------------------------------
 1 file changed, 3 insertions(+), 34 deletions(-)

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 6ab68e058a0a..d64bf7d6c8fe 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -162,7 +162,7 @@ struct cache {
 	 */
 	dm_dblock_t discard_nr_blocks;
 	unsigned long *discard_bitset;
-	uint32_t discard_block_size; /* a power of 2 times sectors per block */
+	uint32_t discard_block_size;
 
 	/*
 	 * Rather than reconstructing the table line for the status we just
@@ -1908,35 +1908,6 @@ static int create_cache_policy(struct cache *cache, struct cache_args *ca,
 	return 0;
 }
 
-/*
- * We want the discard block size to be a power of two, at least the size
- * of the cache block size, and have no more than 2^14 discard blocks
- * across the origin.
- */
-#define MAX_DISCARD_BLOCKS (1 << 14)
-
-static bool too_many_discard_blocks(sector_t discard_block_size,
-				    sector_t origin_size)
-{
-	(void) sector_div(origin_size, discard_block_size);
-
-	return origin_size > MAX_DISCARD_BLOCKS;
-}
-
-static sector_t calculate_discard_block_size(sector_t cache_block_size,
-					     sector_t origin_size)
-{
-	sector_t discard_block_size;
-
-	discard_block_size = roundup_pow_of_two(cache_block_size);
-
-	if (origin_size)
-		while (too_many_discard_blocks(discard_block_size, origin_size))
-			discard_block_size *= 2;
-
-	return discard_block_size;
-}
-
 #define DEFAULT_MIGRATION_THRESHOLD 2048
 
 static int cache_create(struct cache_args *ca, struct cache **result)
@@ -2041,9 +2012,7 @@ static int cache_create(struct cache_args *ca, struct cache **result)
 	}
 	clear_bitset(cache->dirty_bitset, from_cblock(cache->cache_size));
 
-	cache->discard_block_size =
-		calculate_discard_block_size(cache->sectors_per_block,
-					     cache->origin_sectors);
+	cache->discard_block_size = cache->sectors_per_block;
 	cache->discard_nr_blocks = oblock_to_dblock(cache, cache->origin_blocks);
 	cache->discard_bitset = alloc_bitset(from_dblock(cache->discard_nr_blocks));
 	if (!cache->discard_bitset) {
@@ -2630,7 +2599,7 @@ static void set_discard_limits(struct cache *cache, struct queue_limits *limits)
 	/*
 	 * FIXME: these limits may be incompatible with the cache device
 	 */
-	limits->max_discard_sectors = cache->discard_block_size * 1024;
+	limits->max_discard_sectors = cache->discard_block_size;
 	limits->discard_granularity = cache->discard_block_size << SECTOR_SHIFT;
 }
 
-- 
1.9.3

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