[PATCH 3.14 64/83] dm cache: prevent corruption caused by discard_block_size > cache_block_size

Greg Kroah-Hartman
2014-05-11T19:39:22Z

From: Mike Snitzer <snitzer@redhat.com>

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@netapp.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Acked-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/dm-cache-target.c | 37 +++----------------------------------
1 file changed, 3 insertions(+), 34 deletions(-)

--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -239,7 +239,7 @@ 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
@@ -2171,35 +2171,6 @@ static int create_cache_policy(struct 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)
@@ -2321,9 +2292,7 @@ static int cache_create(struct cache_arg
}
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) {
@@ -3120,7 +3089,7 @@ static void set_discard_limits(struct ca
/*
* 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;
}