dm cache: fix spurious cell_defer when dealing with partial block at end of device

We never bother caching a partial block that is at the back end of the
origin device.  No cell ever gets locked, but the calling code was
assuming it was and trying to release it.

Now the code only releases if the cell has been set to a non NULL
value.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 1a090de0c4b8..1e96d7889f51 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -2690,11 +2690,11 @@ static int __cache_map(struct cache *cache, struct bio *bio, struct dm_bio_priso
 static int cache_map(struct dm_target *ti, struct bio *bio)
 {
         int r;
-        struct dm_bio_prison_cell *cell;
+        struct dm_bio_prison_cell *cell = NULL;
         struct cache *cache = ti->private;
 
         r = __cache_map(cache, bio, &cell);
-        if (r == DM_MAPIO_REMAPPED) {
+        if (r == DM_MAPIO_REMAPPED && cell) {
                 inc_ds(cache, bio, cell);
                 cell_defer(cache, cell, false);
         }