[PATCH V2] dm-raid: fix updating of max_discard_sectors limit

[Ming Lei <ming.lei@redhat.com>]

Unit of 'chunk_size' is byte, instead of sector, so fix it.

Without this fix, too big max_discard_sectors is applied on the request queue
of dm-raid, finally raid code has to split the bio again.

This re-split done by raid causes the following nested clone_endio:

1) one big bio 'A' is submitted to dm queue, and served as the original
bio

2) one new bio 'B' is cloned from the original bio 'A', and .map()
is run on this bio of 'B', and B's original bio points to 'A'

3) raid code sees that 'B' is too big, and split 'B' and re-submit
the remainded part of 'B' to dm-raid queue via generic_make_request().

4) now dm will hanlde 'B' as new original bio, then allocate a new
clone bio of 'C' and run .map() on 'C'. Meantime C's original bio
points to 'B'.

5) suppose now 'C' is completed by raid direclty, then the following
clone_endio() is called recursively:

	clone_endio(C)
		->clone_endio(B)		#B is original bio of 'C'
			->bio_endio(A)

'A' can be big enough to make handreds of nested clone_endio(), then
stack can be corrupted easily.

Cc: <stable@vger.kernel.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V2:
	- fix commit log a bit

 drivers/md/dm-raid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c
index 8a60a4a070ac..c26aa4e8207a 100644
--- a/drivers/md/dm-raid.c
+++ b/drivers/md/dm-raid.c
@@ -3749,7 +3749,7 @@ static void raid_io_hints(struct dm_target *ti, struct queue_limits *limits)
 	 */
 	if (rs_is_raid1(rs) || rs_is_raid10(rs)) {
 		limits->discard_granularity = chunk_size;
-		limits->max_discard_sectors = chunk_size;
+		limits->max_discard_sectors = chunk_size >> 9;
 	}
 }
 
-- 
2.20.1