[PATCH 3.19.y-ckt 045/130] block: Do a full clone when splitting discard bios

[Kamal Mostafa]

3.19.8-ckt6 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Martin K. Petersen" <martin.petersen at oracle.com>

commit f3f5da624e0a891c34d8cd513c57f1d9b0c7dadc upstream.

This fixes a data corruption bug when using discard on top of MD linear,
raid0 and raid10 personalities.

Commit 20d0189b1012 "block: Introduce new bio_split()" permits sharing
the bio_vec between the two resulting bios. That is fine for read/write
requests where the bio_vec is immutable. For discards, however, we need
to be able to attach a payload and update the bio_vec so the page can
get mapped to a scatterlist entry. Therefore the bio_vec can not be
shared when splitting discards and we must do a full clone.

Signed-off-by: Martin K. Petersen <martin.petersen at oracle.com>
Reported-by: Seunguk Shin <seunguk.shin at samsung.com>
Tested-by: Seunguk Shin <seunguk.shin at samsung.com>
Cc: Seunguk Shin <seunguk.shin at samsung.com>
Cc: Jens Axboe <axboe at fb.com>
Cc: Kent Overstreet <kent.overstreet at gmail.com>
Reviewed-by: Christoph Hellwig <hch at lst.de>
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 block/bio.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 471d738..dae3483 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1858,8 +1858,9 @@ EXPORT_SYMBOL(bio_endio_nodec);
  * Allocates and returns a new bio which represents @sectors from the start of
  * @bio, and updates @bio to represent the remaining sectors.
  *
- * The newly allocated bio will point to @bio's bi_io_vec; it is the caller's
- * responsibility to ensure that @bio is not freed before the split.
+ * Unless this is a discard request the newly allocated bio will point
+ * to @bio's bi_io_vec; it is the caller's responsibility to ensure that
+ * @bio is not freed before the split.
  */
 struct bio *bio_split(struct bio *bio, int sectors,
 		      gfp_t gfp, struct bio_set *bs)
@@ -1869,7 +1870,15 @@ struct bio *bio_split(struct bio *bio, int sectors,
 	BUG_ON(sectors <= 0);
 	BUG_ON(sectors >= bio_sectors(bio));
 
-	split = bio_clone_fast(bio, gfp, bs);
+	/*
+	 * Discards need a mutable bio_vec to accommodate the payload
+	 * required by the DSM TRIM and UNMAP commands.
+	 */
+	if (bio->bi_rw & REQ_DISCARD)
+		split = bio_clone_bioset(bio, gfp, bs);
+	else
+		split = bio_clone_fast(bio, gfp, bs);
+
 	if (!split)
 		return NULL;
 
-- 
1.9.1