[PATCH] block: Do a full clone when splitting discard bios

[PATCH] block: Do a full clone when splitting discard bios

[Martin K. Petersen]

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@oracle.com>
Reported-by: Seunguk Shin <seunguk.shin@samsung.com>
Tested-by: Seunguk Shin <seunguk.shin@samsung.com>
Cc: Seunguk Shin <seunguk.shin@samsung.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: <stable@vger.kernel.org> # v3.14+
---
 block/bio.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 2a00d349cd68..d18de96919f3 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1831,8 +1831,9 @@ EXPORT_SYMBOL(bio_endio);
  * 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)
@@ -1842,7 +1843,15 @@ 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;
 
-- 
2.4.3

Re: [PATCH] block: Do a full clone when splitting discard bios

[Christoph Hellwig]

On Wed, Jul 22, 2015 at 07:57:12AM -0400, Martin K. Petersen wrote:
> 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.

Eww.  The magic discard payload comes bac to bite us again.  I wish
someone smarter than me could come up with a scheme to avoid this issue.

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>