[PATCH] dm: use cloned bio as head, not remainder, in __split_and_process_bio()

[NeilBrown]

When we use bio_clone_bioset() to split off the front part of a bio
and chain the two together and submit the remainder to
generic_make_request(), it is important that the newly allocated
bio is used as the head to be processed immediately, and the original
bio gets "bio_advance()"d and sent to generic_make_request() as the
remainder.

If the newly allocated bio is used as the remainder, and if it then
needs to be split again, then the next bio_clone_bioset() call will
be made while holding a reference a bio (result of the first clone)
from the same bioset.  This can potentially exhaust the bioset mempool
and result in a memory allocation deadlock.

So the result of the bio_clone_bioset() must be attached to the new
dm_io struct, and the original must be resubmitted.  The current code
is backwards.

Note that there is no race caused by reassigning cio.io->bio after already
calling __map_bio().  This bio will only be dereferenced again after
dec_pending() has found io->io_count to be zero, and this cannot happen
before the dec_pending() call at the end of __split_and_process_bio().

Reported-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---

Hi,
 I think this should resolve the problem Mikulas noticed that the
 bios form a deep chain instead of a wide tree.

Thanks,
NeilBrown

 drivers/md/dm.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 99ec215f7dcb..2e0e10a1c030 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1482,12 +1482,19 @@ static void __split_and_process_bio(struct mapped_device *md,
 				 * Remainder must be passed to generic_make_request()
 				 * so that it gets handled *after* bios already submitted
 				 * have been completely processed.
+				 * We take a clone of the original to store in
+				 * ci.io->bio to be used by end_io_acct() and
+				 * for dec_pending to use for completion handling.
+				 * As this path is not used for REQ_OP_ZONE_REPORT,
+				 * the usage of io->bio in dm_remap_zone_report()
+				 * won't be affected by this reassignment.
 				 */
 				struct bio *b = bio_clone_bioset(bio, GFP_NOIO,
 								 md->queue->bio_split);
-				bio_advance(b, (bio_sectors(b) - ci.sector_count) << 9);
+				ci.io->bio = b;
+				bio_advance(bio, (bio_sectors(bio) - ci.sector_count) << 9);
 				bio_chain(b, bio);
-				generic_make_request(b);
+				generic_make_request(bio);
 				break;
 			}
 		}
-- 
2.14.0.rc0.dirty

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