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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