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Subject[PATCH 5.4 384/388] dm: fix bio splitting and its bio completion order for regular IO
Date
From: Mike Snitzer <snitzer@redhat.com>

commit ee1dfad5325ff1cfb2239e564cd411b3bfe8667a upstream.

dm_queue_split() is removed because __split_and_process_bio() _must_
handle splitting bios to ensure proper bio submission and completion
ordering as a bio is split.

Otherwise, multiple recursive calls to ->submit_bio will cause multiple
split bios to be allocated from the same ->bio_split mempool at the same
time. This would result in deadlock in low memory conditions because no
progress could be made (only one bio is available in ->bio_split
mempool).

This fix has been verified to still fix the loss of performance, due
to excess splitting, that commit 120c9257f5f1 provided.

Fixes: 120c9257f5f1 ("Revert "dm: always call blk_queue_split() in dm_process_bio()"")
Cc: stable@vger.kernel.org # 5.0+, requires custom backport due to 5.9 changes
Reported-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/dm.c | 23 ++---------------------
1 file changed, 2 insertions(+), 21 deletions(-)

--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1720,23 +1720,6 @@ out:
return ret;
}

-static void dm_queue_split(struct mapped_device *md, struct dm_target *ti, struct bio **bio)
-{
- unsigned len, sector_count;
-
- sector_count = bio_sectors(*bio);
- len = min_t(sector_t, max_io_len((*bio)->bi_iter.bi_sector, ti), sector_count);
-
- if (sector_count > len) {
- struct bio *split = bio_split(*bio, len, GFP_NOIO, &md->queue->bio_split);
-
- bio_chain(split, *bio);
- trace_block_split(md->queue, split, (*bio)->bi_iter.bi_sector);
- generic_make_request(*bio);
- *bio = split;
- }
-}
-
static blk_qc_t dm_process_bio(struct mapped_device *md,
struct dm_table *map, struct bio *bio)
{
@@ -1764,14 +1747,12 @@ static blk_qc_t dm_process_bio(struct ma
if (current->bio_list) {
if (is_abnormal_io(bio))
blk_queue_split(md->queue, &bio);
- else
- dm_queue_split(md, ti, &bio);
+ /* regular IO is split by __split_and_process_bio */
}

if (dm_get_md_type(md) == DM_TYPE_NVME_BIO_BASED)
return __process_bio(md, map, bio, ti);
- else
- return __split_and_process_bio(md, map, bio);
+ return __split_and_process_bio(md, map, bio);
}

static blk_qc_t dm_make_request(struct request_queue *q, struct bio *bio)

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