dm: fix bio splitting and its bio completion order for regular IO

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>

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 4a40df8..d948cd5 100644 (file)
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1724,23 +1724,6 @@ 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);
-               submit_bio_noacct(*bio);
-               *bio = split;
-       }
-}
-
 static blk_qc_t dm_process_bio(struct mapped_device *md,
                               struct dm_table *map, struct bio *bio)
 {
@@ -1768,14 +1751,12 @@ static blk_qc_t dm_process_bio(struct mapped_device *md,
        if (current->bio_list) {
                if (is_abnormal_io(bio))
                        blk_queue_split(&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_submit_bio(struct bio *bio)