[PATCH v2 2/3] block: Split and submit bios in LBA order

[Bart Van Assche <bvanassche@xxxxxxx>]

Submit the bio fragment with the lowest LBA first. This approach prevents
write errors when submitting large bios to host-managed zoned block devices.
This patch only modifies the behavior of drivers that call
bio_split_to_limits() directly. This includes DRBD, pktcdvd, dm, md and
the NVMe multipath code.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>
Cc: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 block/blk-merge.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index d6f8552ef209..7281f2d91b2f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -345,8 +345,8 @@ EXPORT_SYMBOL_GPL(bio_split_rw);
  * @nr_segs: returns the number of segments in the returned bio
  *
  * Check if @bio needs splitting based on the queue limits, and if so split off
- * a bio fitting the limits from the beginning of @bio and return it.  @bio is
- * shortened to the remainder and re-submitted.
+ * a bio fitting the limits from the beginning of @bio. @bio is shortened to
+ * the remainder.
  *
  * The split bio is allocated from @q->bio_split, which is provided by the
  * block layer.
@@ -379,10 +379,23 @@ struct bio *__bio_split_to_limits(struct bio *bio,
 		split->bi_opf |= REQ_NOMERGE;
 
 		blkcg_bio_issue_init(split);
-		bio_chain(split, bio);
 		trace_block_split(split, bio->bi_iter.bi_sector);
-		submit_bio_noacct(bio);
-		return split;
+		if (current->bio_list) {
+			/*
+			 * The caller will submit the first half ('split')
+			 * before the second half ('bio').
+			 */
+			bio_chain(split, bio);
+			submit_bio_noacct(bio);
+			return split;
+		}
+		/*
+		 * Submit the first half ('split') let the caller submit the
+		 * second half ('bio').
+		 */
+		*nr_segs = bio_chain_nr_segments(bio, lim);
+		bio_chain(split, bio);
+		submit_bio_noacct(split);
 	}
 	return bio;
 }