[PATCH 4.4 003/117] block: split bios to max possible length

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Keith Busch <keith.busch@xxxxxxxxx>

commit e36f6204288088fda50d1c84830340ccb70f85ff upstream.

This splits bio in the middle of a vector to form the largest possible
bio at the h/w's desired alignment, and guarantees the bio being split
will have some data.

The criteria for splitting is changed from the max sectors to the h/w's
optimal sector alignment if it is provided. For h/w that advertise their
block storage's underlying chunk size, it's a big performance win to not
submit commands that cross them. If sector alignment is not provided,
this patch uses the max sectors as before.

This addresses the performance issue commit d380561113 attempted to
fix, but was reverted due to splitting logic error.

Signed-off-by: Keith Busch <keith.busch@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxx>
Cc: Ming Lei <tom.leiming@xxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 block/blk-merge.c |   19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -81,9 +81,6 @@ static struct bio *blk_bio_segment_split
 	struct bio *new = NULL;
 
 	bio_for_each_segment(bv, bio, iter) {
-		if (sectors + (bv.bv_len >> 9) > queue_max_sectors(q))
-			goto split;
-
 		/*
 		 * If the queue doesn't support SG gaps and adding this
 		 * offset would create a gap, disallow it.
@@ -91,6 +88,22 @@ static struct bio *blk_bio_segment_split
 		if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
 			goto split;
 
+		if (sectors + (bv.bv_len >> 9) >
+				blk_max_size_offset(q, bio->bi_iter.bi_sector)) {
+			/*
+			 * Consider this a new segment if we're splitting in
+			 * the middle of this vector.
+			 */
+			if (nsegs < queue_max_segments(q) &&
+			    sectors < blk_max_size_offset(q,
+						bio->bi_iter.bi_sector)) {
+				nsegs++;
+				sectors = blk_max_size_offset(q,
+						bio->bi_iter.bi_sector);
+			}
+			goto split;
+		}
+
 		if (bvprvp && blk_queue_cluster(q)) {
 			if (seg_size + bv.bv_len > queue_max_segment_size(q))
 				goto new_segment;