block: split bios to max possible length

Keith Busch · axboe · commit e36f62042880 · 2016-01-12T15:10:56.000-07:00
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 d380561 attempted to fix, but was reverted due to splitting logic error. Signed-off-by: Keith Busch <keith.busch@intel.com> Cc: Jens Axboe <axboe@fb.com> Cc: Ming Lei <tom.leiming@gmail.com> Cc: Kent Overstreet <kent.overstreet@gmail.com> Cc: <stable@vger.kernel.org> # 4.4.x- Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-merge.c b/block/blk-merge.c
@@ -83,16 +83,29 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 	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.
 		 */
 		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;
