block: only honor SG gap prevention for merges that contain data

axboe · axboe · commit beefa6ba7bf3 · 2015-05-29T13:10:23.000-06:00
We can safely merge anything that wont generate an SG list entry,
so if the bio is data-less (discard), don't look at potential
SG gaps.

Signed-off-by: Jens Axboe &lt;axboe@fb.com&gt;
diff --git a/block/blk-merge.c b/block/blk-merge.c
@@ -589,7 +589,8 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
 	    !blk_write_same_mergeable(rq->bio, bio))
 		return false;
 
-	if (q->queue_flags & (1 << QUEUE_FLAG_SG_GAPS)) {
+	/* Only check gaps if the bio carries data */
+	if (q->queue_flags & (1 << QUEUE_FLAG_SG_GAPS) && bio_has_data(bio)) {
 		struct bio_vec *bprev;
 
 		bprev = &rq->biotail->bi_io_vec[rq->biotail->bi_vcnt - 1];
