diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 58f3927fd7cc98dda4113571fab3e607842e15f3..0fee0e54d36fa1685746268d9bab5df2d6c8436e 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1607,9 +1607,9 @@ static int dm_merge_bvec(struct request_queue *q,
 	 * Find maximum amount of I/O that won't need splitting
 	 */
 	max_sectors = min(max_io_len(bvm->bi_sector, ti),
-			  (sector_t) BIO_MAX_SECTORS);
+			  (sector_t) queue_max_sectors(q));
 	max_size = (max_sectors << SECTOR_SHIFT) - bvm->bi_size;
-	if (max_size < 0)
+	if (unlikely(max_size < 0)) /* this shouldn't _ever_ happen */
 		max_size = 0;
 
 	/*
@@ -1621,10 +1621,10 @@ static int dm_merge_bvec(struct request_queue *q,
 		max_size = ti->type->merge(ti, bvm, biovec, max_size);
 	/*
 	 * If the target doesn't support merge method and some of the devices
-	 * provided their merge_bvec method (we know this by looking at
-	 * queue_max_hw_sectors), then we can't allow bios with multiple vector
-	 * entries.  So always set max_size to 0, and the code below allows
-	 * just one page.
+	 * provided their merge_bvec method (we know this by looking for the
+	 * max_hw_sectors that dm_set_device_limits may set), then we can't
+	 * allow bios with multiple vector entries.  So always set max_size
+	 * to 0, and the code below allows just one page.
 	 */
 	else if (queue_max_hw_sectors(q) <= PAGE_SIZE >> 9)
 		max_size = 0;