-
-		if (page == bv->bv_page && off == bv->bv_offset + bv->bv_len) {
-			bv->bv_len += len;
-			bio->bi_iter.bi_size += len;
-			return true;
-		}
+		struct request_queue *q = NULL;
+
+		if (page == bv->bv_page && off == (bv->bv_offset + bv->bv_len)
+				&& (off + len) <= PAGE_SIZE)

How could the page struct be the same, but the range beyond PAGE_SIZE
(at least with the existing callers)?

Also no need for the inner btraces, and the && always goes on the
first line.

+		if (bio->bi_disk)
+			q = bio->bi_disk->queue;
+
+		/* disable multi-page bvec too if cluster isn't enabled */
+		if (!q || !blk_queue_cluster(q) ||
+		    ((page_to_phys(bv->bv_page) + bv->bv_offset + bv->bv_len) !=
+		     (page_to_phys(page) + off)))
+			return false;
+ merge:
+		bv->bv_len += len;
+		bio->bi_iter.bi_size += len;
+		return true;

Ok, this is scary, as it will give differen results depending on when
bi_disk is assigned.  But then again we shouldn't really do the cluster
check here, but rather when splitting the bio for the actual low-level
driver.

(and eventually we should kill this clustering setting off in favor
of our normal segment limits).