Btrfs: search parity device wisely

After mapping block with BTRFS_MAP_WRITE, parities have been sorted to
the end position, so this search can start from the first parity
stripe.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ copied changelog as a comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -2231,12 +2231,18 @@ raid56_parity_alloc_scrub_rbio(struct btrfs_fs_info *fs_info, struct bio *bio,
 	ASSERT(!bio->bi_iter.bi_size);
 	rbio->operation = BTRFS_RBIO_PARITY_SCRUB;
 
-	for (i = 0; i < rbio->real_stripes; i++) {
+	/*
+	 * After mapping bbio with BTRFS_MAP_WRITE, parities have been sorted
+	 * to the end position, so this search can start from the first parity
+	 * stripe.
+	 */
+	for (i = rbio->nr_data; i < rbio->real_stripes; i++) {
 		if (bbio->stripes[i].dev == scrub_dev) {
 			rbio->scrubp = i;
 			break;
 		}
 	}
+	ASSERT(i < rbio->real_stripes);
 
 	/* Now we just support the sectorsize equals to page size */
 	ASSERT(fs_info->sectorsize == PAGE_SIZE);