diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index cd4d1315aaa9..8222f6f74147 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4903,10 +4903,17 @@ static void sort_parity_stripes(struct btrfs_bio *bbio, int num_stripes)
 static struct btrfs_bio *alloc_btrfs_bio(int total_stripes, int real_stripes)
 {
 	struct btrfs_bio *bbio = kzalloc(
+		 /* the size of the btrfs_bio */
 		sizeof(struct btrfs_bio) +
+		/* plus the variable array for the stripes */
 		sizeof(struct btrfs_bio_stripe) * (total_stripes) +
+		/* plus the variable array for the tgt dev */
 		sizeof(int) * (real_stripes) +
-		sizeof(u64) * (real_stripes),
+		/*
+		 * plus the raid_map, which includes both the tgt dev
+		 * and the stripes
+		 */
+		sizeof(u64) * (total_stripes),
 		GFP_NOFS);
 	if (!bbio)
 		return NULL;