diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index f870aef..a3798a3a 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7115,6 +7115,10 @@
 	alloc_start = offset & ~mask;
 	alloc_end =  (offset + len + mask) & ~mask;
 
+	/* We only support the FALLOC_FL_KEEP_SIZE mode */
+	if (mode && (mode != FALLOC_FL_KEEP_SIZE))
+		return -EOPNOTSUPP;
+
 	/*
 	 * wait for ordered IO before we have any locks.  We'll loop again
 	 * below with the locks held.