diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 25c422b11bdb..9041f154cc32 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1318,7 +1318,7 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 		i = range->start >> PAGE_CACHE_SHIFT;
 	}
 	if (!max_to_defrag)
-		max_to_defrag = last_index + 1;
+		max_to_defrag = last_index - i + 1;
 
 	/*
 	 * make writeback starts from i, so the defrag range can be