diff --git a/daemon/graphdriver/aufs/aufs.go b/daemon/graphdriver/aufs/aufs.go
index 2d73d282fc..71ea423424 100644
--- a/daemon/graphdriver/aufs/aufs.go
+++ b/daemon/graphdriver/aufs/aufs.go
@@ -233,6 +233,8 @@ func (a *Driver) Create(id, parent, mountLabel string) error {
 	return nil
 }
 
+// createDirsFor creates two directories for the given id.
+// mnt and diff
 func (a *Driver) createDirsFor(id string) error {
 	paths := []string{
 		"mnt",
@@ -243,6 +245,9 @@ func (a *Driver) createDirsFor(id string) error {
 	if err != nil {
 		return err
 	}
+	// Directory permission is 0755.
+	// The path of directories are <aufs_root_path>/mnt/<image_id>
+	// and <aufs_root_path>/diff/<image_id>
 	for _, p := range paths {
 		if err := idtools.MkdirAllAs(path.Join(a.rootPath(), p, id), 0755, rootUID, rootGID); err != nil {
 			return err