code: func (p *Pod) UsesOverlay() bool {
  	f, err := p.openFile(common.OverlayPreparedFilename, syscall.O_RDONLY)
  	defer f.Close()
  	return err == nil
  }
docs: // UsesOverlay returns whether the pod Uses overlayfs.
  // TODO(yifan): Maybe make this function unexported.