def GetMountPoint(self, path=None):
    """Walk back from the path to find the mount point.

    Args:
      path: a Unicode string containing the path or None. If path is None the
        value in self.path is used.

    Returns:
      path string of the mount point
    """
    path = os.path.abspath(
        client_utils.CanonicalPathToLocalPath(path or self.path))

    while not os.path.ismount(path):
      path = os.path.dirname(path)

    return path