code: def add_mount_point(self, path, total_size=None):
          
          path = self.absnormpath(path)
          if path in self.mount_points:
              self.raise_os_error(errno.EEXIST, path)
          self._last_dev += 1
          self.mount_points[path] = {
              'idev': self._last_dev, 'total_size': total_size, 'used_size': 0
          }
          # special handling for root path: has been created before
          root_dir = (self.root if path == self.root.name
                      else self.create_dir(path))
          root_dir.st_dev = self._last_dev
          return self.mount_points[path]
docs: Add a new mount point for a filesystem device.
          The mount point gets a new unique device number.
  
          Args:
              path: The root path for the new mount path.
  
              total_size: The new total size of the added filesystem device
                  in bytes. Defaults to infinite size.
  
          Returns:
              The newly created mount point dict.
  
          Raises:
              OSError: if trying to mount an existing mount point again.