Change the way that reservations keep track of which pages are in use.
  Instead of using the page's PG_CACHED and PG_FREE flags, maintain a bit
  vector within the reservation.  This approach has a couple benefits.
  First, it makes breaking reservations much cheaper because there are
  fewer cache misses to identify the unused pages.  Second, it is a pre-
  requisite for supporting two or more reservation sizes.