It does *not* completely fix the case where massive amounts of data
	are being written, though.  E.g.:

		dd if=/dev/zero of=foo bs=64k
		cat title-key /dvd/video_ts/vts_01_?.vob | efdtt >movie.vob
		cdd -a

	In these cases, it appears that the data to be written backs up very
	badly and causes process (e.g. the X server and my Emacs) to lose
	pages.  Some sort of flow control is badly needed to repair this.

We should be able to fix that by puting UBC pages directly on to the
inactive list as soon as current UBC operation is complete.  As far
as I'm concerned there's no reason UBC pages should ever be `active'
unless they are mmapp()ed into some process' address space.

Eduardo