/proc/xen/balloon?

["Steven M. Bellovin" <smb%cs.columbia.edu@localhost>]

I'm trying to start a domU.  I'm getting the message

        No such file or directory: '/proc/xen/balloon'

As best I can tell, this involves a memory issue on the dom0.  All
machines involved are running 4.0. The dom0 has 128M; there are running
domUs with 1024M, 512M, 128M, and 128M, and the machine has 2GB of
physical RAM.  The domU I can't start should also get 128M.  (I recently
changed one of the domUs to give it 1024M.)

What do I need to change to fix this?

                --Steve Bellovin, http://www.cs.columbia.edu/~smb