However, there can be more than one process on the machine (and there
   normally _is_ with NetBSD :-)).  And the combined virtual address spaces
   of all process may well exceed 4GB.


FWIW, when we rewrote the swap code, we moved to "index by page"
so the address space of these is something like 2^43 or 2^44
(it depends on page size, of course).  and that's per-swap device.
(last i heard, linux had a 128MB limit per-swap device).  so you
should be able to test this out on any machine you have enough
swap space on :-)


of course, why you'd want such a large swap device is a question
i'd rather not know the answer of .... :-)  actually, the data
used to manage such a swap device would probably blow out any
physical memory limits.