Michael van Elst wrote:
> The umass driver requires a big chunk of memory while the hardware
> could live with fragmented memory. When this is fixed the problem
> goes away.

It isn't just a problem for umass; it potentially affects any of the sorts
of devices that can be plugged in and attached after boot time, if their
drivers request contiguous memory.  I ran into it with a Cardbus NIC on a
laptop.  The general problem, as I understood it, is when the kernel needs
to allocate some contiguous memory and there are user processes that have
some pages in the way, the memory manager needs to kick them out.  (They can
always be paged back in.)  It just doesn't do that yet.  The complications
are how to arrange for the kicking-out to be fast enough that things like
drivers can safely do allocation without having to be reschedulable, and
some ideas for that have been floated recently on tech-kern.  Solving the
general problem would reduce the pressure for a bunch of device-specific
workarounds.

-Chap