From: William Lee Irwin III <wli@holomorphy.com>

I've been slab allocating the stack on i386 for some time, and it has gone
without incident in pgcl, -wli, -mjb (?), and so on.  kmalloc() is fine;
there isn't any particularly compelling reason for a dedicated slab as
there's no preconstruction to do, though it can be arranged.

Basically, it works, there's no obvious reason not to, and (even better)
it's not totally invisible to the VM and even makes overhead reportable.