touched any user-space address recently. This is efficient
for things that stay in the kernel for a while, waking up
to handle some I/O then going back to sleep (i.e. nfsd).
If and when such a process returns to user-mode, it will
fault and be given a real context at that time.
This also makes context switch faster, because all we need
to do there for the MMU is slam the context register.