Can't say I've ever got a page fault out of 9fs. Plenty of other
things.

As the kernel does not demand page, page faults should not happen.
You are down to hardware or software bugs. A first step would be
to reveal the hardware involved. If you have a motherboard with
a shared memory vga, for example, then you could be in deep trouble.
You can also check where the page fault occurs in case it is at a
consistent point in the kernel, which would suggest a software bug.
If it wanders around randomly, then it's probably hardware.