Sharing page tables with mshare()

This could also be fun as a sandboxing technology - you could use mshare() to set up two processes such that some of the address space of process A is mirrored over into a sandboxed process B, and so process A can map memory on process B's behalf, and process B doesn't need the ability to memory-map files by itself anymore.

Almost like the KVM interface where host userspace can map memory into the guest with ioctls, but without all the hardware virtualization complications.