userfaultfd: wp: add WP pagetable tracking to x86

Accurate userfaultfd WP tracking is possible by tracking exactly which
virtual memory ranges were writeprotected by userland. We can't relay
only on the RW bit of the mapped pagetable because that information is
destroyed by fork() or KSM or swap. If we were to relay on that, we'd
need to stay on the safe side and generate false positive wp faults
for every swapped out page.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
7 files changed