PG_TABLED
KASSERT((oldpg->flags & PG_TABLED) != 0);
KASSERT((newpg->flags & PG_TABLED) == 0);
KASSERT((pg->flags & PG_TABLED) == 0);
pg->flags |= PG_TABLED;
KASSERT(pg->flags & PG_TABLED);
pg->flags &= ~PG_TABLED;