Question about free_pgd_slow ?

[Rabin Vincent]

2011/3/5 rocky <bill_carson at 126.com>:
> But I still dont get it, why is that  low vector mapping must always
> be active even though the task is going to exit?

exit_mmap() is called by the exiting task.  In this case, the page table
that free_pgtables() operates on is the page table which is currently
in use.  Consider what would happen if we remove the vector mapping here
and an interrupt hits immediately after -- the cpu will (recursively)
fault while trying to find the vector page mapping.

pgd_free() on the other hand is safe because it's called when we've
switched away from this mm, and the page table is no longer in use
(mm->mm_count == 0).

Have a look at exit_mm(), which increases the mm_count before calling
mm_put(), and finish_task_switch(), which does the mmdrop().