Re: [RFC PATCH RESEND 17/28] mm/mmap: prevent pagefault handler from racing with mmu_notifier registration

[Suren Baghdasaryan]

On Fri, Sep 9, 2022 at 7:20 AM Laurent Dufour <ldufour@xxxxxxxxxxxxx> wrote:
>
> Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> > Pagefault handlers might need to fire MMU notifications while a new
> > notifier is being registered. Modify mm_take_all_locks to mark all VMAs
> > as locked and prevent this race with fault handlers that would hold VMA
> > locks.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > ---
> >  mm/mmap.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index b31cc97c2803..1edfcd384f5e 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -3538,6 +3538,7 @@ static void vm_lock_mapping(struct mm_struct *mm, struct address_space *mapping)
> >   *     hugetlb mapping);
> >   *   - all i_mmap_rwsem locks;
> >   *   - all anon_vma->rwseml
> > + *   - all vmas marked locked
>
> IIRC, the anon_vma may be locked during the page fault handling, and this
> happens after the VMA is read lock. I think the same applies to
> i_mmap_rwsem lock.
>
> Thus, the VMA should be marked locked first.

I see. I'll double check and move the locking order. Thanks!

>
> >   *
> >   * We can take all locks within these types randomly because the VM code
> >   * doesn't nest them and we protected from parallel mm_take_all_locks() by
> > @@ -3579,6 +3580,7 @@ int mm_take_all_locks(struct mm_struct *mm)
> >               if (vma->anon_vma)
> >                       list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
> >                               vm_lock_anon_vma(mm, avc->anon_vma);
> > +             vma_mark_locked(vma);
> >       }
> >
> >       return 0;
> > @@ -3636,6 +3638,7 @@ void mm_drop_all_locks(struct mm_struct *mm)
> >       mmap_assert_write_locked(mm);
> >       BUG_ON(!mutex_is_locked(&mm_all_locks_mutex));
> >
> > +     vma_mark_unlocked_all(mm);
> >       for (vma = mm->mmap; vma; vma = vma->vm_next) {
> >               if (vma->anon_vma)
> >                       list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
>