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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 9 Sep 2022 09:12:17 -0700
Message-ID: <CAJuCfpGAt3K51JfaEZxUtJc_h5c5Nj6XYQvdF4x+SbC6teFS0g@mail.gmail.com>
Subject: Re: [RFC PATCH RESEND 17/28] mm/mmap: prevent pagefault handler from
 racing with mmu_notifier registration
To:     Laurent Dufour <ldufour@linux.ibm.com>
Cc:     akpm@linux-foundation.org, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
        hannes@cmpxchg.org, mgorman@suse.de, dave@stgolabs.net,
        willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org,
        laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org,
        songliubraving@fb.com, peterx@redhat.com, david@redhat.com,
        dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de,
        kent.overstreet@linux.dev, rientjes@google.com,
        axelrasmussen@google.com, joelaf@google.com, minchan@google.com,
        kernel-team@android.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
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On Fri, Sep 9, 2022 at 7:20 AM Laurent Dufour <ldufour@linux.ibm.com> wrote=
:
>
> Le 01/09/2022 =C3=A0 19:35, Suren Baghdasaryan a =C3=A9crit :
> > 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@google.com>
> > ---
> >  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 co=
de
> >   * 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, sa=
me_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 =3D mm->mmap; vma; vma =3D vma->vm_next) {
> >               if (vma->anon_vma)
> >                       list_for_each_entry(avc, &vma->anon_vma_chain, sa=
me_vma)
>