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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 8 Sep 2022 17:51:23 -0700
Message-ID: <CAJuCfpFm8irJyhzn-22QR+_4BmU+yDZM97T4S8pPKotxZUmjBg@mail.gmail.com>
Subject: Re: [RFC PATCH RESEND 10/28] mm/mmap: mark VMAs as locked in vma_adjust
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,
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        kent.overstreet@linux.dev, rientjes@google.com,
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        kernel-team@android.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
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On Tue, Sep 6, 2022 at 8:35 AM Laurent Dufour <ldufour@linux.ibm.com> wrote=
:
>
> Le 01/09/2022 =C3=A0 19:34, Suren Baghdasaryan a =C3=A9crit :
> > vma_adjust modifies a VMA and possibly its neighbors. Mark them as lock=
ed
> > before making the modifications.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/mmap.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index f89c9b058105..ed58cf0689b2 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -710,6 +710,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsig=
ned long start,
> >       long adjust_next =3D 0;
> >       int remove_next =3D 0;
> >
> > +     vma_mark_locked(vma);
> > +     if (next)
> > +             vma_mark_locked(next);
> > +
>
> I was wondering if the VMAs insert and expand should be locked too.
>
> For expand, I can't see any valid reason, but for insert, I'm puzzled.
> I would think that it is better to lock the VMA to be inserted but I can'=
t
> really justify that.
>
> It may be nice to detail why this is not need to lock insert and expand h=
ere.

'expand' is always locked before it's passed to __vma_adjust() by
vma_merge(). It has to be locked before we decide "Can it merge with
the predecessor?" here
https://elixir.bootlin.com/linux/latest/source/mm/mmap.c#L1201 because
a change in VMA can affect that decision. I spent many hours tracking
the issue caused by not locking the VMA before making this decision.
It might be good to add a comment about this...

AFAIKT 'insert' is only used by __split_vma() and it's always a brand
new VMA which is not yet linked into mm->mmap. Any reason
__vma_adjust() should lock it?

>
> >       if (next && !insert) {
> >               struct vm_area_struct *exporter =3D NULL, *importer =3D N=
ULL;
> >
> > @@ -754,8 +758,11 @@ int __vma_adjust(struct vm_area_struct *vma, unsig=
ned long start,
> >                        * If next doesn't have anon_vma, import from vma=
 after
> >                        * next, if the vma overlaps with it.
> >                        */
> > -                     if (remove_next =3D=3D 2 && !next->anon_vma)
> > +                     if (remove_next =3D=3D 2 && !next->anon_vma) {
> >                               exporter =3D next->vm_next;
> > +                             if (exporter)
> > +                                     vma_mark_locked(exporter);
> > +                     }
> >
> >               } else if (end > next->vm_start) {
> >                       /*
> > @@ -931,6 +938,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsign=
ed long start,
> >                        * "vma->vm_next" gap must be updated.
> >                        */
> >                       next =3D vma->vm_next;
> > +                     if (next)
> > +                             vma_mark_locked(next);
> >               } else {
> >                       /*
> >                        * For the scope of the comment "next" and
>
> --
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