linux-kernel - Re: [RFC PATCH RESEND 07/28] kernel/fork: mark VMAs as locked before copying pages during fork

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Message-ID: <CAJuCfpFDFzCB7zuOGyd-gqovyhwvcQaUMOUS0E8+1QxLqD-Gdg@mail.gmail.com>
Date:   Thu, 8 Sep 2022 16:57:17 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...e.de, dave@...olabs.net,
        willy@...radead.org, liam.howlett@...cle.com, peterz@...radead.org,
        laurent.dufour@...ibm.com, paulmck@...nel.org, luto@...nel.org,
        songliubraving@...com, peterx@...hat.com, david@...hat.com,
        dhowells@...hat.com, hughd@...gle.com, bigeasy@...utronix.de,
        kent.overstreet@...ux.dev, rientjes@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        kernel-team@...roid.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH RESEND 07/28] kernel/fork: mark VMAs as locked before
 copying pages during fork

On Tue, Sep 6, 2022 at 7:38 AM Laurent Dufour <ldufour@...ux.ibm.com> wrote:
>
> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> > Protect VMAs from concurrent page fault handler while performing
> > copy_page_range for VMAs having VM_WIPEONFORK flag set.
>
> I'm wondering why is that necessary.
> The copied mm is write locked, and the destination one is not reachable.
> If any other readers are using the VMA, this is only for page fault handling.

Correct, this is done to prevent page faulting in the VMA being
duplicated. I assume we want to prevent the pages in that VMA from
changing when we are calling copy_page_range(). Am I wrong?

> I should have miss something because I can't see any need to mark the lock
> VMA here.
>
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > ---
> >  kernel/fork.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index bfab31ecd11e..1872ad549fed 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -709,8 +709,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> >               rb_parent = &tmp->vm_rb;
> >
> >               mm->map_count++;
> > -             if (!(tmp->vm_flags & VM_WIPEONFORK))
> > +             if (!(tmp->vm_flags & VM_WIPEONFORK)) {
> > +                     vma_mark_locked(mpnt);
> >                       retval = copy_page_range(tmp, mpnt);
> > +             }
> >
> >               if (tmp->vm_ops && tmp->vm_ops->open)
> >                       tmp->vm_ops->open(tmp);
>