linux-kernel - Re: [RFC PATCH RESEND 08/28] mm/khugepaged: mark VMA as locked while collapsing a hugepage

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Date:   Thu, 8 Sep 2022 17:15:31 -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 08/28] mm/khugepaged: mark VMA as locked while
 collapsing a hugepage

On Tue, Sep 6, 2022 at 7:43 AM Laurent Dufour <ldufour@...ux.ibm.com> wrote:
>
> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> > Protect VMA from concurrent page fault handler while modifying it in
> > collapse_huge_page.
>
> Is the goal to protect changes in the anon_vma structure?
>
> AFAICS, the vma it self is not impacted here, only the anon_vma and the
> PMD/PTE are touched, and they have their own protection mechanism, isn't it?

Yes, I think you are right about not needing to lock VMA here as all
modified components are already protected. Thanks!

>
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > ---
> >  mm/khugepaged.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 01f71786d530..030680633989 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1072,6 +1072,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> >       if (mm_find_pmd(mm, address) != pmd)
> >               goto out_up_write;
> >
> > +     vma_mark_locked(vma);
> >       anon_vma_lock_write(vma->anon_vma);
> >
> >       mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm,
>