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Date:   Fri, 13 Jan 2023 03:10:37 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Yang Shi <shy828301@gmail.com>
Cc:     Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Zach O'Keefe <zokeefe@google.com>,
        linux-kernel@vger.kernel.org, David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm/khugepaged: Fix ->anon_vma race
Message-ID: <20230113001037.adqha6lxj7lh75at@box.shutemov.name>
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 <CAHbLzkpAVTys9dBSodHBB3ovKhwP8imUsyXy=aPqY5SyXCT7ww@mail.gmail.com>
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On Thu, Jan 12, 2023 at 10:12:53AM -0800, Yang Shi wrote:
> > This is totally wrong direction. Or I don't understand the race.
> >
> > At this point we already paid nearly all price of of pagetable retraction.
> > I don't see any correctness reason to stop here, except for the assert.
> 
> Isn't it possible that collapse_and_free_pmd() clear the pmd which may
> point to a PTE which maps the COW'ed anon page if this race happens?

No. At this point we have huge page in the place in the page cache and it
is locked. COW fault would serialize on the page lock.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov