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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 4 Jun 2021 21:19:55 -0700
Message-ID: <CALvZod5EWuUu7MfJ12_+QTY2MUinmShoS+0hoRGVcn5Q1G6eHw@mail.gmail.com>
Subject: Re: [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to_unmap
To:     Liam Howlett <liam.howlett@oracle.com>
Cc:     Alistair Popple <apopple@nvidia.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
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        Christoph Hellwig <hch@lst.de>
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On Fri, Jun 4, 2021 at 8:39 PM Liam Howlett <liam.howlett@oracle.com> wrote:
>
> > Particularly the following paragraph:
> > ...
> >     Vlastimil Babka points out another race which this patch protects against.
> >      try_to_unmap_one() might reach its mlock_vma_page() TestSetPageMlocked a
> >     moment after munlock_vma_pages_all() did its Phase 1 TestClearPageMlocked:
> >     leaving PageMlocked and unevictable when it should be evictable.  mmap_sem
> >     is ineffective because exit_mmap() does not hold it; page lock ineffective
> >     because __munlock_pagevec() only takes it afterwards, in Phase 2; pte lock
> >     is effective because __munlock_pagevec_fill() takes it to get the page,
> >     after VM_LOCKED was cleared from vm_flags, so visible to try_to_unmap_one.
> > ...
>
> So this is saying the race with exit_mmap() isn't benign after all?
>

Yes, not benign at all.