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Date:   Mon, 2 Oct 2023 11:23:05 -0400
From:   Peter Xu <peterx@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Suren Baghdasaryan <surenb@google.com>, akpm@linux-foundation.org,
        viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org,
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        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
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Subject: Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation
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On Mon, Oct 02, 2023 at 04:42:50PM +0200, David Hildenbrand wrote:
> On 23.09.23 03:31, Suren Baghdasaryan wrote:
> > From: Andrea Arcangeli <aarcange@redhat.com>
> > 
> > As far as the rmap code is concerned, UFFDIO_REMAP only alters the
> > page->mapping and page->index. It does it while holding the page
> > lock. However folio_referenced() is doing rmap walks without taking the
> > folio lock first, so folio_lock_anon_vma_read() must be updated to
> > re-check that the folio->mapping didn't change after we obtained the
> > anon_vma read lock.
> 
> I'm curious: why don't we need this for existing users of
> page_move_anon_rmap()? What's special about UFFDIO_REMAP?

Totally no expert on anon vma so I'm prone to errors, but IIUC the
difference here is root anon vma cannot change in page_move_anon_rmap(),
while UFFDIO_REMAP can.

Thanks,

-- 
Peter Xu