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Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce
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To:     Linus Torvalds <torvalds@linux-foundation.org>
CC:     Andrea Arcangeli <aarcange@redhat.com>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 1/7/21 2:24 PM, Linus Torvalds wrote:
> On Thu, Jan 7, 2021 at 2:20 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> Hmm, add a check for the page being PageAnon(), perhaps?
>>
>> If it's a shared vma, then the page can be pinned shared with multiple
>> mappings, I agree.
> 
> Or check the vma directly for whether it's a COW vma. That's probably
> a more obvious test, but would have to be done outside of
> page_maybe_dma_pinned().
> 
> For example, in copy_present_page(), we've already done that COW-vma
> test, so if we want to strengthen just _that_ test, then it would be
> sufficient to just add a
> 
>          /* This cannot be a pinned page if it has more than one mapping */
>          if (page_mappings(page) != 1)
>                  return 1;
> 
> to copy_present_page() along with the existing page_maybe_dma_pinned() test.
> 
> No?
> 
>              Linus

That approach makes me a lot happier, yes. Because it doesn't add constraints
to the RDMA cases, but still does what we need here.


thanks,
-- 
John Hubbard
NVIDIA