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To:     Peter Xu <peterx@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation
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On 02.10.23 17:23, Peter Xu wrote:
> 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.

That does sound reasonable, thanks.

Probably we can do better with the patch description (once [1] is used 
to move the folio to the other anon_vma).

"mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()

For now, folio_move_anon_rmap() was only used to move a folio to a 
different anon_vma after fork(), whereby the root anon_vma stayed 
unchanged. For that, it was sufficient to hold the page lock when 
calling folio_move_anon_rmap().

However, we want to make use of folio_move_anon_rmap() to move folios 
between VMAs that have a different root anon_vma. As folio_referenced() 
performs an RMAP walk without holding the page lock but only holding the 
anon_vma in read mode, holding the page lock is insufficient.

When moving to an anon_vma with a different root anon_vma, we'll have to 
hold both, the page lock and the anon_vma lock in write mode. 
Consequently, whenever we succeeded in folio_lock_anon_vma_read() to 
read-lock the anon_vma, we have to re-check if the mapping was changed 
in the meantime. If that was the case, we have to retry.

Note that folio_move_anon_rmap() must only be called if the anon page is 
exclusive to a process, and must not be called on KSM folios.

This is a preparation for UFFDIO_REMAP, which will hold the page lock, 
the anon_vma lock in write mode, and the mmap_lock in read mode.
"

In addition, we should document these locking details for 
folio_move_anon_rmap() and probably not mention UFFDIO_REMAP in the 
comment in folio_lock_anon_vma_read(), but instead say 
"folio_move_anon_rmap() might have changed the anon_vma as we might not 
hold the page lock here."


[1] https://lkml.kernel.org/r/20231002142949.235104-3-david@redhat.com

-- 
Cheers,

David / dhildenb