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@xxxxxxxxxx>

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@xxxxxxxxxx

--
Cheers,

David / dhildenb





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