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
