On Mon, Oct 2, 2023 at 10:30 AM David Hildenbrand david@redhat.com wrote:
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).
I'll develop the next version with your patches in the baseline. Hopefully by the time of my posting your patches will be in the mm-unstable.
"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. "
Thanks for taking time to write this up! Looks really clear to me. I'll reuse it.
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."
Sounds good. Will add.
[1] https://lkml.kernel.org/r/20231002142949.235104-3-david@redhat.com
-- Cheers,
David / dhildenb