On Mon, Oct 2, 2023 at 10:30 AM David Hildenbrand <david@xxxxxxxxxx> 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@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). 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@xxxxxxxxxx > > -- > Cheers, > > David / dhildenb >
