Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation

Peter Xu
2023-10-02T15:24:11Z

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.

Thanks,

--
Peter Xu