Re: [PATCH v3 1/3] mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()

[Suren Baghdasaryan]

On Fri, Oct 13, 2023 at 1:04 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 13.10.23 00:01, Peter Xu wrote:
> > On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote:
> >> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> >>
> >> 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 folio 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 folio lock but only holding the
> >> anon_vma in read mode, holding the folio lock is insufficient.
> >>
> >> When moving to an anon_vma with a different root anon_vma, we'll have to
> >> hold both, the folio 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_MOVE, which will hold the folio lock,
> >> the anon_vma lock in write mode, and the mmap_lock in read mode.
> >>
> >> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> >> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> >> ---
> >>   mm/rmap.c | 24 ++++++++++++++++++++++++
> >>   1 file changed, 24 insertions(+)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index c1f11c9dbe61..f9ddc50269d2 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
> >>      struct anon_vma *root_anon_vma;
> >>      unsigned long anon_mapping;
> >>
> >> +retry:
> >>      rcu_read_lock();
> >> +retry_under_rcu:
> >>      anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
> >>      if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
> >>              goto out;
> >> @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
> >>      anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> >>      root_anon_vma = READ_ONCE(anon_vma->root);
> >>      if (down_read_trylock(&root_anon_vma->rwsem)) {
> >> +            /*
> >> +             * folio_move_anon_rmap() might have changed the anon_vma as we
> >> +             * might not hold the folio lock here.
> >> +             */
> >> +            if (unlikely((unsigned long)READ_ONCE(folio->mapping) !=
> >> +                         anon_mapping)) {
> >> +                    up_read(&root_anon_vma->rwsem);
> >> +                    goto retry_under_rcu;
> >
> > Is adding this specific label worthwhile?  How about rcu unlock and goto
> > retry (then it'll also be clear that we won't hold rcu read lock for
> > unpredictable time)?
>
> +1, sounds good to me

Sorry for the delay, I was travelling for a week.

I was hesitant about RCU unlocking and then immediately re-locking but
your point about holding it for unpredictable time makes sense. Will
change. Thanks!

>
> --
> Cheers,
>
> David / dhildenb
>