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

Suren Baghdasaryan
2023-09-28T20:03:58Z

On Thu, Sep 28, 2023 at 9:23 AM Peter Xu <peterx@redhat.com> wrote:
>
> Suren,
>
> Sorry to review so late.
>
> On Fri, Sep 22, 2023 at 06:31:44PM -0700, Suren Baghdasaryan wrote:
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index ec7f8e6c9e48..c1ebbd23fa61 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -542,6 +542,7 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
> > struct anon_vma *root_anon_vma;
> > unsigned long anon_mapping;
> >
> > +repeat:
> > rcu_read_lock();
> > anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
> > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
> > @@ -586,6 +587,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
> > rcu_read_unlock();
> > anon_vma_lock_read(anon_vma);
> >
> > + /*
> > + * Check if UFFDIO_REMAP changed the anon_vma. This is needed
> > + * because we don't assume the folio was locked.
> > + */
> > + if (unlikely((unsigned long) READ_ONCE(folio->mapping) !=
> > + anon_mapping)) {
> > + anon_vma_unlock_read(anon_vma);
> > + put_anon_vma(anon_vma);
> > + anon_vma = NULL;
> > + goto repeat;
> > + }
>
> We have an open-coded fast path above this:
>
> if (down_read_trylock(&root_anon_vma->rwsem)) {
> /*
> * If the folio is still mapped, then this anon_vma is still
> * its anon_vma, and holding the mutex ensures that it will
> * not go away, see anon_vma_free().
> */
> if (!folio_mapped(folio)) {
> up_read(&root_anon_vma->rwsem);
> anon_vma = NULL;
> }
> goto out;
> }
>
> Would that also need such check?

Yes, I think they should be handled the same way. Will fix. Thanks!

>
> > +
> > if (atomic_dec_and_test(&anon_vma->refcount)) {
> > /*
> > * Oops, we held the last refcount, release the lock
> > --
> > 2.42.0.515.g380fc7ccd1-goog
> >
>
> --
> Peter Xu
>