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

[Suren Baghdasaryan]

On Thu, Sep 28, 2023 at 9:23 AM Peter Xu <peterx@xxxxxxxxxx> 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
>