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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 28 Sep 2023 13:03:42 -0700
Message-ID: <CAJuCfpEPzJZmrMsD2JkcUjnhhW4sm9i=-1U9n-87YPLWopgm2w@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation
To:     Peter Xu <peterx@redhat.com>
Cc:     akpm@linux-foundation.org, viro@zeniv.linux.org.uk,
        brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com,
        lokeshgidra@google.com, david@redhat.com, hughd@google.com,
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        zhangpeng362@huawei.com, bgeffon@google.com,
        kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
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On Thu, Sep 28, 2023 at 9:23=E2=80=AFAM 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 fo=
lio *folio,
> >       struct anon_vma *root_anon_vma;
> >       unsigned long anon_mapping;
> >
> > +repeat:
> >       rcu_read_lock();
> >       anon_mapping =3D (unsigned long)READ_ONCE(folio->mapping);
> >       if ((anon_mapping & PAGE_MAPPING_FLAGS) !=3D PAGE_MAPPING_ANON)
> > @@ -586,6 +587,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct f=
olio *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) !=3D
> > +                  anon_mapping)) {
> > +             anon_vma_unlock_read(anon_vma);
> > +             put_anon_vma(anon_vma);
> > +             anon_vma =3D 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 st=
ill
>                  * its anon_vma, and holding the mutex ensures that it wi=
ll
>                  * not go away, see anon_vma_free().
>                  */
>                 if (!folio_mapped(folio)) {
>                         up_read(&root_anon_vma->rwsem);
>                         anon_vma =3D 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
>