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In-Reply-To: <cd64f4f5-e77b-d51d-637e-51cf8dba8277@intel.com>
From:   Yu Zhao <yuzhao@google.com>
Date:   Mon, 24 Jul 2023 23:22:16 -0600
Message-ID: <CAOUHufbnsPhjX2nD95dXQmRokHRSZ3hQAmhcVymO9qXUJ-2xpQ@mail.gmail.com>
Subject: Re: [RFC PATCH v2 1/4] madvise: not use mapcount() against large
 folio for sharing check
To:     "Yin, Fengwei" <fengwei.yin@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, minchan@kernel.org, willy@infradead.org,
        david@redhat.com, ryan.roberts@arm.com, shy828301@gmail.com
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On Sun, Jul 23, 2023 at 6:27=E2=80=AFAM Yin, Fengwei <fengwei.yin@intel.com=
> wrote:
>
>
>
> On 7/22/2023 2:57 AM, Yu Zhao wrote:
> > On Fri, Jul 21, 2023 at 3:41=E2=80=AFAM Yin Fengwei <fengwei.yin@intel.=
com> wrote:
> >>
> >> The commit
> >> 07e8c82b5eff ("madvise: convert madvise_cold_or_pageout_pte_range() to
> >> use folios") replaced the page_mapcount() with folio_mapcount() to
> >> check whether the folio is shared by other mapping.
> >>
> >> But it's not correct for large folio. folio_mapcount() returns the
> >> total mapcount of large folio which is not suitable to detect whether
> >> the folio is shared.
> >>
> >> Use folio_estimated_sharers() which returns a estimated number of
> >> shares. That means it's not 100% correct. But it should be OK for
> >> madvise case here.
> >>
> >> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
> >
> > Fixes:
> > Cc: stable
> OK
>
> >
> >> @@ -383,7 +383,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t=
 *pmd,
> >>                 folio =3D pfn_folio(pmd_pfn(orig_pmd));
> >>
> >>                 /* Do not interfere with other mappings of this folio =
*/
> >> -               if (folio_mapcount(folio) !=3D 1)
> >> +               if (folio_estimated_sharers(folio) !=3D 1)
> >
> > Strictly speaking, this isn't a bug. But it may be ok to include it in
> > the same patch.
> OK. I will drop the change for pmd.
>
> >
> >>                         goto huge_unlock;
> >>
> >>                 if (pageout_anon_only_filter && !folio_test_anon(folio=
))
> >> @@ -459,7 +459,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t=
 *pmd,
> >>                 if (folio_test_large(folio)) {
> >>                         int err;
> >>
> >> -                       if (folio_mapcount(folio) !=3D 1)
> >> +                       if (folio_estimated_sharers(folio) !=3D 1)
> >>                                 break;
> >>                         if (pageout_anon_only_filter && !folio_test_an=
on(folio))
> >>                                 break;
> >> @@ -682,7 +682,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsi=
gned long addr,
> >
> > What about madvise_free_huge_pmd()? Should it be changed as well so
> > that it's consistent with the first change? Either change both or neith=
er.
> >
> >>                 if (folio_test_large(folio)) {
> >>                         int err;
> >>
> >> -                       if (folio_mapcount(folio) !=3D 1)
> >> +                       if (folio_estimated_sharers(folio) !=3D 1)
> >
> > This is another bug fix and should be in a separate patch.
> OK. Will split to two patches.
>
> >
> >>                                 break;
> >>                         if (!folio_trylock(folio))
> >>                                 break;
> >
> > Please send two separate fixes, and then:
> >
> > Reviewed-by: Yu Zhao <yuzhao@google.com>
> Thanks a lot. I will drop the mapcount() change for pmd and sent to patch=
es
> for madvise_cold_or_pageout_pte_range() and madvise_free_pte_range().

I don't mind including the PMD changes. Either way works for me :)