Re: [RFC PATCH v2 1/4] madvise: not use mapcount() against large folio for sharing check

[Yu Zhao]

On Sun, Jul 23, 2023 at 6:27 AM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>
>
>
> On 7/22/2023 2:57 AM, Yu Zhao wrote:
> > On Fri, Jul 21, 2023 at 3:41 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> 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@xxxxxxxxx>
> >
> > Fixes:
> > Cc: stable
> OK
>
> >
> >> @@ -383,7 +383,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >>                 folio = pfn_folio(pmd_pfn(orig_pmd));
> >>
> >>                 /* Do not interfere with other mappings of this folio */
> >> -               if (folio_mapcount(folio) != 1)
> >> +               if (folio_estimated_sharers(folio) != 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) != 1)
> >> +                       if (folio_estimated_sharers(folio) != 1)
> >>                                 break;
> >>                         if (pageout_anon_only_filter && !folio_test_anon(folio))
> >>                                 break;
> >> @@ -682,7 +682,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned 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 neither.
> >
> >>                 if (folio_test_large(folio)) {
> >>                         int err;
> >>
> >> -                       if (folio_mapcount(folio) != 1)
> >> +                       if (folio_estimated_sharers(folio) != 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@xxxxxxxxxx>
> Thanks a lot. I will drop the mapcount() change for pmd and sent to patches
> 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 :)