Re: [PATCH] madvise:madvise_cold_or_pageout_pte_range(): allow split while folio_estimated_sharers = 0

[Barry Song <21cnbao@gmail.com>]

On Tue, Feb 27, 2024 at 2:46 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 21/02/2024 08:50, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > The purpose is stopping splitting large folios whose mapcount are 2 or
> > above. Folios whose estimated_shares = 0 should be still perfect and
> > even better candidates than estimated_shares = 1.
> >
> > Consider a pte-mapped large folio with 16 subpages, if we unmap 1-15,
> > the current code will split folios and reclaim them while madvise goes
> > on this folio; but if we unmap subpage 0, we will keep this folio and
> > break. This is weird.
> >
> > For pmd-mapped large folios, we can still use "= 1" as the condition
> > as anyway we have the entire map for it. So this patch doesn't change
> > the condition for pmd-mapped large folios.
> > This also explains why we had been using "= 1" for both pmd-mapped and
> > pte-mapped large folios before commit 07e8c82b5eff ("madvise: convert
> > madvise_cold_or_pageout_pte_range() to use folios"), because in the
> > past, we used the mapcount of the specific subpage, since the subpage
> > had pte present, its mapcount wouldn't be 0.
> >
> > The problem can be quite easily reproduced by writing a small program,
> > unmapping the first subpage of a pte-mapped large folio vs. unmapping
> > anyone other than the first subpage.
> >
> > Fixes: 2f406263e3e9 ("madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check")
> > Cc: Yin Fengwei <fengwei.yin@intel.com>
> > Cc: Yu Zhao <yuzhao@google.com>
> > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Minchan Kim <minchan@kernel.org>
> > Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> > Cc: Yang Shi <shy828301@gmail.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >  mm/madvise.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index cfa5e7288261..abde3edb04f0 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -453,7 +453,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >               if (folio_test_large(folio)) {
> >                       int err;
> >
> > -                     if (folio_estimated_sharers(folio) != 1)
> > +                     if (folio_estimated_sharers(folio) > 1)
> >                               break;
> >                       if (pageout_anon_only_filter && !folio_test_anon(folio))
> >                               break;
>
> I wonder if we should change all the instances:
>
> folio_estimated_sharers() != 1   ->   folio_estimated_sharers() > 1
> folio_estimated_sharers() == 1   ->   folio_estimated_sharers() <= 1
>
> It shouldn't cause a problem for the pmd case, and there are definitely other
> cases where it will help. e.g. madvise_free_pte_range().

right. My test case covered PAGEOUT only and I agree madvise_free and
others have
exactly the same issue. for pmd case, it doesn't matter whether we
change the condition
or not because we have already pmd-mapped in the page table.

And good to know David will have a wrapper in folio_mapped_shared()  to more
widely address this issue.

>
> Regardless:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
>

Thanks though we might have missed your tag as this one has been
in mm-stable.

Best regards,
Barry