linux-kernel - Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

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Message-ID: <CAOUHufY9EQ70Pn-n2zVa9=Gm3-WHxxphp7VHia4qv9x2domdbg@mail.gmail.com>
Date:   Thu, 3 Aug 2023 14:46:23 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     Ryan Roberts <ryan.roberts@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        akpm@...ux-foundation.org, willy@...radead.org,
        vishal.moola@...il.com, wangkefeng.wang@...wei.com,
        minchan@...nel.org, david@...hat.com, shy828301@...il.com
Subject: Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

On Wed, Aug 2, 2023 at 6:56 AM Yin, Fengwei <fengwei.yin@...el.com> wrote:
>
>
>
> On 8/2/2023 8:49 PM, Ryan Roberts wrote:
> > On 02/08/2023 13:42, Yin, Fengwei wrote:
> >>
> >>
> >> On 8/2/2023 8:40 PM, Ryan Roberts wrote:
> >>> On 02/08/2023 13:35, Yin, Fengwei wrote:
> >>>>
> >>>>
> >>>> On 8/2/2023 6:27 PM, Ryan Roberts wrote:
> >>>>> On 28/07/2023 17:13, Yin Fengwei wrote:
> >>>>>> In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(),
> >>>>>> folio_mapcount() is used to check whether the folio is shared. But it's
> >>>>>> not correct as folio_mapcount() returns total mapcount of large folio.
> >>>>>>
> >>>>>> Use folio_estimated_sharers() here as the estimated number is enough.
> >>>>>>
> >>>>>> Yin Fengwei (2):
> >>>>>>   madvise: don't use mapcount() against large folio for sharing check
> >>>>>>   madvise: don't use mapcount() against large folio for sharing check
> >>>>>>
> >>>>>>  mm/huge_memory.c | 2 +-
> >>>>>>  mm/madvise.c     | 6 +++---
> >>>>>>  2 files changed, 4 insertions(+), 4 deletions(-)
> >>>>>>
> >>>>>
> >>>>> As a set of fixes, I agree this is definitely an improvement, so:
> >>>>>
> >>>>> Reviewed-By: Ryan Roberts
> >>>> Thanks.
> >>>>
> >>>>>
> >>>>>
> >>>>> But I have a couple of comments around further improvements;
> >>>>>
> >>>>> Once we have the scheme that David is working on to be able to provide precise
> >>>>> exclusive vs shared info, we will probably want to move to that. Although that
> >>>>> scheme will need access to the mm_struct of a process known to be mapping the
> >>>>> folio. We have that info, but its not passed to folio_estimated_sharers() so we
> >>>>> can't just reimplement folio_estimated_sharers() - we will need to rework these
> >>>>> call sites again.
> >>>> Yes. This could be extra work. Maybe should delay till David's work is done.
> >>>
> >>> What you have is definitely an improvement over what was there before. And is
> >>> probably the best we can do without David's scheme. So I wouldn't delay this.
> >>> Just pointing out that we will be able to make it even better later on (if
> >>> David's stuff goes in).
> >> Yes. I agree that we should wait for David's work ready and do fix based on that.
> >
> > I was suggesting the opposite - not waiting. Then we can do separate improvement
> > later.
> Let's wait for David's work ready.

Waiting is fine as long as we don't miss the next merge window -- we
don't want these two bugs to get into another release. Also I think we
should cc stable, since as David mentioned, they have been causing
selftest failures.