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From:   Barry Song <21cnbao@gmail.com>
Date:   Thu, 14 Dec 2023 09:34:35 +0800
Message-ID: <CAGsJ_4zYq2d757BBeQ=2HhksdChvK_i+Srp_Rxw3tcu4YWEcNQ@mail.gmail.com>
Subject: Re: [PATCH] mm: compaction: avoid fast_isolate_freepages blindly
 choose improper pageblock
To:     Mel Gorman <mgorman@techsingularity.net>, akpm@linux-foundation.org
Cc:     baolin.wang@linux.alibaba.com, linux-mm@kvack.org,
        david@redhat.com, hannes@cmpxchg.org, huzhanyuan@oppo.com,
        linux-kernel@vger.kernel.org, shikemeng@huaweicloud.com,
        v-songbaohua@oppo.com, willy@infradead.org
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On Wed, Dec 13, 2023 at 9:02=E2=80=AFPM Mel Gorman <mgorman@techsingularity=
.net> wrote:
>
> On Thu, Dec 07, 2023 at 12:00:54AM +1300, Barry Song wrote:
> > Testing shows fast_isolate_freepages can blindly choose an unsuitable
> > pageblock from time to time particularly while the min mark is used
> > from XXX path:
> >  if (!page) {
> >          cc->fast_search_fail++;
> >          if (scan_start) {
> >                  /*
> >                   * Use the highest PFN found above min. If one was
> >                   * not found, be pessimistic for direct compaction
> >                   * and use the min mark.
> >                   */
> >                  if (highest >=3D min_pfn) {
> >                          page =3D pfn_to_page(highest);
> >                          cc->free_pfn =3D highest;
> >                  } else {
> >                          if (cc->direct_compaction && pfn_valid(min_pfn=
)) { /* XXX */
> >                                  page =3D pageblock_pfn_to_page(min_pfn=
,
> >                                          min(pageblock_end_pfn(min_pfn)=
,
> >                                              zone_end_pfn(cc->zone)),
> >                                          cc->zone);
> >                                  cc->free_pfn =3D min_pfn;
> >                          }
> >                  }
> >          }
> >  }
> >
> > The reason is that no code is doing any check on the min_pfn
> >  min_pfn =3D pageblock_start_pfn(cc->free_pfn - (distance >> 1));
> >
> > In contrast, slow path of isolate_freepages() is always skipping unsuit=
able
> > pageblocks in a decent way.
> >
> > This issue doesn't happen quite often. When running 25 machines with 16=
GiB
> > memory for one night, most of them can hit this unexpected code path.
> > However the frequency isn't like many times per second. It might be one
> > time in a couple of hours. Thus, it is very hard to measure the visible
> > performance impact in my machines though the affection of choosing the
> > unsuitable migration_target should be negative in theory.
> >
> > I feel it's still worth fixing this to at least make the code theoretic=
ally
> > self-explanatory as it is quite odd an unsuitable migration_target can =
be
> > still migration_target.
> >
> > Reported-by: Zhanyuan Hu <huzhanyuan@oppo.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>
> Acked-by: Mel Gorman <mgorman@techsingularity.net>

Hi Mel,
Thanks!

Hi Andrew,
Given this patch has been in mm-stable,
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?h=3Dmm-=
stable&id=3Dd19b1a1797
does it still have a chance to collect Mel's tag=EF=BC=9F

>
> --
> Mel Gorman
> SUSE Labs

Thanks
Barry