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References: <20240224190724.45681-1-sj@kernel.org> <20240224201221.46450-1-sj@kernel.org>
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From: Barry Song <21cnbao@gmail.com>
Date: Sun, 25 Feb 2024 04:33:25 +0800
Message-ID: <CAGsJ_4w4wnbryJaAKRtNGNMg_YTGkeG3QwL9_JkT=wtwq1+_+Q@mail.gmail.com>
Subject: Re: [PATCH RFC] mm: madvise: pageout: ignore references rather than
 clearing young
To: SeongJae Park <sj@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>, akpm@linux-foundation.org, damon@lists.linux.dev, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org, mhocko@suse.com, 
	hannes@cmpxchg.org, Barry Song <v-songbaohua@oppo.com>
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On Sun, Feb 25, 2024 at 4:12=E2=80=AFAM SeongJae Park <sj@kernel.org> wrote=
:
>
> On Sat, 24 Feb 2024 11:07:23 -0800 SeongJae Park <sj@kernel.org> wrote:
>
> > Hi Barry,
> >
> > On Sat, 24 Feb 2024 12:37:59 +0800 Barry Song <21cnbao@gmail.com> wrote=
:
> >
> > [...]
> > >
> > > BTW\uff0c
> > > Hi SeongJae,
> > > I am not quite sure if damon also needs this, so I have kept damon as=
 is by
> > > setting ignore_references =3D false.  MADV_PAGEOUT is an explicit hin=
t users
> > > don't want the memory to be reclaimed, I don't know if it is true for=
 damon as
> > > well. If you have some comments, please chime in.
> >
> > Thank you for calling my name :)
> >
> > For DAMON's usecase, the document simply says the behavior would be sam=
e to
> > MADV_PAGEOUT, so if we conclude to change MADV_PAGEOUT, I think same ch=
ange
> > should be made for DAMON's usecase, or update DAMON document.
>
> Thanks to Barry's nice explanation on my other reply to the patch, now I =
think
> the change is modest, and therefore I'd prefer the first way: Changing DA=
MON's
> usecase, and keep the document as is.

Hi SeongJae,

thanks! I actually blindly voted for keeping DAMON's behaviour but
slightly updated the
document as I set ignore_references to false for the DAMON case in the RFC =
:-)

--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -249,7 +249,7 @@ static unsigned long damon_pa_pageout(struct
damon_region *r, struct damos *s)
 put_folio:
  folio_put(folio);
  }
- applied =3D reclaim_pages(&folio_list);
+ applied =3D reclaim_pages(&folio_list, false);
  cond_resched();
  return applied * PAGE_SIZE;
 }

MADV_PAGEOUT comes from userspace by a specific process to tell the kernel
to reclaim its own memory(actually focus on non-shared memory as it
skips folios with
mapcount>1).
The range is a virtual address and the app does know it doesn't want
to access the
range in the foreseeable future.  and the affected app is itself not global=
.

In the DAMON case,  it seems the range is the physical address.  if
the pa is mapped
by more than one process, it seems safer to double-check in the kernel
as it might
affect multiple processes?

Please correct me if I am wrong.

>
>
> Thanks,
> SJ
>
> >
> >
> > Thanks,
> > SJ

Thanks
 Barry