linux-kernel - Re: [PATCH RFC] mm: madvise: pageout: ignore references rather than clearing young

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Date: Sun, 25 Feb 2024 04:33:25 +0800
From: Barry Song <21cnbao@...il.com>
To: SeongJae Park <sj@...nel.org>
Cc: Minchan Kim <minchan@...nel.org>, akpm@...ux-foundation.org, damon@...ts.linux.dev, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, mhocko@...e.com, 
	hannes@...xchg.org, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH RFC] mm: madvise: pageout: ignore references rather than
 clearing young

On Sun, Feb 25, 2024 at 4:12 AM SeongJae Park <sj@...nel.org> wrote:
>
> On Sat, 24 Feb 2024 11:07:23 -0800 SeongJae Park <sj@...nel.org> wrote:
>
> > Hi Barry,
> >
> > On Sat, 24 Feb 2024 12:37:59 +0800 Barry Song <21cnbao@...il.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 = false.  MADV_PAGEOUT is an explicit hint 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 same to
> > MADV_PAGEOUT, so if we conclude to change MADV_PAGEOUT, I think same change
> > 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 DAMON'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 = reclaim_pages(&folio_list);
+ applied = 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