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

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Date: Fri, 23 Feb 2024 15:24:53 -0800
From: Minchan Kim <minchan@...nel.org>
To: Barry Song <21cnbao@...il.com>
Cc: sj@...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 Sat, Feb 24, 2024 at 11:20:36AM +1300, Barry Song wrote:
> On Sat, Feb 24, 2024 at 11:09 AM Minchan Kim <minchan@...nel.org> wrote:
> >
> > Hi Barry,
> >
> > On Fri, Feb 23, 2024 at 05:15:50PM +1300, Barry Song wrote:
> > > From: Barry Song <v-songbaohua@...o.com>
> > >
> > > While doing MADV_PAGEOUT, the current code will clear PTE young
> > > so that vmscan won't read young flags to allow the reclamation
> > > of madvised folios to go ahead.
> >
> > Isn't it good to accelerate reclaiming? vmscan checks whether the
> > page was accessed recenlty by the young bit from pte and if it is,
> > it doesn't reclaim the page. Since we have cleared the young bit
> > in pte in madvise_pageout, vmscan is likely to reclaim the page
> > since it wouldn't see the ferencecd_ptes from folio_check_references.
> 
> right, but the proposal is asking vmscan to skip the folio_check_references
> if this is a PAGEOUT. so we remove both pte_clear_young and rmap
> of folio_check_references.
> 
> >
> > Could you clarify if I miss something here?
> 
> guest you missed we are skipping folio_check_references now.
> we remove both, thus, make MADV_PAGEOUT 6% faster.

This makes sense to me.

Only concern was race with mlock during the reclaim but the race was already
there for normal page reclaming. Thus, mlock would already handle it.

Thanks.