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

[SeongJae Park <sj@kernel.org>]

On Sun, 25 Feb 2024 03:50:48 +0800 Barry Song <21cnbao@gmail.com> wrote:

> On Sun, Feb 25, 2024 at 3:02 AM SeongJae Park <sj@kernel.org> wrote:
> >
> > On Fri, 23 Feb 2024 17:15:50 +1300 Barry Song <21cnbao@gmail.com> wrote:
> >
> > > From: Barry Song <v-songbaohua@oppo.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.
> > > It seems we can do it by directly ignoring references, thus we
> > > can remove tlb flush in madvise and rmap overhead in vmscan.
> > >
> > > Regarding the side effect, in the original code, if a parallel
> > > thread runs side by side to access the madvised memory with the
> > > thread doing madvise, folios will get a chance to be re-activated
> > > by vmscan. But with the patch, they will still be reclaimed. But
> > > this behaviour doing PAGEOUT and doing access at the same time is
> > > quite silly like DoS. So probably, we don't need to care.
> >
> > I think we might need to take care of the case, since users may use just a
> > best-effort estimation like DAMON for the target pages.  In such cases, the
> > page granularity re-check of the access could be helpful.  So I concern if this
> > could be a visible behavioral change for some valid use cases.
> 
> Hi SeongJae,
> 
> If you read the code of MADV_PAGEOUT,  you will find it is not the best-effort.

I'm not saying about MADV_PAGEOUT, but the logic of ther user of MADV_PAGEOUT,
which being used for finding the pages to reclaim.

> It does clearing pte  young and immediately after the ptes are cleared, it reads
> pte and checks if the ptes are young. If not, reclaim it. So the
> purpose of clearing
> PTE young is helping the check of young in folio_references to return false.
> The gap between clearing ptes and re-checking ptes is quite small at
> microseconds
> level.
> 
> >
> > >
> > > A microbench as below has shown 6% decrement on the latency of
> > > MADV_PAGEOUT,
> >
> > I assume some of the users may use MADV_PAGEOUT for proactive reclamation of
> > the memory.  In the use case, I think latency of MADV_PAGEOUT might be not that
> > important.
> >
> > Hence I think the cons of the behavioral change might outweigh the pros of the
> > latench improvement, for such best-effort proactive reclamation use case.  Hope
> > to hear and learn from others' opinions.
> 
> I don't see  the behavioral change for MADV_PAGEOUT as just the ping-pong
> is removed. The only chance is in that very small time gap, somebody accesses
> the cleared ptes and makes it young again, considering this time gap
> is so small,
> i don't think it is worth caring.  thus, i don't see pros for MADV_PAGEOUT
> case, but we improve the efficiency of MADV_PAGEOUT and save the power of
> Android phones.

Ok, I agree the time gap is small enough and the benefit could be significant
on such use case.  Thank you for enlightening me with the nice examples and the
numbers :)


Thanks,
SJ

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