linux-kernel - Re: [RFC PATCH v3 1/7] mm/damon/paddr: refactor DAMOS_PAGEOUT with migration

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Message-Id: <20240511201617.292811-1-sj@kernel.org>
Date: Sat, 11 May 2024 13:16:17 -0700
From: SeongJae Park <sj@...nel.org>
To: SeongJae Park <sj@...nel.org>
Cc: Honggyu Kim <honggyu.kim@...com>,
	damon@...ts.linux.dev,
	linux-mm@...ck.org,
	akpm@...ux-foundation.org,
	apopple@...dia.com,
	baolin.wang@...ux.alibaba.com,
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	mathieu.desnoyers@...icios.com,
	mhiramat@...nel.org,
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	art.jeongseob@...il.com
Subject: Re: [RFC PATCH v3 1/7] mm/damon/paddr: refactor DAMOS_PAGEOUT with migration_mode

On Fri,  5 Apr 2024 12:19:07 -0700 SeongJae Park <sj@...nel.org> wrote:

> On Fri,  5 Apr 2024 15:08:50 +0900 Honggyu Kim <honggyu.kim@...com> wrote:
> 
> > This is a preparation patch that introduces migration modes.
> > 
> > The damon_pa_pageout is renamed to damon_pa_migrate and it receives an
> > extra argument for migration_mode.
> 
> I personally think keeping damon_pa_pageout() as is and adding a new function
> (damon_pa_migrate()) with some duplicated code is also ok, but this approach
> also looks fine to me.  So I have no strong opinion here, but just letting you
> know I would have no objection at both approaches.

Meanwhile, we added one more logic in damon_pa_pageout() for doing page
idleness double check on its own[1].  It makes reusing damon_pa_pageout() for
multiple reason a bit complex.  I think the complexity added a problem in this
patch that I also missed before due to the complexity.  Show below comment in
line.  Hence now I think it would be better to do the suggested way.

If we use the approach, this patch is no more necessary, and therefore can be
dropped.

[1] https://lore.kernel.org/20240426195247.100306-1-sj@kernel.org


Thanks,
SJ

[...]
> 
> > 
> > No functional changes applied.
> > 
> > Signed-off-by: Honggyu Kim <honggyu.kim@...com>
> > ---
> >  mm/damon/paddr.c | 18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
> > index 081e2a325778..277a1c4d833c 100644
> > --- a/mm/damon/paddr.c
> > +++ b/mm/damon/paddr.c
> > @@ -224,7 +224,12 @@ static bool damos_pa_filter_out(struct damos *scheme, struct folio *folio)
> >  	return false;
> >  }
> >  
> > -static unsigned long damon_pa_pageout(struct damon_region *r, struct damos *s)
> > +enum migration_mode {
> > +	MIG_PAGEOUT,
> > +};
> 
> To avoid name conflicts, what about renaming to 'damos_migration_mode' and
> 'DAMOS_MIG_PAGEOUT'?
> 
> > +
> > +static unsigned long damon_pa_migrate(struct damon_region *r, struct damos *s,
> > +				      enum migration_mode mm)
> 
> My poor brain has a bit confused with the name.  What about calling it 'mode'?
> 
> >  {
> >  	unsigned long addr, applied;
> >  	LIST_HEAD(folio_list);
> > @@ -249,7 +254,14 @@ static unsigned long damon_pa_pageout(struct damon_region *r, struct damos *s)

Before this line, damon_pa_pageout() calls folio_clear_referenced() and
folio_test_clear_young() for the folio, because this is pageout code.  Changed
function, damon_pa_migrate() is not only for cold pages but general migrations.
Hence it should also be handled based on the migration mode, but not handled.

I think this problem came from the increased complexity of this function.
Hence I think it is better to keep damon_pa_pageout() as is and adding a new
function for migration.


Thanks,
SJ

[...]