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Date:	Wed, 18 Jun 2014 17:40:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Chen Yucong <slaoub@...il.com>
Cc:	minchan@...nel.org, mgorman@...e.de, hannes@...xchg.org,
	mhocko@...e.cz, riel@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan.c: fix an implementation flaw in proportional
 scanning

On Thu, 19 Jun 2014 08:04:32 +0800 Chen Yucong <slaoub@...il.com> wrote:

> On Wed, 2014-06-18 at 15:27 -0700, Andrew Morton wrote:
> > On Tue, 17 Jun 2014 12:55:02 +0800 Chen Yucong <slaoub@...il.com> wrote:
> > 
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index a8ffe4e..2c35e34 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -2087,8 +2086,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> > >  	blk_start_plug(&plug);
> > >  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> > >  					nr[LRU_INACTIVE_FILE]) {
> > > -		unsigned long nr_anon, nr_file, percentage;
> > > -		unsigned long nr_scanned;
> > > +		unsigned long nr_anon, nr_file, file_percent, anon_percent;
> > > +		unsigned long nr_to_scan, nr_scanned, percentage;
> > >  
> > >  		for_each_evictable_lru(lru) {
> > >  			if (nr[lru]) {
> > 
> > The increased stack use is a slight concern - we can be very deep here.
> > I suspect the "percent" locals are more for convenience/clarity, and
> > they could be eliminated (in a separate patch) at some cost of clarity?
> > 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a8ffe4e..2c35e34 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2057,8 +2057,7 @@ out:
>  static void shrink_lruvec(struct lruvec *lruvec, struct scan_control
> *sc)
>  {
>         unsigned long nr[NR_LRU_LISTS];
> -       unsigned long targets[NR_LRU_LISTS];
> -       unsigned long nr_to_scan;
> +       unsigned long file_target, anon_target;
> 
> >From the above snippet, we can know that the "percent" locals come from
> targets[NR_LRU_LISTS]. So this fix does not increase the stack.

OK.  But I expect the stack use could be decreased by using more
complex expressions.

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