linux-kernel - Re: [PATCH] mmotm: memcgvmscan-do-not-break-out-targeted-reclaim-without-reclaimed-pages.patch fix

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Date:	Wed, 30 Jan 2013 17:37:50 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Ying Han <yinghan@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Tejun Heo <htejun@...il.com>,
	Glauber Costa <glommer@...allels.com>, linux-mm@...ck.org,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH] mmotm:
 memcgvmscan-do-not-break-out-targeted-reclaim-without-reclaimed-pages.patch
 fix

On Wed 30-01-13 11:22:57, Johannes Weiner wrote:
> On Tue, Jan 29, 2013 at 09:51:04AM +0100, Michal Hocko wrote:
> > Ying has noticed me (via private email) that the patch is bogus because
> > the break out condition is incorrect. She said she would post a fix
> > but she's been probably too busy. If she doesn't oppose, could you add
> > the follow up fix, please?
> > 
> > I am really sorry about this mess.
> > ---
> > >From 6d23b59e96b8173fae2d0d397cb5e99f16899874 Mon Sep 17 00:00:00 2001
> > From: Ying Han <yinghan@...gle.com>
> > Date: Tue, 29 Jan 2013 09:42:28 +0100
> > Subject: [PATCH] mmotm:
> >  memcgvmscan-do-not-break-out-targeted-reclaim-without-reclaimed-pages.patch
> >  fix
> > 
> > We should break out of the hierarchy loop only if nr_reclaimed exceeded
> > nr_to_reclaim and not vice-versa. This patch fixes the condition.
> > 
> > Signed-off-by: Ying Han <yinghan@...gle.com>
> > ---
> >  mm/vmscan.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index d75c1ec..7528eae 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1985,7 +1985,7 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
> >  			 * whole hierarchy is not sufficient.
> >  			 */
> >  			if (!global_reclaim(sc) &&
> > -					sc->nr_to_reclaim >= sc->nr_reclaimed) {
> > +					sc->nr_to_reclaim <= sc->nr_reclaimed) {
> 
> This is just a really weird ordering of the operands, isn't it?  You
> compare the constant to the variable, like if (42 == foo->nr_pages).
> 
>     if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> 
> would be less surprising.

No objections from me.
-- 
Michal Hocko
SUSE Labs
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