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

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Message-ID: <20130130162257.GB21614@cmpxchg.org>
Date:	Wed, 30 Jan 2013 11:22:57 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
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 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.
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