linux-kernel - Re: [PATCH] memcg: change behavior of moving charges at task move

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Message-Id: <20120322173000.f078a43f.akpm@linux-foundation.org>
Date:	Thu, 22 Mar 2012 17:30:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	"n-horiguchi@...jp.nec.com" <n-horiguchi@...jp.nec.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Glauber Costa <glommer@...allels.com>
Subject: Re: [PATCH] memcg: change behavior of moving charges at task move

On Fri, 23 Mar 2012 09:18:46 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> >> +#ifdef CONFIG_SWAP
> >> +	/*
> >> +	 * Avoid lookup_swap_cache() not to update statistics.
> >> +	 */
> > 
> > I don't understand this comment - what is it trying to tell us?
> > 
> 
> 
> High Dickins advised me to use find_get_page() rather than lookup_swap_cache()
> because lookup_swap_cache() has some statistics with swap.

ah.

--- a/mm/memcontrol.c~memcg-change-behavior-of-moving-charges-at-task-move-fix
+++ a/mm/memcontrol.c
@@ -5137,7 +5137,8 @@ static struct page *mc_handle_swap_pte(s
 		return NULL;
 #ifdef CONFIG_SWAP
 	/*
-	 * Avoid lookup_swap_cache() not to update statistics.
+	 * Use find_get_page() rather than lookup_swap_cache() because the
+	 * latter alters statistics.
 	 */
 	page = find_get_page(&swapper_space, ent.val);
 #endif

> >> +	page = find_get_page(&swapper_space, ent.val);
> > 
> > The code won't even compile if CONFIG_SWAP=n?
> > 
> 
> mm/built-in.o: In function `mc_handle_swap_pte':
> /home/kamezawa/Kernel/next/linux/mm/memcontrol.c:5172: undefined reference to `swapper_space'
> make: *** [.tmp_vmlinux1] Error 1
> 
> Ah...but I think this function (mc_handle_swap_pte) itself should be under CONFIG_SWAP.
> I'll post v2.

Confused.  The new reference to swapper_space is already inside #ifdef
CONFIG_SWAP.
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