linux-kernel - Re: [BUGFIX][PATCH] memcg: fix memory migration of shmem swapcache

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Message-ID: <AANLkTikCQbzQcUjxtgLrSVtF76Jr9zTmXUhO_yDWss5k@mail.gmail.com>
Date:	Wed, 5 Jan 2011 13:48:50 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [BUGFIX][PATCH] memcg: fix memory migration of shmem swapcache

Hi,

On Wed, Jan 5, 2011 at 1:00 PM, Daisuke Nishimura
<nishimura@....nes.nec.co.jp> wrote:
> Hi.
>
> This is a fix for a problem which has bothered me for a month.
>
> ===
> From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
>
> In current implimentation, mem_cgroup_end_migration() decides whether the page
> migration has succeeded or not by checking "oldpage->mapping".
>
> But if we are tring to migrate a shmem swapcache, the page->mapping of it is
> NULL from the begining, so the check would be invalid.
> As a result, mem_cgroup_end_migration() assumes the migration has succeeded
> even if it's not, so "newpage" would be freed while it's not uncharged.
>
> This patch fixes it by passing mem_cgroup_end_migration() the result of the
> page migration.
>
> Signed-off-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Reviewed-by: Minchan Kim <minchan.kim@...il.com>

Nice catch. I don't oppose the patch.
But as looking the code in unmap_and_move, I feel part of mem cgroup
migrate is rather awkward.

int unmap_and_move()
{
   charge = mem_cgroup_prepare_migration(xxx);
   ..
   BUG_ON(charge); <-- BUG if it is charged?
   ..
uncharge:
   if (!charge)    <-- why do we have to uncharge !charge?
      mem_group_end_migration(xxx);
   ..
}

'charge' local variable isn't good. How about changing "uncharge" or whatever?
Of course, It would be another patch.
If you don't mind, I will send the patch or you may send the patch.

Thanks,

-- 
Kind regards,
Minchan Kim
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