linux-kernel - Re: [-mm][PATCH 9/10] memcg: fix mem_cgroup_end

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Date:	Wed, 25 Jun 2008 19:53:55 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [-mm][PATCH 9/10]  memcg: fix mem_cgroup_end_migration() race

On Wed, 25 Jun 2008 19:10:11 +0900, KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> =
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> In general, mem_cgroup's charge on ANON page is removed when page_remove_rmap()
> is called.
> 
> At migration, the newpage is remapped again by remove_migration_ptes(). But
> pte may be already changed (by task exits).
> It is charged at page allocation but have no chance to be uncharged in that
> case because it is never added to rmap.
> 
> Handle that corner case in mem_cgroup_end_migration().
> 
> 
Sorry for late reply.

I've confirmed that this patch fixes the bad page problem
I had been seeing on my test(survived more than 28h w/o errors).

> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Acked-by: Balbir Singh <balbir@...ux.vnet.ibm.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> 

Tested-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>


Thanks,
Daisuke Nishimura.

> ---
>  mm/memcontrol.c |   14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> Index: b/mm/memcontrol.c
> ===================================================================
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -747,10 +747,22 @@ int mem_cgroup_prepare_migration(struct 
>  /* remove redundant charge if migration failed*/
>  void mem_cgroup_end_migration(struct page *newpage)
>  {
> -	/* At success, page->mapping is not NULL and nothing to do. */
> +	/*
> +	 * At success, page->mapping is not NULL.
> +	 * special rollback care is necessary when
> +	 * 1. at migration failure. (newpage->mapping is cleared in this case)
> +	 * 2. the newpage was moved but not remapped again because the task
> +	 *    exits and the newpage is obsolete. In this case, the new page
> +	 *    may be a swapcache. So, we just call mem_cgroup_uncharge_page()
> +	 *    always for avoiding mess. The  page_cgroup will be removed if
> +	 *    unnecessary. File cache pages is still on radix-tree. Don't
> +	 *    care it.
> +	 */
>  	if (!newpage->mapping)
>  		__mem_cgroup_uncharge_common(newpage,
>  					 MEM_CGROUP_CHARGE_TYPE_FORCE);
> +	else if (PageAnon(newpage))
> +		mem_cgroup_uncharge_page(newpage);
>  }
>  
>  /*
> 
> 
> --
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