linux-kernel - Re: [patch 5/8] mm: memcg: remove unneeded checks from newpage

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Date:	Thu, 24 Nov 2011 11:30:49 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <bsingharora@...il.com>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 5/8] mm: memcg: remove unneeded checks from
 newpage_charge()

On Thu 24-11-11 10:04:09, Johannes Weiner wrote:
> On Thu, Nov 24, 2011 at 09:04:43AM +0900, KAMEZAWA Hiroyuki wrote:
> > On Wed, 23 Nov 2011 16:42:28 +0100
> > Johannes Weiner <hannes@...xchg.org> wrote:
> > 
> > > From: Johannes Weiner <jweiner@...hat.com>
> > > 
> > > All callsites pass in freshly allocated pages and a valid mm.  As a
> > > result, all checks pertaining the page's mapcount, page->mapping or
> > > the fallback to init_mm are unneeded.
> > > 
> > > Signed-off-by: Johannes Weiner <jweiner@...hat.com>
> > 
> > Hmm, it's true now. But for making clear our assumption to all readers of code,
> > 
> > could you add
> > VM_BUG_ON(!mm || page_mapped(page) || (page->mapping && !PageAnon(page)) ?
> 
> Of course.  Please find the delta patch below.  I broke them up into
> three separate checks to make the problem easier to find if the BUG
> triggers.

sounds reasonable.

> 
> > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> Thank you.
> 
> ---
> From: Johannes Weiner <jweiner@...hat.com>
> Subject: mm: memcg: remove unneeded checks from newpage_charge() fix
> 
> Document page state assumptions and catch if they are violated in
> newpage_charge().
> 
> Signed-off-by: Johannes Weiner <jweiner@...hat.com>

I assume you are going to fold it into the previous one.
Acked-by: Michal Hocko <mhocko@...e.cz>

> ---
>  mm/memcontrol.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0d10be4..f338018 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2679,6 +2679,9 @@ int mem_cgroup_newpage_charge(struct page *page,
>  {
>  	if (mem_cgroup_disabled())
>  		return 0;
> +	VM_BUG_ON(page_mapped(page));
> +	VM_BUG_ON(page->mapping && !PageAnon(page));
> +	VM_BUG_ON(!mm);
>  	return mem_cgroup_charge_common(page, mm, gfp_mask,
>  					MEM_CGROUP_CHARGE_TYPE_MAPPED);
>  }
> -- 
> 1.7.6.4

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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