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

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Date:	Thu, 24 Nov 2011 09:04:43 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	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 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)) ?

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

> ---
>  mm/memcontrol.c |   13 +------------
>  1 files changed, 1 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d4d139a..0d10be4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2679,19 +2679,8 @@ int mem_cgroup_newpage_charge(struct page *page,
>  {
>  	if (mem_cgroup_disabled())
>  		return 0;
> -	/*
> -	 * If already mapped, we don't have to account.
> -	 * If page cache, page->mapping has address_space.
> -	 * But page->mapping may have out-of-use anon_vma pointer,
> -	 * detecit it by PageAnon() check. newly-mapped-anon's page->mapping
> -	 * is NULL.
> -  	 */
> -	if (page_mapped(page) || (page->mapping && !PageAnon(page)))
> -		return 0;
> -	if (unlikely(!mm))
> -		mm = &init_mm;
>  	return mem_cgroup_charge_common(page, mm, gfp_mask,
> -				MEM_CGROUP_CHARGE_TYPE_MAPPED);
> +					MEM_CGROUP_CHARGE_TYPE_MAPPED);
>  }
>  
>  static void
> -- 
> 1.7.6.4
> 
> --
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