linux-kernel - Re: [PATCH] memcg: avoid overflow in memcg_hierarchical_free

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Message-Id: <20101109124426.312f9979.nishimura@mxp.nes.nec.co.jp>
Date:	Tue, 9 Nov 2010 12:44:26 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Wu Fengguang <fengguang.wu@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: avoid overflow in
 memcg_hierarchical_free_pages()

On Mon,  8 Nov 2010 17:15:20 -0800
Greg Thelen <gthelen@...gle.com> wrote:

> Use page counts rather than byte counts to avoid overflowing
> unsigned long local variables.
> 
> Signed-off-by: Greg Thelen <gthelen@...gle.com>
> ---
>  mm/memcontrol.c |   10 +++++-----
>  1 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6c7115d..b287afd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1345,17 +1345,17 @@ memcg_hierarchical_free_pages(struct mem_cgroup *mem)
>  {
>  	unsigned long free, min_free;
>  
hmm, the default value of RES_LIMIT is LLONG_MAX, so I think we must declare
"free" as unsinged long long to avoid overflow.

Thanks,
Daisuke Nishimura.

> -	min_free = global_page_state(NR_FREE_PAGES) << PAGE_SHIFT;
> +	min_free = global_page_state(NR_FREE_PAGES);
>  
>  	while (mem) {
> -		free = res_counter_read_u64(&mem->res, RES_LIMIT) -
> -			res_counter_read_u64(&mem->res, RES_USAGE);
> +		free = (res_counter_read_u64(&mem->res, RES_LIMIT) -
> +			res_counter_read_u64(&mem->res, RES_USAGE)) >>
> +			PAGE_SHIFT;
>  		min_free = min(min_free, free);
>  		mem = parent_mem_cgroup(mem);
>  	}
>  
> -	/* Translate free memory in pages */
> -	return min_free >> PAGE_SHIFT;
> +	return min_free;
>  }
>  
>  /*
> -- 
> 1.7.3.1
> 
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