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

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Date:	Mon, 8 Nov 2010 19:52:20 -0800
From:	Greg Thelen <gthelen@...gle.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>,
	Johannes Weiner <hannes@...xchg.org>,
	Wu Fengguang <fengguang.wu@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: avoid overflow in memcg_hierarchical_free_pages()

On Mon, Nov 8, 2010 at 7:44 PM, Daisuke Nishimura
<nishimura@....nes.nec.co.jp> wrote:
> 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.

Agreed.  I am testing a fix for that issue now.  I do not want
complicate this patch with the RES_LIMIT issue you mention.  The fix
will be in a separate patch.

> 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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