Message-ID: <53614F3C.8020009@redhat.com>
Date: Wed, 30 Apr 2014 15:30:04 -0400
From: Rik van Riel <riel@redhat.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0
MIME-Version: 1.0
To: Andrew Morton <akpm@linux-foundation.org>
CC: Michal Hocko <mhocko@suse.cz>, Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com,
        jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com,
        fengguang.wu@intel.com, mpatlasov@parallels.com,
        Motohiro.Kosaki@us.fujitsu.com
Subject: Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom
References: <20140429151910.53f740ef@annuminas.surriel.com>	<5360C9E7.6010701@jp.fujitsu.com>	<20140430093035.7e7226f2@annuminas.surriel.com>	<20140430134826.GH4357@dhcp22.suse.cz>	<20140430104114.4bdc588e@cuia.bos.redhat.com> <20140430120001.b4b95061ac7252a976b8a179@linux-foundation.org>
In-Reply-To: <20140430120001.b4b95061ac7252a976b8a179@linux-foundation.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org

On 04/30/2014 03:00 PM, Andrew Morton wrote:
> On Wed, 30 Apr 2014 10:41:14 -0400 Rik van Riel <riel@redhat.com> wrote:
>
>> It is possible for "limit - setpoint + 1" to equal zero, leading to a
>> divide by zero error. Blindly adding 1 to "limit - setpoint" is not
>> working, so we need to actually test the divisor before calling div64.
>>
>> ...
>>
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
>>   					  unsigned long limit)
>>   {
>>   	long long pos_ratio;
>> +	long divisor;
>>   	long x;
>>
>> +	divisor = limit - setpoint;
>> +	if (!(s32)divisor)
>> +		divisor = 1;	/* Avoid div-by-zero */
>> +
>>   	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
>> -		    limit - setpoint + 1);
>> +		    (s32)divisor);
>
> Doesn't this just paper over the bug one time in four billion?  The
> other 3999999999 times, pos_ratio_polynom() returns an incorect result?
>
> If it is indeed the case that pos_ratio_polynom() callers are
> legitimately passing a setpoint which is more than 2^32 less than limit
> then it would be better to handle that input correctly.

The easy way would be by calling div64_s64 and div64_u64,
which are 64 bit all the way through.

Any objections?

The inlined bits seem to be stubs calling the _rem variants
of the functions, and discarding the remainder.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/