On Mon, Dec 7, 2015 at 4:26 PM, Tejun Heo <t...@kernel.org> wrote:
> Hello, Andrey.
> On Mon, Dec 07, 2015 at 05:17:06PM +0300, Andrey Ryabinin wrote:
>> I've hit undefined shift in wb_update_dirty_ratelimit() which does some
>> mysterious 'step' calculations:
>>
>> /*
>> * Don't pursue 100% rate matching. It's impossible since the balanced
>> * rate itself is constantly fluctuating. So decrease the track speed
>> * when it gets close to the target. Helps eliminate pointless tremors.
>> */
>> step >>= dirty_ratelimit / (2 * step + 1);
>>
>>
>> dirty_ratelimit = INIT_BW and step = 0 results in this:
>>
>> [ 5006.957366]
>> ================================================================================
>> [ 5006.957798] UBSAN: Undefined behaviour in ../mm/page-writeback.c:1286:7
>> [ 5006.958091] shift exponent 25600 is too large for 64-bit type 'long
>> unsigned int'
>
> We prolly should do sth like
>
> shift = dirty_ratelimit / (2 * step = 1);
> if (shift < BITS_PER_LONG) {
> step = (step >> shift) + 7 / 8;
7/8 = 0. Missed parens?
In that case it's DIV_ROUND_UP(step >> shift, 8)
> } else {
> step = 0;
> }
--
With Best Regards,
Andy Shevchenko
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