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Date: Sun, 13 Dec 2015 01:17:12 +0200
Message-ID: <CAHp75VdHCSmw9tgd7ZC_n=N09wQHMHk8T5oK7jS7cM+zqBQ-_Q@mail.gmail.com>
Subject: Re: undefined shift in wb_update_dirty_ratelimit()
From: Andy Shevchenko <andy.shevchenko@gmail.com>
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Sender: owner-linux-mm@kvack.org
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To: Tejun Heo <tj@kernel.org>
Cc: Andrey Ryabinin <aryabinin@odin.com>, Wu Fengguang <fengguang.wu@intel.com>, Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>, Dmitry Vyukov <dvyukov@google.com>, Sasha Levin <sasha.levin@oracle.com>

On Mon, Dec 7, 2015 at 4:26 PM, Tejun Heo <tj@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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