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;
} else {
step = 0;
}
Thanks.
--
tejun
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