On Sun, Oct 6, 2013 at 10:08 PM, Toralf Förster <[email protected]> wrote:
> On 10/06/2013 08:38 PM, Geert Uytterhoeven wrote:
>> On Sun, Oct 6, 2013 at 4:17 PM, Toralf Förster <[email protected]>
>> wrote:
>>> The UML stopped here :
>>> ...
>>> if (unlikely(task_ratelimit == 0)) {
>>> period = max_pause;
>>> pause = max_pause;
>>> BUG_ON(pause < 0);
>>> goto pause;
>>> }
>>> BUG_ON(pages_dirtied < 0);
>>> BUG_ON(task_ratelimit < 0);
>>> period = HZ * pages_dirtied / task_ratelimit;
>>> BUG_ON(period < 0); <----------------------here
>>
>> So pages_dirtied becomes that big compared to task_ratelimit (both are
>> "unsigned long"), that period (which is "long", just like "pause") overflows
>> into a negative number.
>>
>> This is indeed much more likely to happen on 32-bit.
>>
>>> The back trace is :
>>
>>> #9 0x08411c64 in balance_dirty_pages (pages_dirtied=9, mapping=<optimized
>>> out>) at mm/page-writeback.c:1471
>>
>> But here pages_dirtied is only 9??
> Well, this points to an overflow or ? :
Negative indicates an overflow, but pages_dirtied doesn't.
> tfoerste@n22 ~/devel/linux $ nl -ba mm/page-writeback.c | grep -A 5 -B 5 1468
> 1463 BUG_ON(pause < 0);
> 1464 goto pause;
> 1465 }
> 1466 period = HZ * pages_dirtied / task_ratelimit;
> 1467 pause = period;
> 1468 BUG_ON(pause < 0 && pages_dirtied > 0 &&
> task_ratelimit > 0);
> 1469 if (current->dirty_paused_when)
> 1470 pause -= now - current->dirty_paused_when;
> 1471 /*
> 1472 * For less than 1s think time (ext3/4 may block the
> dirtier
> 1473 * for up to 800ms from time to time on 1-HDD; so
> does xfs,
>
>
> and the back trace is :
>
> #9 0x08411c6c in balance_dirty_pages (pages_dirtied=0, mapping=<optimized
> out>) at mm/page-writeback.c:1468
Hmm, now pages_dirtied is zero, according to the backtrace, but the BUG_ON()
asserts its strict positive?!?
Can you please try the following instead of the BUG_ON():
if (pause < 0) {
printk("pages_dirtied = %lu\n", pages_dirtied);
printk("task_ratelimit = %lu\n", task_ratelimit);
printk("pause = %ld\n", pause);
}
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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