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On Sun, Oct 6, 2013 at 10:08 PM, Toralf Förster <toralf.foerster@xxxxxx> wrote:
> On 10/06/2013 08:38 PM, Geert Uytterhoeven wrote:
>> On Sun, Oct 6, 2013 at 4:17 PM, Toralf Förster <toralf.foerster@xxxxxx> 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 -- geert@xxxxxxxxxxxxxx

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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