On 09/24/2012 03:53 PM, Borislav Petkov wrote:

Hi all,

we're able to trigger the oops below when doing CPU hotplug tests.

I hit this problem as well, which I reported here, a few days ago:
https://lkml.org/lkml/2012/9/13/222

<snip>

	...

and from looking at the register dump below, the dividend, which should
be in %rdx:%rax is 0 and the divisor (denominator) we've got from
bdi_writeout_fraction and is in %rdi is also 0. Which is strange because
fprop_fraction_percpu guards for division by zero by setting denominator
to 1 if it were zero but what about the case where den > num? Can that
even happen?

And also, what happens if num is 0? Which it kinda is by looking at %rcx
where there's copy of it.

Going by the usage of percpu_counter_read_positive() (which is used to get
both the values of num and den), the least value that num or den can have
is zero. So, the C code to guard against divide-by-zero looks OK to me.
Which unfortunately keeps the mystery unsolved :(

Regards,
Srivatsa S. Bhat
 

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>