Re: 4.3-rc1 dirty page count underflow (cgroup-related?)

[Greg Thelen]

Dave Hansen wrote:

> On 09/17/2015 11:09 PM, Greg Thelen wrote:
>> I'm not denying the issue, bug the WARNING splat isn't necessarily
>> catching a problem.  The corresponding code comes from your debug patch:
>> +		WARN_ONCE(__this_cpu_read(memcg->stat->count[MEM_CGROUP_STAT_DIRTY]) > (1UL<<30), "MEM_CGROUP_STAT_DIRTY bogus");
>> 
>> This only checks a single cpu's counter, which can be negative.  The sum
>> of all counters is what matters.
>> Imagine:
>> cpu1) dirty page: inc
>> cpu2) clean page: dec
>> The sum is properly zero, but cpu2 is -1, which will trigger the WARN.
>> 
>> I'll look at the code and also see if I can reproduce the failure using
>> mem_cgroup_read_stat() for all of the new WARNs.
>
> D'oh.  I'll replace those with the proper mem_cgroup_read_stat() and
> test with your patch to see if anything still triggers.

Thanks Dave.

Here's what I think we should use to fix the issue.  I tagged this for
v4.2 stable given the way that unpatched performance falls apart without
warning or workaround (besides deleting and recreating affected memcg).
Feedback welcome.