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

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Date:	Fri, 18 Sep 2015 07:50:42 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
	Jens Axboe <axboe@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>,
	"open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" 
	<cgroups@...r.kernel.org>,
	"open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" 
	<linux-mm@...ck.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: 4.3-rc1 dirty page count underflow (cgroup-related?)

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.

> Did you notice if the global /proc/meminfo:Dirty count also underflowed?

It did not underflow.  It was one of the first things I looked at and it
looked fine, went down near 0 at 'sync', etc...
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