linux-kernel - Re: mem_cgroup_get_limit() return type, was [patch] memcg: fix unit mismatch in memcg oom limit calculation

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Date:	Tue, 9 Nov 2010 15:21:50 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Greg Thelen <gthelen@...gle.com>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <balbir@...ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: mem_cgroup_get_limit() return type, was [patch] memcg: fix unit
 mismatch in memcg oom limit calculation

On Tue, 9 Nov 2010, Greg Thelen wrote:

> >> > Adding the number of swap pages to the byte limit of a memory control
> >> > group makes no sense.  Convert the pages to bytes before adding them.
> >> >
> >> > The only user of this code is the OOM killer, and the way it is used
> >> > means that the error results in a higher OOM badness value.  Since the
> >> > cgroup limit is the same for all tasks in the cgroup, the error should
> >> > have no practical impact at the moment.
> >> >
> >> > But let's not wait for future or changing users to trip over it.
> >> 
> >> Thanks for the fix.
> >> 
> >
> > Nice catch, but it's done in the opposite way: the oom killer doesn't use 
> > byte limits but page limits.  So this needs to be
> >
> > 	(res_counter_read_u64(&memcg->res, RES_LIMIT) >> PAGE_SHIFT) +
> > 			total_swap_pages;
> 
> In -mm, the oom killer queries memcg for a byte limit using
> mem_cgroup_get_limit(). The following is from
> mem_cgroup_out_of_memory():
> 
> 	limit = mem_cgroup_get_limit(mem) >> PAGE_SHIFT;
> 

Oops, I missed that.  I think Johannes' patch is better because 
mem_cgroup_get_limit() may eventually be used elsewhere and the subsystem 
has byte granularity.

Thanks!
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