linux-kernel - Re: [PATCH] memcg: make mem_cgroup_read

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Message-Id: <20150922152423.5751d932aebfe12cdd40a618@linux-foundation.org>
Date:	Tue, 22 Sep 2015 15:24:23 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: make mem_cgroup_read_stat() unsigned

On Tue, 22 Sep 2015 15:16:32 -0700 Greg Thelen <gthelen@...gle.com> wrote:

> mem_cgroup_read_stat() returns a page count by summing per cpu page
> counters.  The summing is racy wrt. updates, so a transient negative sum
> is possible.  Callers don't want negative values:
> - mem_cgroup_wb_stats() doesn't want negative nr_dirty or nr_writeback.
> - oom reports and memory.stat shouldn't show confusing negative usage.
> - tree_usage() already avoids negatives.
> 
> Avoid returning negative page counts from mem_cgroup_read_stat() and
> convert it to unsigned.

Someone please remind me why this code doesn't use the existing
percpu_counter library which solved this problem years ago.

>  	for_each_possible_cpu(cpu)

and which doesn't iterate across offlined CPUs.


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