linux-kernel - Re: [PATCH -mm 2/2] memcg: fix css reference leak and endless loop in mem_cgroup

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Date:	Tue, 21 Jan 2014 11:42:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Greg Thelen <gthelen@...gle.com>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 2/2] memcg: fix css reference leak and endless loop
 in mem_cgroup_iter

On Tue, 21 Jan 2014 11:45:43 +0100 Michal Hocko <mhocko@...e.cz> wrote:

> 19f39402864e (memcg: simplify mem_cgroup_iter) has reorganized
> mem_cgroup_iter code in order to simplify it. A part of that change was
> dropping an optimization which didn't call css_tryget on the root of
> the walked tree. The patch however didn't change the css_put part in
> mem_cgroup_iter which excludes root.
> This wasn't an issue at the time because __mem_cgroup_iter_next bailed
> out for root early without taking a reference as cgroup iterators
> (css_next_descendant_pre) didn't visit root themselves.
> 
> Nevertheless cgroup iterators have been reworked to visit root by
> bd8815a6d802 (cgroup: make css_for_each_descendant() and friends include
> the origin css in the iteration) when the root bypass have been dropped
> in __mem_cgroup_iter_next. This means that css_put is not called for
> root and so css along with mem_cgroup and other cgroup internal object
> tied by css lifetime are never freed.
> 
> Fix the issue by reintroducing root check in __mem_cgroup_iter_next
> and do not take css reference for it.
> 
> This reference counting magic protects us also from another issue, an
> endless loop reported by Hugh Dickins when reclaim races with root
> removal and css_tryget called by iterator internally would fail. There
> would be no other nodes to visit so __mem_cgroup_iter_next would return
> NULL and mem_cgroup_iter would interpret it as "start looping from root
> again" and so mem_cgroup_iter would loop forever internally.

I grabbed these two patches but I will sit on them for a week or so,
pending review-n-test.

> Cc: stable@...r.kernel.org # mem_leak part 3.12+

What does this mean?
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