Re: [PATCH -mm 2/2] memcg: fix css reference leak and endless loopin mem_cgroup_iter

From: Michal Hocko
Date: Wed Jan 22 2014 - 03:12:21 EST

On Tue 21-01-14 11:42:19, Andrew Morton wrote:
> On Tue, 21 Jan 2014 11:45:43 +0100 Michal Hocko <mhocko@xxxxxxx> 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.

Yes, there is no rush and this needs a proper review.

> > Cc: stable@xxxxxxxxxxxxxxx # mem_leak part 3.12+
> What does this mean?

Dohh. I had both patches in one but then I decided to split it. This is
just left over. Should be 3.12+.

Sorry about the confusion.

Michal Hocko
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