linux-kernel - Re: [PATCH] mm: memcontrol: fix NR_WRITEBACK leak in memcg and system stats

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Date:   Wed, 7 Feb 2018 11:55:47 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...com
Subject: Re: [PATCH] mm: memcontrol: fix NR_WRITEBACK leak in memcg and
 system stats

Hi Shakeel,

On Wed, Feb 07, 2018 at 07:44:08AM -0800, Shakeel Butt wrote:
> On Sat, Feb 3, 2018 at 12:23 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> > After the ("a983b5ebee57 mm: memcontrol: fix excessive complexity in
> > memory.stat reporting"), we observed slowly upward creeping
> > NR_WRITEBACK counts over the course of several days, both the
> > per-memcg stats as well as the system counter in e.g. /proc/meminfo.
> >
> > The conversion from full per-cpu stat counts to per-cpu cached atomic
> > stat counts introduced an irq-unsafe RMW operation into the updates.
> >
> > Most stat updates come from process context, but one notable exception
> > is the NR_WRITEBACK counter. While writebacks are issued from process
> > context, they are retired from (soft)irq context.
> >
> > When writeback completions interrupt the RMW counter updates of new
> > writebacks being issued, the decs from the completions are lost.
> >
> > Since the global updates are routed through the joint lruvec API, both
> > the memcg counters as well as the system counters are affected.
> >
> > This patch makes the joint stat and event API irq safe.
> >
> > Fixes: a983b5ebee57 ("mm: memcontrol: fix excessive complexity in memory.stat reporting")
> > Debugged-by: Tejun Heo <tj@...nel.org>
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Should this be considered for stable?

The stable tree is only for fixes to already released kernels, but
there is no release containing the faulty patch:

$ git describe --tags a983b5ebee57
v4.15-3322-ga983b5ebee57

nor was the faulty patch itself marked for stable.

So as long as this fix makes it into 4.16 we should be good.