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

[Johannes Weiner]

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@xxxxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> 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.