Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup

Michal Hocko
2016-08-03T11:27:58Z

On Wed 03-08-16 12:50:49, Vladimir Davydov wrote:
> On Tue, Aug 02, 2016 at 06:00:26PM +0200, Michal Hocko wrote:
> > On Tue 02-08-16 18:00:48, Vladimir Davydov wrote:
> ...
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 3be791afd372..4ae12effe347 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -4036,6 +4036,24 @@ static void mem_cgroup_id_get(struct mem_cgroup *memcg)
> > > atomic_inc(&memcg->id.ref);
> > > }
> > >
> > > +static struct mem_cgroup *mem_cgroup_id_get_active(struct mem_cgroup *memcg)
> > > +{
> > > + while (!atomic_inc_not_zero(&memcg->id.ref)) {
> > > + /*
> > > + * The root cgroup cannot be destroyed, so it's refcount must
> > > + * always be >= 1.
> > > + */
> > > + if (memcg == root_mem_cgroup) {
> > > + VM_BUG_ON(1);
> > > + break;
> > > + }
> >
> > why not simply VM_BUG_ON(memcg == root_mem_cgroup)?
>
> Because with DEBUG_VM disabled we could wind up looping forever here if
> the refcount of the root_mem_cgroup got screwed up. On production
> kernels, it's better to break the loop and carry on closing eyes on
> diverging counters rather than getting a lockup.

Wouldn't this just paper over a real bug? Anyway I will not insist but
making the code more complex just to pretend we can handle a situation
gracefully doesn't sound right to me.
--
Michal Hocko
SUSE Labs