Re: [External] Re: [PATCH 2/7] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm

[Muchun Song]

On Wed, Apr 14, 2021 at 6:15 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 14-04-21 18:04:35, Muchun Song wrote:
> > On Wed, Apr 14, 2021 at 5:24 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Tue 13-04-21 14:51:48, Muchun Song wrote:
> > > > When mm is NULL, we do not need to hold rcu lock and call css_tryget for
> > > > the root memcg. And we also do not need to check !mm in every loop of
> > > > while. So bail out early when !mm.
> > >
> > > mem_cgroup_charge and other callers unconditionally drop the reference
> > > so how come this does not underflow reference count?
> >
> > For the root memcg, the CSS_NO_REF flag is set, so css_get
> > and css_put do not get or put reference.
>
> Ohh, right you are. I must have forgot about that special case. I am
> pretty sure I (and likely few more) will stumble over that in the future
> again. A small comment explaining that the reference can be safely
> ignore would be helpful.

OK. Will do.

>
> Anyway
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks.

>
> Thanks!
>
> --
> Michal Hocko
> SUSE Labs