Messages in this thread | | | From | Muchun Song <> | Date | Thu, 15 Apr 2021 11:13:16 +0800 | Subject | Re: [External] Re: [PATCH 2/7] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm |
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On Wed, Apr 14, 2021 at 6:15 PM Michal Hocko <mhocko@suse.com> wrote: > > On Wed 14-04-21 18:04:35, Muchun Song wrote: > > On Wed, Apr 14, 2021 at 5:24 PM Michal Hocko <mhocko@suse.com> 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@suse.com>
Thanks.
> > Thanks! > > -- > Michal Hocko > SUSE Labs
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