Re: [External] Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero

[Muchun Song]

On Mon, Feb 15, 2021 at 5:39 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Sat 13-02-21 01:01:58, Muchun Song wrote:
> > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id,
> > so idr_find() is pointless and wastes CPU cycles.
>
> Is this possible at all to happen? If not why should we add a test for
> _all_ invocations?

Yeah, this indeed can happen. If we allocate a new swap cache page
and charge it via mem_cgroup_charge, then the page will uncharge
the swap counter via mem_cgroup_uncharge_swap. When the swap
entry is indeed freed, we will call mem_cgroup_uncharge_swap again,
In this routine, we can pass zero to mem_cgroup_from_id. Right?


>
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  mm/memcontrol.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index a3f26522765a..68ed4b297c13 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg)
> >  struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
> >  {
> >       WARN_ON_ONCE(!rcu_read_lock_held());
> > +     /* The memcg ID cannot be zero. */
> > +     if (id == 0)
> > +             return NULL;
> >       return idr_find(&mem_cgroup_idr, id);
> >  }
> >
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs