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    SubjectRe: [External] Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero
    On Mon, Feb 15, 2021 at 5:39 PM Michal Hocko <mhocko@suse.com> 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@bytedance.com>
    > > ---
    > > 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

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