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?
>
> 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
