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?
> Signed-off-by: Muchun Song <songmuc...@bytedance.com>
> Acked-by: Johannes Weiner <han...@cmpxchg.org>
> Reviewed-by: Shakeel Butt <shake...@google.com>
> ---
> mm/memcontrol.c | 21 ++++++++++-----------
> 1 file changed, 10 insertions(+), 11 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f229de925aa5..9cbfff59b171 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -901,20 +901,19 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct
> mm_struct *mm)
> if (mem_cgroup_disabled())
> return NULL;
>
> + /*
> + * Page cache insertions can happen without an
> + * actual mm context, e.g. during disk probing
> + * on boot, loopback IO, acct() writes etc.
> + */
> + if (unlikely(!mm))
> + return root_mem_cgroup;
> +
> rcu_read_lock();
> do {
> - /*
> - * Page cache insertions can happen without an
> - * actual mm context, e.g. during disk probing
> - * on boot, loopback IO, acct() writes etc.
> - */
> - if (unlikely(!mm))
> + memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> + if (unlikely(!memcg))
> memcg = root_mem_cgroup;
> - else {
> - memcg =
> mem_cgroup_from_task(rcu_dereference(mm->owner));
> - if (unlikely(!memcg))
> - memcg = root_mem_cgroup;
> - }
> } while (!css_tryget(&memcg->css));
> rcu_read_unlock();
> return memcg;
> --
> 2.11.0
--
Michal Hocko
SUSE Labs
