Re: [PATCH 3/3] mm, memcg: Ensure valid memcg from objcg within a RCU critical section

[Shakeel Butt <shakeelb@google.com>]

On Fri, Oct 1, 2021 at 12:10 PM Waiman Long <longman@redhat.com> wrote:
>
> To ensure that a to-be-offlined memcg fetched from objcg remains
> valid (has non-zero reference count) within a RCU critical section,
> a synchronize_rcu() call is inserted at the end of memcg_offline_kmem().
>
> With that change, we no longer need to use css_tryget()
> in get_mem_cgroup_from_objcg() as the final css_put() in
> css_killed_work_fn() would not have been called yet.
>
> The obj_cgroup_uncharge_pages() function is simplifed to perform
> the whole uncharge operation within a RCU critical section saving a
> css_get()/css_put() pair.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  mm/memcontrol.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8177f253a127..1dbb37d96e49 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2769,10 +2769,8 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
>         struct mem_cgroup *memcg;
>
>         rcu_read_lock();
> -retry:
>         memcg = obj_cgroup_memcg(objcg);
> -       if (unlikely(!css_tryget(&memcg->css)))
> -               goto retry;
> +       css_get(&memcg->css);
>         rcu_read_unlock();
>
>         return memcg;
> @@ -2947,13 +2945,14 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
>  {
>         struct mem_cgroup *memcg;
>
> -       memcg = get_mem_cgroup_from_objcg(objcg);
> +       rcu_read_lock();
> +       memcg = obj_cgroup_memcg(objcg);
>
>         if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
>                 page_counter_uncharge(&memcg->kmem, nr_pages);
>         refill_stock(memcg, nr_pages);
>
> -       css_put(&memcg->css);
> +       rcu_read_unlock();
>  }
>
>  /*
> @@ -3672,6 +3671,13 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
>         memcg_drain_all_list_lrus(kmemcg_id, parent);
>
>         memcg_free_cache_id(kmemcg_id);
> +
> +       /*
> +        * To ensure that a to-be-offlined memcg fetched from objcg remains
> +        * valid within a RCU critical section, we need to wait here until
> +        * the a grace period has elapsed.
> +        */
> +       synchronize_rcu();

This is called with cgroup_mutex held from css_offline path and
synchronize_rcu() can be very expensive on a busy system, so, this
will indirectly impact all the code paths which take cgroup_mutex.

>  }
>  #else
>  static int memcg_online_kmem(struct mem_cgroup *memcg)
> --
> 2.18.1
>