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

[Waiman Long]

On 10/1/21 4:24 PM, Shakeel Butt wrote:
> On Fri, Oct 1, 2021 at 12:10 PM Waiman Long <longman@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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.
Yes, you are right. Just don't consider this patch for the time being. I 
will need to find a way to work around that.

Thanks,
Longman