Re: [RFC PATCH bpf-next 03/10] mm, memcg: Add new helper obj_cgroup_from_current()

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On Thu, Jun 23, 2022 at 11:01 AM Roman Gushchin
<roman.gushchin@xxxxxxxxx> wrote:
>
> On Sun, Jun 19, 2022 at 03:50:25PM +0000, Yafang Shao wrote:
> > The difference between get_obj_cgroup_from_current() and obj_cgroup_from_current()
> > is that the later one doesn't add objcg's refcnt.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > ---
> >  include/linux/memcontrol.h |  1 +
> >  mm/memcontrol.c            | 24 ++++++++++++++++++++++++
> >  2 files changed, 25 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index cf074156c6ac..402b42670bcd 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -1703,6 +1703,7 @@ bool mem_cgroup_kmem_disabled(void);
> >  int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order);
> >  void __memcg_kmem_uncharge_page(struct page *page, int order);
> >
> > +struct obj_cgroup *obj_cgroup_from_current(void);
> >  struct obj_cgroup *get_obj_cgroup_from_current(void);
> >  struct obj_cgroup *get_obj_cgroup_from_page(struct page *page);
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index abec50f31fe6..350a7849dac3 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2950,6 +2950,30 @@ struct obj_cgroup *get_obj_cgroup_from_page(struct page *page)
> >       return objcg;
> >  }
> >
> > +__always_inline struct obj_cgroup *obj_cgroup_from_current(void)
> > +{
> > +     struct obj_cgroup *objcg = NULL;
> > +     struct mem_cgroup *memcg;
> > +
> > +     if (memcg_kmem_bypass())
> > +             return NULL;
> > +
> > +     rcu_read_lock();
> > +     if (unlikely(active_memcg()))
> > +             memcg = active_memcg();
> > +     else
> > +             memcg = mem_cgroup_from_task(current);
> > +
> > +     for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
> > +             objcg = rcu_dereference(memcg->objcg);
> > +             if (objcg)
> > +                     break;
> > +     }
> > +     rcu_read_unlock();
>
> Hm, what prevents the objcg from being released here? Under which conditions
> it's safe to call it?

obj_cgroup_from_current() is used when we know the objcg's refcnt has
already been incremented.
For example in my case, it is called after we have already call get_
parent_mem_cgroup().
I should add a comment or a WARN_ON() in this function.

-- 
Regards
Yafang



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