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

Yafang Shao
2022-06-25T13:54:58Z

On Thu, Jun 23, 2022 at 11:01 AM Roman Gushchin
<roman.gushchin@linux.dev> 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@gmail.com>
> > ---
> > 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