Re: [PATCH bpf-next 13/15] mm, memcg: Add new helper get_obj_cgroup_from_cgroup

[Roman Gushchin <roman.gushchin@linux.dev>]

On Sat, Aug 13, 2022 at 07:56:54AM +0800, Yafang Shao wrote:
> On Sat, Aug 13, 2022 at 1:40 AM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> >
> > On Fri, Aug 12, 2022 at 08:35:19AM +0800, Yafang Shao wrote:
> > > On Fri, Aug 12, 2022 at 12:16 AM Roman Gushchin
> > > <roman.gushchin@linux.dev> wrote:
> > > >
> > > > On Wed, Aug 10, 2022 at 03:18:38PM +0000, Yafang Shao wrote:
> > > > > Introduce new helper get_obj_cgroup_from_cgroup() to get obj_cgroup from
> > > > > a specific cgroup.
> > > > >
> > > > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > > > > ---
> > > > >  include/linux/memcontrol.h |  1 +
> > > > >  mm/memcontrol.c            | 41 +++++++++++++++++++++++++++++++++++++++++
> > > > >  2 files changed, 42 insertions(+)
> > > > >
> > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > > > index 2f0a611..901a921 100644
> > > > > --- a/include/linux/memcontrol.h
> > > > > +++ b/include/linux/memcontrol.h
> > > > > @@ -1713,6 +1713,7 @@ static inline void set_shrinker_bit(struct mem_cgroup *memcg,
> > > > >  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 *get_obj_cgroup_from_cgroup(struct cgroup *cgrp);
> > > > >  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 618c366..762cffa 100644
> > > > > --- a/mm/memcontrol.c
> > > > > +++ b/mm/memcontrol.c
> > > > > @@ -2908,6 +2908,47 @@ static struct obj_cgroup *__get_obj_cgroup_from_memcg(struct mem_cgroup *memcg)
> > > > >       return objcg;
> > > > >  }
> > > > >
> > > > > +static struct obj_cgroup *get_obj_cgroup_from_memcg(struct mem_cgroup *memcg)
> > > > > +{
> > > > > +     struct obj_cgroup *objcg;
> > > > > +
> > > > > +     if (memcg_kmem_bypass())
> > > > > +             return NULL;
> > > > > +
> > > > > +     rcu_read_lock();
> > > > > +     objcg = __get_obj_cgroup_from_memcg(memcg);
> > > > > +     rcu_read_unlock();
> > > > > +     return objcg;
> > > >
> > > > This code doesn't make sense to me. What does rcu read lock protect here?
> > >
> > > To protect rcu_dereference(memcg->objcg);.
> > > Doesn't it need the read rcu lock ?
> >
> > No, it's not how rcu works. Please, take a look at the docs here:
> > https://docs.kernel.org/RCU/whatisRCU.html#whatisrcu .
> > In particular, it describes this specific case very well.
> >
> > In 2 words, you don't protect the rcu_dereference() call, you protect the pointer
> 
> I just copied and pasted rcu_dereference(memcg->objcg) there to make it clear.
> Actually it protects memcg->objcg, doesn't it ?
> 
> > you get, cause it's valid only inside the rcu read section. After rcu_read_unlock()
> > it might point at a random data, because the protected object can be already freed.
> >
> 
> Are you sure?
> Can't the obj_cgroup_tryget(objcg) prevent it from being freed ?

Ok, now I see where it comes from. You copy-pasted it from get_obj_cgroup_from_current()?
There rcu read lock section protects memcg, not objcg.
In your case you don't need it, because memcg is passed as a parameter to the function,
so it's the duty of the caller to ensure the lifetime of memcg.

Thanks!