Re: [PATCH bpf-next v2 10/12] mm, memcg: Add new helper get_obj_cgroup_from_cgroup

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Aug 19, 2022 at 4:38 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Thu, Aug 18, 2022 at 7:32 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > We want to open a cgroup directory and pass the fd into kernel, and then
> > in the kernel we can charge the allocated memory into the open cgroup if it
> > has valid memory subsystem. In the bpf subsystem, the opened cgroup will
> > be store as a struct obj_cgroup pointer, so a new helper
> > get_obj_cgroup_from_cgroup() is introduced.
> >
> > It also add a comment on why the helper  __get_obj_cgroup_from_memcg()
> > must be protected by rcu read lock.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> >  include/linux/memcontrol.h |  1 +
> >  mm/memcontrol.c            | 47 ++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 48 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..0409cc4 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2895,6 +2895,14 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
> >         return page_memcg_check(folio_page(folio, 0));
> >  }
> >
> > +/*
> > + * Pls. note that the memg->objcg can be freed after it is deferenced,
> > + * that can happen when the memcg is changed from online to offline.
> > + * So this helper must be protected by read rcu lock.
> > + *
> > + * After obj_cgroup_tryget(), it is safe to use the objcg outside of the rcu
> > + * read-side critical section.
> > + */
>
> The comment is too verbose. My suggestion would be to add
> WARN_ON_ONCE(!rcu_read_lock_held()) in the function and if you want to
> add a comment, just say that the caller should have a reference on
> memcg.
>

Sure, I will change it.

> >  static struct obj_cgroup *__get_obj_cgroup_from_memcg(struct mem_cgroup *memcg)
> >  {
> >         struct obj_cgroup *objcg = NULL;
> > @@ -2908,6 +2916,45 @@ 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;
> > +}
> > +
> > +struct obj_cgroup *get_obj_cgroup_from_cgroup(struct cgroup *cgrp)
>
> Since this function is exposed to other files, it would be nice to
> have a comment similar to get_mem_cgroup_from_mm() for this function.
> I know other get_obj_cgroup variants do not have such a comment yet
> but let's start with this.
>

Sure, I will add it.

> > +{
> > +       struct cgroup_subsys_state *css;
> > +       struct mem_cgroup *memcg;
> > +       struct obj_cgroup *objcg;
> > +
> > +       rcu_read_lock();
> > +       css = rcu_dereference(cgrp->subsys[memory_cgrp_id]);
> > +       if (!css || !css_tryget_online(css)) {
>
> Any reason to use css_tryget_online() instead of css_tryget()?

Because in this case, the cgroup is from an opened cgroup dir, which
should be online.
But considering this is a generic helper, which may be used by others,
it would be more reasonable to use css_tryget().
I will change it.

-- 
Regards
Yafang