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

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Date:   Thu, 18 Aug 2022 13:38:44 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>, kpsingh@...nel.org,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, jolsa@...nel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Cgroups <cgroups@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH bpf-next v2 10/12] mm, memcg: Add new helper get_obj_cgroup_from_cgroup

On Thu, Aug 18, 2022 at 7:32 AM Yafang Shao <laoar.shao@...il.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@...il.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.

>  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.

> +{
> +       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()?