Re: [PATCH 8/8] memcg: get rid of mem_cgroup_from_task

Vladimir Davydov
2015-07-08T17:43:52Z

On Wed, Jul 08, 2015 at 02:27:52PM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.cz>
>
> mem_cgroup_from_task has always been a tricky API. It was added
> by 78fb74669e80 ("Memory controller: accounting setup") for
> mm_struct::mem_cgroup initialization. Later on it gained new callers
> mostly due to mm_struct::mem_cgroup -> mem_cgroup::owner transition and
> most users had to do mem_cgroup_from_task(mm->owner) to get the
> resulting memcg. Now that mm_struct::owner is gone this is not
> necessary, yet the API is still confusing.
>
> One tricky part has always been that the API sounds generic but it is
> not really. mem_cgroup_from_task(current) doesn't necessarily mean the
> same thing as current->mm->memcg (resp.
> mem_cgroup_from_task(current->mm->owner) previously) because mm might be
> associated with a different cgroup than the process.
>
> Another tricky part is that p->mm->memcg is unsafe if p!=current
> as pointed by Oleg because nobody is holding a reference on that
> mm. This is not a problem right now because we have only 2 callers in
> the tree. sock_update_memcg operates on current and task_in_mem_cgroup
> is providing non-NULL task so it is always using task_css.
>
> Let's ditch this function and use current->mm->memcg for
> sock_update_memcg and use task_css for task_in_mem_cgroup. This doesn't
> have any functional effect.
>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
> mm/memcontrol.c | 24 +++++++-----------------
> 1 file changed, 7 insertions(+), 17 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4069ec8f52be..fb8e9bd04a29 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -294,18 +294,6 @@ static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
> return mem_cgroup_from_css(css);
> }
>
> -static struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
> -{
> - if (p->mm)
> - return rcu_dereference(p->mm->memcg);
> -
> - /*
> - * If the process doesn't have mm struct anymore we have to fallback
> - * to the task_css.
> - */
> - return mem_cgroup_from_css(task_css(p, memory_cgrp_id));
> -}
> -
> /* Writing them here to avoid exposing memcg's inner layout */
> #if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
>
> @@ -332,7 +320,7 @@ void sock_update_memcg(struct sock *sk)
> }
>
> rcu_read_lock();
> - memcg = mem_cgroup_from_task(current);
> + memcg = rcu_dereference(current->mm->memcg);
> cg_proto = sk->sk_prot->proto_cgroup(memcg);
> if (cg_proto && memcg_proto_active(cg_proto) &&
> css_tryget_online(&memcg->css)) {
> @@ -1091,12 +1079,14 @@ bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg)
> task_unlock(p);
> } else {
> /*
> - * All threads may have already detached their mm's, but the oom
> - * killer still needs to detect if they have already been oom
> - * killed to prevent needlessly killing additional tasks.
> + * All threads have already detached their mm's but we should
> + * still be able to at least guess the original memcg from the
> + * task_css. These two will match most of the time but there are
> + * corner cases where task->mm and task_css refer to a different
> + * cgroups.
> */
> rcu_read_lock();
> - task_memcg = mem_cgroup_from_task(task);
> + task_memcg = mem_cgroup_from_css(task_css(task, memory_cgrp_id));
> css_get(&task_memcg->css);

I wonder why it's safe to call css_get here.

The patch itself looks good though,

Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>

> rcu_read_unlock();
> }
> --
> 2.1.4
>