Re: [PATCH rfc 2/5] mm: kmem: add direct objcg pointer to task_struct

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Sep 27, 2023 at 08:08:29AM -0700, Roman Gushchin wrote:
> @@ -3001,6 +3001,47 @@ static struct obj_cgroup *__get_obj_cgroup_from_memcg(struct mem_cgroup *memcg)
>  	return objcg;
>  }
>  
> +static DEFINE_SPINLOCK(current_objcg_lock);
> +
> +static struct obj_cgroup *current_objcg_update(struct obj_cgroup *old)
> +{
> +	struct mem_cgroup *memcg;
> +	struct obj_cgroup *objcg;
> +	unsigned long flags;
> +
> +	old = current_objcg_clear_update_flag(old);
> +	if (old)
> +		obj_cgroup_put(old);
> +
> +	spin_lock_irqsave(&current_objcg_lock, flags);
> +	rcu_read_lock();
> +	memcg = mem_cgroup_from_task(current);
> +	for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
> +		objcg = rcu_dereference(memcg->objcg);
> +		if (objcg && obj_cgroup_tryget(objcg))
> +			break;
> +		objcg = NULL;
> +	}
> +	rcu_read_unlock();

Can this tryget() actually fail when this is called on the current
task during fork() and attach()? A cgroup cannot be offlined while
there is a task in it.

> @@ -6345,6 +6393,22 @@ static void mem_cgroup_move_task(void)
>  		mem_cgroup_clear_mc();
>  	}
>  }
> +
> +#ifdef CONFIG_MEMCG_KMEM
> +static void mem_cgroup_fork(struct task_struct *task)
> +{
> +	task->objcg = (struct obj_cgroup *)0x1;

dup_task_struct() will copy this pointer from the old task. Would it
be possible to bump the refcount here instead? That would save quite a
bit of work during fork().