Re: [RFC 3/3] memcg: get rid of mm_struct::owner

Oleg Nesterov
2015-05-26T16:37:52Z

On 05/26, Michal Hocko wrote:
>
> @@ -426,17 +426,7 @@ struct mm_struct {
> struct kioctx_table __rcu *ioctx_table;
> #endif
> #ifdef CONFIG_MEMCG
> - /*
> - * "owner" points to a task that is regarded as the canonical
> - * user/owner of this mm. All of the following must be true in
> - * order for it to be changed:
> - *
> - * current == mm->owner
> - * current->mm != mm
> - * new_owner->mm == mm
> - * new_owner->alloc_lock is held
> - */
> - struct task_struct __rcu *owner;
> + struct mem_cgroup __rcu *memcg;

Yes, thanks, this is what I tried to suggest ;)

But I can't review this series. Simply because I know nothing about
memcs. I don't even know how to use it.

Just one question,

> +static struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
> +{
> + if (!p->mm)
> + return NULL;
> + return rcu_dereference(p->mm->memcg);
> +}

Probably I missed something, but it seems that the callers do not
expect it can return NULL. Perhaps sock_update_memcg() is fine, but
task_in_mem_cgroup() calls it when find_lock_task_mm() fails, and in
this case ->mm is NULL.

And in fact I can't understand what mem_cgroup_from_task() actually
means, with or without these changes.

And another question. I can't understand what happens when a task
execs... IOW, could you confirm that exec_mmap() does not need
mm_set_memcg(mm, oldmm->memcg) ?

Oleg.