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

[Michal Hocko <mhocko@suse.cz>]

On Tue 26-05-15 18:36:46, Oleg Nesterov wrote:
> 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.

This hasn't changed by this patch. mem_cgroup_from_task was allowed to
return NULL even before. I've just made it static because it doesn't
have any external users anymore. I will double check whether we can ever
get NULL there in the real life. We have this code like that for quite
some time. Maybe this is just a heritage from the past...

> 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.

It performs task_struct->mem_cgroup mapping. We cannot use cgroup
mapping here because the charges are bound to mm_struct rather than
task.

> 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) ?

Right you are! Fixed thanks!
---
diff --git a/fs/exec.c b/fs/exec.c
index 2cd4def4b1d6..ea00d5a47aad 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -867,6 +867,7 @@ static int exec_mmap(struct mm_struct *mm)
 		up_read(&old_mm->mmap_sem);
 		BUG_ON(active_mm != old_mm);
 		setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm);
+		mm_set_memcg(mm, old_mm->memcg);
 		mmput(old_mm);
 		return 0;
 	}
-- 
Michal Hocko
SUSE Labs