On 05/26, Michal Hocko wrote:
>
> On Tue 26-05-15 18:36:46, Oleg Nesterov wrote:
> >
> > > +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 see, but it could only return NULL if mem_cgroup_from_css() returns
NULL. Now it returns NULL for sure if the caller is task_in_mem_cgroup(),
// called when task->mm == NULL
task_memcg = mem_cgroup_from_task(task);
css_get(&task_memcg->css);
and this css_get() doesn't look nice if task_memcg == NULL ;)
> I will double check
Yes, please. Perhaps I missed something.
> > 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.
Sure, this is what I can understand. I meant... OK, lets ignore
"without these changes", because without these changes there are
much more oddities ;) With these changes only ->mm == NULL case
looks unclear.
And btw,
if (!p->mm)
return NULL;
return rcu_dereference(p->mm->memcg);
perhaps this needs a comment. It is not clear what protects ->mm.
But. After this series "p" is always current (if ->mm != NULL), so
this is fine.
Nevermind. Please forget. I feel this needs a bit of cleanup, but
we can always do this later.
Oleg.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>