Re: [PATCH 3/6] mm, oom_adj: make sure processes sharing mm have same view of oom_score_adj

[Michal Hocko]

On Fri 27-05-16 19:18:21, Vladimir Davydov wrote:
> On Fri, May 27, 2016 at 01:18:03PM +0200, Michal Hocko wrote:
> ...
> > @@ -1087,7 +1105,25 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
> >  	unlock_task_sighand(task, &flags);
> >  err_put_task:
> >  	put_task_struct(task);
> > +
> > +	if (mm) {
> > +		struct task_struct *p;
> > +
> > +		rcu_read_lock();
> > +		for_each_process(p) {
> > +			task_lock(p);
> > +			if (!p->vfork_done && process_shares_mm(p, mm)) {
> > +				p->signal->oom_score_adj = oom_adj;
> > +				if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
> > +					p->signal->oom_score_adj_min = (short)oom_adj;
> > +			}
> > +			task_unlock(p);
> 
> I.e. you write to /proc/pid1/oom_score_adj and get
> /proc/pid2/oom_score_adj updated if pid1 and pid2 share mm?
> IMO that looks unexpected from userspace pov.

How much different it is from threads in the same thread group?
Processes sharing the mm without signals is a rather weird threading
model isn't it? Currently we just lie to users about their oom_score_adj
in this weird corner case. The only exception was OOM_SCORE_ADJ_MIN
where we really didn't kill the task but all other values are simply
ignored in practice.

> May be, we'd better add mm->oom_score_adj and set it to the min
> signal->oom_score_adj over all processes sharing it? This would
> require iterating over all processes every time oom_score_adj gets
> updated, but that's a slow path.

Not sure I understand. So you would prefer that mm->oom_score_adj might
disagree with p->signal->oom_score_adj?

-- 
Michal Hocko
SUSE Labs