Re: [PATCH 6/6] mm, oom: fortify task_will_free_mem

[Michal Hocko]

On Wed 01-06-16 00:29:33, Oleg Nesterov wrote:
> On 05/31, Michal Hocko wrote:
> >
> > On Mon 30-05-16 19:35:05, Oleg Nesterov wrote:
> > >
> > > Well, let me suggest this again. I think it should do
> > >
> > >
> > > 	if (SIGNAL_GROUP_COREDUMP)
> > > 		return false;
> > >
> > > 	if (SIGNAL_GROUP_EXIT)
> > > 		return true;
> > >
> > > 	if (thread_group_empty() && PF_EXITING)
> > > 		return true;
> > >
> > > 	return false;
> > >
> > > we do not need fatal_signal_pending(), in this case SIGNAL_GROUP_EXIT should
> > > be set (ignoring some bugs with sub-namespaces which we need to fix anyway).
> >
> > OK, so we shouldn't care about race when the fatal_signal is set on the
> > task until it reaches do_group_exit?
> 
> if fatal_signal() is true then (ignoring exec and coredump) SIGNAL_GROUP_EXIT
> is already set (again, ignoring the bugs with sub-namespace inits).
> 
> At the same time, SIGKILL can be already dequeued when the task exits, so
> fatal_signal_pending() can be "false negative".

Thanks for the clarification. I guess I got the point but this is a land
of surprises so one can never be sure...

> > > And. I think this needs smp_rmb() at the end of the loop (assuming we have the
> > > process_shares_mm() check here). We need it to ensure that we read p->mm before
> > > we read next_task(), to avoid the race with exit() + clone(CLONE_VM).
> >
> > Why don't we need the same barrier in oom_kill_process?
> 
> Because it calls do_send_sig_info() which takes ->siglock and copy_process()
> takes the same lock. Not a barrier, but acts the same way.

Ahh ok, so an implicit barrier.

> > Which barrier it
> > would pair with?
> 
> With the barrier implied by list_add_tail_rcu(&p->tasks, &init_task.tasks).

Ahh I see. rcu_assign_pointer that is, right?

> > Anyway I think this would deserve it's own patch.
> > Barriers are always tricky and it is better to have them in a small
> > patch with a full explanation.
> 
> OK, agreed.

cool

> I am not sure I can read the new patch correctly, it depends on the previous
> changes... but afaics it looks good.
> 
> Cosmetic/subjective nit, feel free to ignore,
> 
> > +bool task_will_free_mem(struct task_struct *task)
> > +{
> > +	struct mm_struct *mm = NULL;
> 
> unnecessary initialization ;)

fixed

> > +	struct task_struct *p;
> > +	bool ret;
> > +
> > +	/*
> > +	 * If the process has passed exit_mm we have to skip it because
> > +	 * we have lost a link to other tasks sharing this mm, we do not
> > +	 * have anything to reap and the task might then get stuck waiting
> > +	 * for parent as zombie and we do not want it to hold TIF_MEMDIE
> > +	 */
> > +	p = find_lock_task_mm(task);
> > +	if (!p)
> > +		return false;
> > +
> > +	if (!__task_will_free_mem(p)) {
> > +		task_unlock(p);
> > +		return false;
> > +	}
> 
> We can call the 1st __task_will_free_mem(p) before find_lock_task_mm(). In the
> likely case (I think) it should return false.

OK

> 
> And since __task_will_free_mem() has no other callers perhaps it should go into
> oom_kill.c too.

ok

I will resend the whole series with the fixups later during this week.
Thanks again for your review.
-- 
Michal Hocko
SUSE Labs