Re: [patch 06/18] oom: avoid sending exiting tasks a SIGKILL

[Oleg Nesterov <oleg@redhat.com>]

On 06/10, KAMEZAWA Hiroyuki wrote:
>
> On Wed, 9 Jun 2010 22:14:30 +0200
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > > in this case since it would
> > > not be allocating memory without p->mm.
> >
> > This thread will not allocate the memory, yes. But its sub-threads can.
> > And select_bad_process() can constantly return the same (dead) thread P,
> > badness() inspects ->mm under find_lock_task_mm() which finds the thread
> > with the valid ->mm.
> >
> > OK. Probably this doesn't matter. I don't know if task_in_mem_cgroup(task)
> > was fixed or not, but currently it also looks at task->mm and thus have
> > the same boring problem: it is trivial to make the memory-hog process
> > invisible to oom. Unless I missed something, of course.
>
> HmHm...your concern is that there is a case when mem_cgroup_out_of_memory()
> can't kill anything ?

Or it can kill the wrong task. But once again, I am only speculating
looking at the current code.

> Now, memcg doesn't return -ENOMEM in usual case.
> So, it loops until there are some available memory under its limit.
> Then, if memory_cgroup_out_of_memory() can kill a process in several trial,
> we'll not have terrible problem. (even if it's slow.)
>
> Hmm. What I can't understand is whether there is a case when PF_EXITING
> thread never exit. If so, we need some care (in memcg?)

	void *thread_func(void *)
	{
		for (;;)
			malloc();
	}

	int main(void)
	{
		pthread_create(..., thread_func, ...);
		pthread_exit();
	}

This process runs with the dead group-leader (PF_EXITING is set, ->mm == NULL).
mem_cgroup_out_of_memory()->select_bad_process() can't see it due to
task_in_mem_cgroup() check.

Afaics

	- task_in_mem_cgroup() should use find_lock_task_mm() too

	- oom_kill_process() should check "PF_EXITING && p->mm",
	  like select_bad_process() does.

Oleg.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>