Re: [RFC 3/4] mm, oom: do not rely on TIF_MEMDIE for exit_oom_victim

[Tetsuo Handa]

Michal Hocko wrote:
> mark_oom_victim and exit_oom_victim are used for oom_killer_disable
> which should block as long as there any any oom victims alive. Up to now
> we have relied on TIF_MEMDIE task flag to count how many oom victim
> we have. This is not optimal because only one thread receives this flag
> at the time while the whole process (thread group) is killed and should
> die. As a result we do not thaw the whole thread group and so a multi
> threaded process can leave some threads behind in the fridge. We really
> want to thaw all the threads.
> 
> This is not all that easy because there is no reliable way to count
> threads in the process as the oom killer might race with copy_process.

What is wrong with racing with copy_process()? Threads doing copy_process()
are not frozen and thus we don't need to thaw such threads. Also, being
OOM-killed implies receiving SIGKILL. Thus, newly created thread will also
enter do_exit().

> So marking all threads with TIF_MEMDIE and increment oom_victims
> accordingly is not safe. Also TIF_MEMDIE flag should just die so
> we should better come up with a different approach.
> 
> All we need to guarantee is that exit_oom_victim is called at the time
> when no further access to (possibly suspended) devices or generate other
> IO (which would clobber suspended image and only once per process)
> is possible. It seems we can rely on exit_notify for that because we
> already have to detect the last thread to do a cleanup. Let's propagate
> that information up to do_exit and only call exit_oom_victim for such
> a thread. With this in place we can safely increment oom_victims only
> once per thread group and thaw all the threads from the process.
> freezing_slow_path can also rely on tsk_is_oom_victim as well now.

If marking all threads which belong to tsk thread group with TIF_MEMDIE
is not safe (due to possible race with copy_process()), how can

	rcu_read_lock();
	for_each_thread(tsk, t)
		__thaw_task(t);
	rcu_read_unlock();

in mark_oom_victim() guarantee that all threads which belong to tsk
thread group are thawed?

Unless all threads which belong to tsk thread group in __refrigerator()
are guaranteed to be thawed, they might fail to leave __refrigerator()
in order to enter do_exit() which means that exit_oom_victim() won't be
called.

Do we want to thaw OOM victims from the beginning? If the freezer
depends on CONFIG_MMU=y , we don't need to thaw OOM victims.