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.

--
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>