On Tue 21-06-16 20:03:17, Tetsuo Handa wrote: > Michal Hocko wrote: > > On Mon 13-06-16 13:19:43, Michal Hocko wrote: > > [...] > > > I am trying to remember why we are disabling oom killer before kernel > > > threads are frozen but not really sure about that right away. > > > > OK, I guess I remember now. Say that a task would depend on a freezable > > kernel thread to get to do_exit (stuck in wait_event etc...). We would > > simply get stuck in oom_killer_disable for ever. So we need to address > > it a different way. > > > > One way would be what you are proposing but I guess it would be more > > systematic to never call exit_oom_victim on a remote task. After [1] we > > have a solid foundation to rely only on MMF_REAPED even when TIF_MEMDIE > > is set. It is more code than your patch so I can see a reason to go with > > yours if the following one seems too large or ugly. > > > > [1] http://lkml.kernel.org/r/1466426628-15074-1-git-send-email-mhocko@xxxxxxxxxx > > > > What do you think about the following? > > I'm OK with not clearing TIF_MEMDIE from a remote task. But this patch is racy. > > > @@ -567,40 +612,23 @@ static void oom_reap_task(struct task_struct *tsk) > > while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task(tsk)) > > schedule_timeout_idle(HZ/10); > > > > - if (attempts > MAX_OOM_REAP_RETRIES) { > > - struct task_struct *p; > > + tsk->oom_reaper_list = NULL; > > > > + if (attempts > MAX_OOM_REAP_RETRIES) { > > attempts > MAX_OOM_REAP_RETRIES would mean that down_read_trylock() > continuously failed. But it does not guarantee that the offending task > shall not call up_write(&mm->mmap_sem) and arrives at mmput() from exit_mm() > (as well as other threads which are blocked at down_read(&mm->mmap_sem) in > exit_mm() by the offending task arrive at mmput() from exit_mm()) when the > OOM reaper was preempted at this point. > > Therefore, find_lock_task_mm() in requeue_oom_victim() could return NULL and > the OOM reaper could fail to set MMF_OOM_REAPED (and find_lock_task_mm() in > oom_scan_process_thread() could return NULL and the OOM killer could fail to > select next OOM victim as well) when __mmput() got stuck. Fair enough. As this would break no-lockup requirement we cannot go that way. Let me think about it more. > So, from the point of view of correctness, there remains an unhandled race > window as long as you depend on find_lock_task_mm() not returning NULL. > You will again ask "does it really matter/occur", and I can't make progress. Sigh... -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>