On 2018/06/20 20:55, Michal Hocko wrote: > On Wed 20-06-18 20:20:38, Tetsuo Handa wrote: >> Sleeping with oom_lock held can cause AB-BA lockup bug because >> __alloc_pages_may_oom() does not wait for oom_lock. Since >> blocking_notifier_call_chain() in out_of_memory() might sleep, sleeping >> with oom_lock held is currently an unavoidable problem. > > Could you be more specific about the potential deadlock? Sleeping while > holding oom lock is certainly not nice but I do not see how that would > result in a deadlock assuming that the sleeping context doesn't sleep on > the memory allocation obviously. "A" is "owns oom_lock" and "B" is "owns CPU resources". It was demonstrated at "mm,oom: Don't call schedule_timeout_killable() with oom_lock held." proposal. But since you don't accept preserving the short sleep which is a heuristic for reducing the possibility of AB-BA lockup, the only way we would accept will be wait for the owner of oom_lock (e.g. by s/mutex_trylock/mutex_lock/ or whatever) which is free of heuristic and free of AB-BA lockup. > >> As a preparation for not to sleep with oom_lock held, this patch brings >> OOM notifier callbacks to outside of OOM killer, with two small behavior >> changes explained below. > > Can we just eliminate this ugliness and remove it altogether? We do not > have that many notifiers. Is there anything fundamental that would > prevent us from moving them to shrinkers instead? > For long term, it would be possible. But not within this patch. For example, I think that virtio_balloon wants to release memory only when we have no choice but OOM kill. If virtio_balloon trivially releases memory, it will increase the risk of killing the entire guest by OOM-killer from the host side.
