On Mon 16-07-18 19:38:21, Tetsuo Handa wrote: > On 2018/07/16 16:44, Michal Hocko wrote: > >> If setting MMF_OOM_SKIP is guarded by oom_lock, we can enforce > >> last second allocation attempt like below. > >> > >> CPU 0 CPU 1 > >> > >> mutex_trylock(&oom_lock) in __alloc_pages_may_oom() succeeds. > >> get_page_from_freelist() fails. > >> Enters out_of_memory(). > >> > >> __oom_reap_task_mm() reclaims > >> some memory. > >> mutex_lock(&oom_lock); > >> > >> select_bad_process() does not select new victim because MMF_OOM_SKIP is > >> not yet set. > >> Leaves out_of_memory(). > >> mutex_unlock(&oom_lock) in __alloc_pages_may_oom() is called. > >> > >> Sets MMF_OOM_SKIP. > >> mutex_unlock(&oom_lock); > >> > >> get_page_from_freelist() likely succeeds before reaching > >> __alloc_pages_may_oom() again. > >> Saved one OOM victim from being needlessly killed. > >> > >> That is, guarding setting MMF_OOM_SKIP works as if synchronize_rcu(); it > >> waits for anybody > >> who already acquired (or started waiting for) oom_lock to release > >> oom_lock, in order to > >> prevent select_bad_process() from needlessly selecting new OOM victim. > > > > Hmm, is this a practical problem though? Do we really need to have a > > broader locking context just to defeat this race? > > Yes, for you think that select_bad_process() might take long time. It is > possible > that MMF_OOM_SKIP is set while the owner of oom_lock is preempted. It is not > such > a small window that select_bad_process() finds an mm which got MMF_OOM_SKIP > immediately before examining that mm.
I only do care if the race is practical to hit. And that is why I would like a simplification first (so drop the oom_lock in the oom_reaper path) and then follow up with some decent justification on top. -- Michal Hocko SUSE Labs
