Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap

[Michal Hocko <mhocko@kernel.org>]

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