On Fri 26-10-18 10:25:31, Johannes Weiner wrote: > On Mon, Oct 22, 2018 at 09:13:23AM +0200, Michal Hocko wrote: > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > Tetsuo has reported [1] that a single process group memcg might easily > > swamp the log with no-eligible oom victim reports due to race between > > the memcg charge and oom_reaper > > > > Thread 1 Thread2 oom_reaper > > try_charge try_charge > > mem_cgroup_out_of_memory > > mutex_lock(oom_lock) > > mem_cgroup_out_of_memory > > mutex_lock(oom_lock) > > out_of_memory > > select_bad_process > > oom_kill_process(current) > > wake_oom_reaper > > oom_reap_task > > MMF_OOM_SKIP->victim > > mutex_unlock(oom_lock) > > out_of_memory > > select_bad_process # no task > > > > If Thread1 didn't race it would bail out from try_charge and force the > > charge. We can achieve the same by checking tsk_is_oom_victim inside > > the oom_lock and therefore close the race. > > > > [1] http://lkml.kernel.org/r/bb2074c0-34fe-8c2c-1c7d-db71338f1e7f@xxxxxxxxxxxxxxxxxxx > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > --- > > mm/memcontrol.c | 14 +++++++++++++- > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index e79cb59552d9..a9dfed29967b 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -1380,10 +1380,22 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask, > > .gfp_mask = gfp_mask, > > .order = order, > > }; > > - bool ret; > > + bool ret = true; > > > > mutex_lock(&oom_lock); > > + > > + /* > > + * multi-threaded tasks might race with oom_reaper and gain > > + * MMF_OOM_SKIP before reaching out_of_memory which can lead > > + * to out_of_memory failure if the task is the last one in > > + * memcg which would be a false possitive failure reported > > + */ > > + if (tsk_is_oom_victim(current)) > > + goto unlock; > > + > > ret = out_of_memory(&oc); > > We already check tsk_is_oom_victim(current) in try_charge() before > looping on the OOM killer, so at most we'd have a single "no eligible > tasks" message from such a race before we force the charge - right? Not really. You can have many threads blocked on the oom_lock and being reaped while they are waiting. So the check without the lock will always be racy. This is what Tetsuo's test case actually triggers I believe. > While that's not perfect, I don't think it warrants complicating this > code even more. I honestly find it near-impossible to follow the code > and the possible scenarios at this point. I do agree that the code is quite far from easy to follow. The set of events that might happen in a different context is not trivial. > out_of_memory() bails on task_will_free_mem(current), which > specifically *excludes* already reaped tasks. Why are we then adding a > separate check before that to bail on already reaped victims? 696453e66630a has introduced the bail out. > Do we want to bail if current is a reaped victim or not? > > I don't see how we could skip it safely in general: the current task > might have been killed and reaped and gotten access to the memory > reserve and still fail to allocate on its way out. It needs to kill > the next task if there is one, or warn if there isn't another > one. Because we're genuinely oom without reclaimable tasks. Yes, this would be the case for the global case which is a real OOM situation. Memcg oom is somehow more relaxed because the oom is local. > There is of course the scenario brought forward in this thread, where > multiple threads of a process race and the second one enters oom even > though it doesn't need to anymore. What the global case does to catch > this is to grab the oom lock and do one last alloc attempt. Should > memcg lock the oom_lock and try one more time to charge the memcg? That would be another option. I agree that making it more towards the global case makes it more attractive. My tsk_is_oom_victim is more towards "plug this particular case". So does this look better to you? diff --git a/mm/memcontrol.c b/mm/memcontrol.c index e79cb59552d9..4abb66efe806 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1380,10 +1380,22 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask, .gfp_mask = gfp_mask, .order = order, }; - bool ret; + bool ret = true; mutex_lock(&oom_lock); + + /* + * Make the last moment check while we were waiting for the oom_lock + * just in case the oom_reaper could have freed released some + * memory in the meantime. This mimics the lalst moment allocation + * in __alloc_pages_may_oom + */ + if (mem_cgroup_margin(mem_over_limit) >= 1 << order) + goto unlock; + ret = out_of_memory(&oc); + +unlock: mutex_unlock(&oom_lock); return ret; } > Some simplification in this area would really be great. I'm reluctant > to ack patches like the above, even if they have some optical benefits > for the user, because the code is already too tricky for what it does. I am open to different ideas, unless they are crazy timeout here and timeout there wrapped with a duct tape. -- Michal Hocko SUSE Labs
