Re: [patch 6/6] mm: memcg: do not trap chargers with full callstack on OOM

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jul 29, 2013 at 04:12:50PM +0200, Michal Hocko wrote:
> On Fri 26-07-13 17:28:09, Johannes Weiner wrote:
> > On Fri, Jul 26, 2013 at 04:43:10PM +0200, Michal Hocko wrote:
> > > On Thu 25-07-13 18:25:38, Johannes Weiner wrote:
> > > > @@ -2189,31 +2191,20 @@ static void memcg_oom_recover(struct mem_cgroup *memcg)
> > > >  }
> > > >  
> > > >  /*
> > > > - * try to call OOM killer. returns false if we should exit memory-reclaim loop.
> > > > + * try to call OOM killer
> > > >   */
> > > > -static bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask,
> > > > -				  int order)
> > > > +static void mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
> > > >  {
> > > > -	struct oom_wait_info owait;
> > > > -	bool locked, need_to_kill;
> > > > +	bool locked, need_to_kill = true;
> > > >  
> > > > -	owait.memcg = memcg;
> > > > -	owait.wait.flags = 0;
> > > > -	owait.wait.func = memcg_oom_wake_function;
> > > > -	owait.wait.private = current;
> > > > -	INIT_LIST_HEAD(&owait.wait.task_list);
> > > > -	need_to_kill = true;
> > > > -	mem_cgroup_mark_under_oom(memcg);
> > > 
> > > You are marking memcg under_oom only for the sleepers. So if we have
> > > no sleepers then the memcg will never report it is under oom which
> > > is a behavior change. On the other hand who-ever relies on under_oom
> > > under such conditions (it would basically mean a busy loop reading
> > > memory.oom_control) would be racy anyway so it is questionable it
> > > matters at all. At least now when we do not have any active notification
> > > that under_oom has changed.
> > > 
> > > Anyway, this shouldn't be a part of this patch so if you want it because
> > > it saves a pointless hierarchy traversal then make it a separate patch
> > > with explanation why the new behavior is still OK.
> > 
> > This made me think again about how the locking and waking in there
> > works and I found a bug in this patch.
> > 
> > Basically, we have an open-coded sleeping lock in there and it's all
> > obfuscated by having way too much stuffed into the memcg_oom_lock
> > section.
> > 
> > Removing all the clutter, it becomes clear that I can't remove that
> > (undocumented) final wakeup at the end of the function.  As with any
> > lock, a contender has to be woken up after unlock.  We can't rely on
> > the lock holder's OOM kill to trigger uncharges and wakeups, because a
> > contender for the OOM lock could show up after the OOM kill but before
> > the lock is released.  If there weren't any more wakeups, the
> > contender would sleep indefinitely.
> 
> I have checked that path again and I still do not see how wakeup_oom
> helps here. What prevents us from the following race then?
> 
> spin_lock(&memcg_oom_lock)
> locked = mem_cgroup_oom_lock(memcg) # true
> spin_unlock(&memcg_oom_lock)

                                                prepare_to_wait()

> 						spin_lock(&memcg_oom_lock)
> 						locked = mem_cgroup_oom_lock(memcg) # false
> 						spin_unlock(&memcg_oom_lock)
> 						<resched>
> mem_cgroup_out_of_memory()
> 			<uncharge & memcg_oom_recover>
> spin_lock(&memcg_oom_lock)
> mem_cgroup_oom_unlock(memcg)
> memcg_wakeup_oom(memcg)
> 						schedule()
> spin_unlock(&memcg_oom_lock)
> mem_cgroup_unmark_under_oom(memcg)

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