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

[Michal Hocko]

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)
						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)

> It also becomes clear that I can't remove the
> mem_cgroup_mark_under_oom() like that because it is key in receiving
> wakeups.  And as with any sleeping lock, we need to listen to wakeups
> before attempting the trylock, or we might miss the wakeup from the
> unlock.
> 
> It definitely became a separate patch, which cleans up this unholy
> mess first before putting new things on top:

I will check the patch tomorrow with a clean head.
[...]
-- 
Michal Hocko
SUSE Labs
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