Hello, On Wed, Aug 07, 2013 at 01:28:25PM +0200, Michal Hocko wrote: > There is no limit for the maximum number of threshold events registered > per memcg. This might lead to an user triggered memory depletion if a > regular user is allowed to register on memory.[memsw.]usage_in_bytes > eventfd interface. > > Let's be more strict and cap the number of events that might be > registered. MAX_THRESHOLD_EVENTS value is more or less random. The > expectation is that it should be high enough to cover reasonable > usecases while not too high to allow excessive resources consumption. > 1024 events consume something like 16KB which shouldn't be a big deal > and it should be good enough. I don't think the memory consumption per-se is the issue to be handled here (as kernel memory consumption is a different generic problem) but rather that all listeners, regardless of their priv level, cgroup membership and so on, end up contributing to this single shared contiguous table, which makes it quite easy to do DoS attack on it if the event control is actually delegated to untrusted security domain, which BTW kinda makes all these complexities kinda pointless as it nullifies the only use case (many un-coordinated listeners watching different thresholds) which the event mechanism can actually do better. A proper fix would be making it build sorted data structure, be it list or tree, and letting each listener insert its own probe at the appropriate position and updating the event generation maintain cursor in the tree and fire events as appropriate, but given that the whole usage model is being obsoleted, it probably isn't worth doing that and this fixed limit is better than just letting things go and allow allocation to fail at some point, I suppose. Can you please update the patch description to reflect the actual problem? Thanks. -- tejun -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>