On Thu 30-01-14 12:29:06, Johannes Weiner wrote: > On Tue, Dec 17, 2013 at 04:45:29PM +0100, Michal Hocko wrote: > > The current charge path might race with memcg offlining because holding > > css reference doesn't stop css offline. As a result res counter might be > > charged after mem_cgroup_reparent_charges (called from memcg css_offline > > callback) and so the charge would never be freed. This has been worked > > around by 96f1c58d8534 (mm: memcg: fix race condition between memcg > > teardown and swapin) which tries to catch such a leaked charges later > > during css_free. It is more optimal to heal this race in the long term > > though. > > We already deal with the race, so IMO the only outstanding improvement > is to take advantage of the teardown synchronization provided by the > cgroup core and get rid of our one-liner workaround in .css_free. I am not sure I am following you here. Which teardown synchronization do you have in mind? rcu_read_lock & css_tryget? > > In order to make this raceless we would need to hold rcu_read_lock since > > css_tryget until res_counter_charge. This is not so easy unfortunately > > because mem_cgroup_do_charge might sleep so we would need to do drop rcu > > lock and do css_tryget tricks after each reclaim. > > Yes, why not? Although css_tryget is cheap these days I thought that a simple flag check would be even heaper in this hot path. Changing the patch to use css_tryget rather than offline check is trivial if you really think it is better? Btw. I plan to repost the series as soon as Andrew releases his mmotm tree. Thanks -- Michal Hocko SUSE Labs -- 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>