Messages in this thread | | | Date | Wed, 20 Apr 2016 17:29:22 -0400 | From | Tejun Heo <> | Subject | Re: [PATCH for-4.6-fixes] memcg: remove lru_add_drain_all() invocation from mem_cgroup_move_charge() |
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Hello, Michal.
On Sun, Apr 17, 2016 at 08:07:48AM -0400, Michal Hocko wrote: > On Fri 15-04-16 15:17:19, Tejun Heo wrote: > > mem_cgroup_move_charge() invokes lru_add_drain_all() so that the pvec > > pages can be moved too. lru_add_drain_all() schedules and flushes > > work items on system_wq which depends on being able to create new > > kworkers to make forward progress. Since 1ed1328792ff ("sched, > > cgroup: replace signal_struct->group_rwsem with a global > > percpu_rwsem"), a new task can't be created while in the cgroup > > migration path and the described lru_add_drain_all() invocation can > > easily lead to a deadlock. > > > > Charge moving is best-effort and whether the pvec pages are migrated > > or not doesn't really matter. Don't call it during charge moving. > > Eventually, we want to move the actual charge moving outside the > > migration path. > > > > Signed-off-by: Tejun Heo <tj@kernel.org> > > Reported-by: Johannes Weiner <hannes@cmpxchg.org> > > I guess > Debugged-by: Petr Mladek <pmladek@suse.com> > Reported-by: Cyril Hrubis <chrubis@suse.cz>
Yeah, definitely. Sorry about missing them.
> > Suggested-by: Michal Hocko <mhocko@kernel.org> > > Fixes: 1ed1328792ff ("sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem") > > Cc: stable@vger.kernel.org > > --- > > Hello, > > > > So, this deadlock seems pretty easy to trigger. We'll make the charge > > moving asynchronous eventually but let's not hold off fixing an > > immediate problem. > > Although this looks rather straightforward and it fixes the immediate > problem I am little bit nervous about it. As already pointed out in > other email mem_cgroup_move_charge still depends on mmap_sem for > read and we might hit an even more subtle lockup if the current holder > of the mmap_sem for write depends on the task creation (e.g. some of the > direct reclaim path uses WQ which is really hard to rule out and I even > think that some shrinkers do this). > > I liked your proposal when mem_cgroup_move_charge would be called from a > context which doesn't hold the problematic rwsem much more. Would that > be too intrusive for the stable backport?
Yeah, I'm working on the fix but let's plug this one first as it seems really easy to trigger. I got a couple off-list reports (in and outside fb) of this triggering.
Thanks.
-- tejun
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