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 <t...@kernel.org> > Reported-by: Johannes Weiner <han...@cmpxchg.org>
I guess Debugged-by: Petr Mladek <pmla...@suse.com> Reported-by: Cyril Hrubis <chru...@suse.cz> would be due > Suggested-by: Michal Hocko <mho...@kernel.org> > Fixes: 1ed1328792ff ("sched, cgroup: replace signal_struct->group_rwsem with > a global percpu_rwsem") > Cc: sta...@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? > Thanks. > > mm/memcontrol.c | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 36db05f..56060c7 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -4859,7 +4859,6 @@ static void mem_cgroup_move_charge(struct mm_struct *mm) > .mm = mm, > }; > > - lru_add_drain_all(); > /* > * Signal lock_page_memcg() to take the memcg's move_lock > * while we're moving its pages to another memcg. Then wait -- Michal Hocko SUSE Labs