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    SubjectRe: [PATCH for-4.6-fixes] memcg: remove lru_add_drain_all() invocation from mem_cgroup_move_charge()
    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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