Re: [RFC 4/5] memcg: make sure that memcg is not offline whencharging

[Johannes Weiner]

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.

> 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?

> This patch addresses the issue by introducing memcg->offline flag
> which is set from mem_cgroup_css_offline callback before the pages are
> reparented. mem_cgroup_do_charge checks the flag before res_counter
> is charged inside rcu read section. mem_cgroup_css_offline uses
> synchronize_rcu to let all preceding chargers finish while all the new
> ones will see the group offline already and back out.
>
> Callers are then updated to retry with a new memcg which is fallback to
> mem_cgroup_from_task(current).
> 
> The only exception is mem_cgroup_do_precharge which should never see
> this race because it is called from cgroup {can_}attach callbacks and so
> the whole cgroup cannot go away.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> ---
>  mm/memcontrol.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 55 insertions(+), 3 deletions(-)

That makes no sense to me.  It's a lateral move in functionality and
cgroup integration, but more complicated.
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