Re: [PATCH v2] vmpressure: make sure memcg stays alive until all users are signaled

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2013/7/12 0:22, Michal Hocko wrote:
> On Thu 11-07-13 08:44:08, Tejun Heo wrote:
>> Hello, Michal.
>>
>> On Thu, Jul 11, 2013 at 11:33:00AM +0200, Michal Hocko wrote:
>>> +static inline
>>> +struct mem_cgroup *vmpressure_to_mem_cgroup(struct vmpressure *vmpr)
>>> +{
>>> +	return container_of(vmpr, struct mem_cgroup, vmpressure);
>>> +}
>>> +
>>> +void vmpressure_pin_memcg(struct vmpressure *vmpr)
>>> +{
>>> +	struct mem_cgroup *memcg = vmpressure_to_mem_cgroup(vmpr);
>>> +
>>> +	css_get(&memcg->css);
>>> +}
>>> +
>>> +void vmpressure_unpin_memcg(struct vmpressure *vmpr)
>>> +{
>>> +	struct mem_cgroup *memcg = vmpressure_to_mem_cgroup(vmpr);
>>> +
>>> +	css_put(&memcg->css);
>>> +}
>>
>> So, while this *should* work, can't we just cancel/flush the work item
>> from offline? 
> 
> I would rather not put vmpressure clean up code into memcg offlining.
> We have reference counting for exactly this purposes so it feels strange
> to overcome it like that.

I'd agree with Tejun here. Asynchrously should be avoided if not necessary,
and the change would be simpler. There's already a vmpressure_init() in
mem_cgroup_css_alloc(), so it doesn't seem bad to do vmpressure cleanup
in memcg.