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 17:29, Michal Hocko wrote:
> On Fri 12-07-13 17:20:09, Li Zefan wrote:
> [...]
>> But if I read the code correctly, even no one registers a vmpressure event,
>> vmpressure() is always running and queue the work item.
> 
> True but checking there is somebody is rather impractical. First we
> would have to take a events_lock to check this and then drop it after
> scheduling the work. Which doesn't guarantee that the registered event
> wouldn't go away.
> And even trickier, we would have to do the same for all parents up the
> hierarchy.
> 

The thing is, we can forget about eventfd. eventfd is checked in
vmpressure_work_fn(), while vmpressure() is always called no matter what.

vmpressure()
  queue_work()
                      cgroup_diput()
                        call_rcu(cgroup_free_rcu)                        
                      ...
                      queue_work(destroy_cgroup)
                      ...
                      cgroup_free_fn()
                        mem_cgroup_destroy()
  ...
  vmpress_work_fn()

There's no guarantee that vmpressure work is run before cgroup destroy
work, I think.