On 2013/7/12 17:29, Michal Hocko wrote:

On Fri 12-07-13 17:20:09, Li Zefan wrote:
[...]

quoted

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.