On Sat 10-01-15 16:43:16, Tejun Heo wrote: > Currently, if a hierarchy doesn't have any live children when it's > unmounted, the hierarchy starts dying by killing its refcnt. The > expectation is that even if there are lingering dead children which > are lingering due to remaining references, they'll be put in a finite > amount of time. When the children are finally released, the hierarchy > is destroyed and all controllers bound to it also are released. > > However, for memcg, the premise that the lingering refs will be put in > a finite amount time is not true. In the absense of memory pressure, > dead memcg's may hang around indefinitely pinned by its pages. This > unfortunately may lead to indefinite hang on the next mount attempt > involving memcg as the mount logic waits for it to get released. > > While we can change hierarchy destruction logic such that a hierarchy > is only destroyed when it's not mounted anywhere and all its children, > live or dead, are gone, this makes whether the hierarchy gets > destroyed or not to be determined by factors opaque to userland. > Userland may or may not get a new hierarchy on the next mount attempt. > Worse, if it explicitly wants to create a new hierarchy with different > options or controller compositions involving memcg, it will fail in an > essentially arbitrary manner. > > We want to guarantee that a hierarchy is destroyed once the > conditions, unmounted and no visible children, are met. To aid it, > this patch introduces a new callback cgroup_subsys->unbind() which is > invoked right before the hierarchy a subsystem is bound to starts > dying. memcg can implement this callback and initiate draining of > remaining refs so that the hierarchy can eventually be released in a > finite amount of time. > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > Cc: Li Zefan <lizefan@xxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxx> > Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> Ohh, I have missed this one as I wasn't on the CC list. FWIW this approach makes sense to me. I just think that we should have a way to fail. E.g. kmem pages are impossible to reclaim because there might be some objects lingering somewhere not bound to a task context and reparenting is hard as Vladimir has pointed out several times already. Normal LRU pages should be reclaimable or reparented to the root easily. I cannot judge the implementation but I agree with the fact that memcg controller should be the one to take an action. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>