Re: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction

[Roman Gushchin]

On Thu, Nov 28, 2019 at 10:43:12AM +0100, Michal Hocko wrote:
> On Wed 27-11-19 17:27:29, Roman Gushchin wrote:
> > On Wed, Nov 27, 2019 at 01:32:25PM +0100, Michal Hocko wrote:
> > > On Tue 26-11-19 18:41:41, Roman Gushchin wrote:
> > > > On Tue, Nov 26, 2019 at 10:29:18AM +0100, Michal Hocko wrote:
> > > > > On Mon 25-11-19 10:54:53, Roman Gushchin wrote:
> > > [...]
> > > > > > So in a rare case when not all children kmem_caches are destroyed
> > > > > > at the moment when the root kmem_cache is about to be gone, we need
> > > > > > to wait another rcu grace period before destroying the root
> > > > > > kmem_cache.
> > > > > 
> > > > > Could you explain how rare this really is please?
> > > > 
> > > > It seems that we don't destroy root kmem_caches with enabled memcg
> > > > accounting that often, but maybe I'm biased here.
> > > 
> > > So this happens each time a root kmem_cache is destroyed? Which would
> > > imply that only dynamically created ones?
> > 
> > Yes, only dynamically created and only in those cases when destruction
> > of the root cache happens immediately after the deactivation of the
> > non-root cache.
> > Tbh I can't imagine any other case except rmmod after
> > removing the cgroup.
> 
> Thanks for the confirmation! Could you please make this explicit in the
> changelog please? Maybe it is obvious to you but it took me quite some
> time to grasp what the hell is going on here. Both memcg and kmem_cache
> destruction are quite complex and convoluted.

Sure, will send v2 shorty.

Thanks!