linux-kernel - Re: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem

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Date:   Fri, 29 Nov 2019 02:28:11 +0000
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing
 on root kmem_cache destruction

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!