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

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Date:   Wed, 27 Nov 2019 17:27:29 +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 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!