linux-kernel - Re: [PATCH v5 6/7] mm: reparent slab memory on cgroup removal

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Date:   Tue, 28 May 2019 21:33:02 +0300
From:   Vladimir Davydov <vdavydov.dev@...il.com>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Christoph Lameter <cl@...ux.com>, cgroups@...r.kernel.org,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v5 6/7] mm: reparent slab memory on cgroup removal

On Tue, May 21, 2019 at 01:07:34PM -0700, Roman Gushchin wrote:
> Let's reparent memcg slab memory on memcg offlining. This allows us
> to release the memory cgroup without waiting for the last outstanding
> kernel object (e.g. dentry used by another application).
> 
> So instead of reparenting all accounted slab pages, let's do reparent
> a relatively small amount of kmem_caches. Reparenting is performed as
> a part of the deactivation process.
> 
> Since the parent cgroup is already charged, everything we need to do
> is to splice the list of kmem_caches to the parent's kmem_caches list,
> swap the memcg pointer and drop the css refcounter for each kmem_cache
> and adjust the parent's css refcounter. Quite simple.
> 
> Please, note that kmem_cache->memcg_params.memcg isn't a stable
> pointer anymore. It's safe to read it under rcu_read_lock() or
> with slab_mutex held.
> 
> We can race with the slab allocation and deallocation paths. It's not
> a big problem: parent's charge and slab global stats are always
> correct, and we don't care anymore about the child usage and global
> stats. The child cgroup is already offline, so we don't use or show it
> anywhere.
> 
> Local slab stats (NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE)
> aren't used anywhere except count_shadow_nodes(). But even there it
> won't break anything: after reparenting "nodes" will be 0 on child
> level (because we're already reparenting shrinker lists), and on
> parent level page stats always were 0, and this patch won't change
> anything.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>

This one looks good to me. I can't see why anything could possibly go
wrong after this change.