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

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Date:   Tue, 28 May 2019 18:16:14 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        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" <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 28, 2019 at 07:58:17PM +0000, Roman Gushchin wrote:
> On Tue, May 28, 2019 at 09:33:02PM +0300, Vladimir Davydov wrote:
> > 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.
> 
> Hi Vladimir!
> 
> Thank you for looking into the series. Really appreciate it!
> 
> It looks like outstanding questions are:
> 1) synchronization around the dying flag
> 2) removing CONFIG_SLOB in 2/7
> 3) early sysfs_slab_remove()
> 4) mem_cgroup_from_kmem in 7/7
> 
> Please, let me know if I missed anything.
> 
> I'm waiting now for Johanness's review, so I'll address these issues
> in background and post the next (and hopefully) final version.

The todo items here aside, the series looks good to me - although I'm
glad that Vladimir gave it a much more informed review than I could.