Re: [PATCH v7 10/10] mm: reparent memcg kmem_caches on cgroup removal

[Vladimir Davydov]

On Tue, Jun 11, 2019 at 04:18:13PM -0700, Roman Gushchin wrote:
> Let's reparent non-root kmem_caches 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).
> 
> 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, drop the css refcounter for each kmem_cache
> and adjust the parent's css refcounter.
> 
> Please, note that kmem_cache->memcg_params.memcg isn't a stable
> pointer anymore. It's safe to read it under rcu_read_lock(),
> cgroup_mutex held, or any other way that protects the memory cgroup
> from being released.
> 
> 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@xxxxxx>

Acked-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>