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

[Roman Gushchin]

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@xxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> 
> 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.

Thanks!