Re: [PATCH 4/5] mm: rework non-root kmem_cache lifecycle management

[Roman Gushchin <guro@fb.com>]

On Wed, Apr 17, 2019 at 04:41:01PM -0700, Shakeel Butt wrote:
> On Wed, Apr 17, 2019 at 2:55 PM Roman Gushchin <guroan@gmail.com> wrote:
> >
> > This commit makes several important changes in the lifecycle
> > of a non-root kmem_cache, which also affect the lifecycle
> > of a memory cgroup.
> >
> > Currently each charged slab page has a page->mem_cgroup pointer
> > to the memory cgroup and holds a reference to it.
> > Kmem_caches are held by the cgroup. On offlining empty kmem_caches
> > are freed, all other are freed on cgroup release.
> 
> No, they are not freed (i.e. destroyed) on offlining, only
> deactivated. All memcg kmem_caches are freed/destroyed on memcg's
> css_free.

You're right, my bad. I was thinking about the corresponding sysfs entry
when was writing it. We try to free it from the deactivation path too.

> 
> >
> > So the current scheme can be illustrated as:
> > page->mem_cgroup->kmem_cache.
> >
> > To implement the slab memory reparenting we need to invert the scheme
> > into: page->kmem_cache->mem_cgroup.
> >
> > Let's make every page to hold a reference to the kmem_cache (we
> > already have a stable pointer), and make kmem_caches to hold a single
> > reference to the memory cgroup.
> 
> What about memcg_kmem_get_cache()? That function assumes that by
> taking reference on memcg, it's kmem_caches will stay. I think you
> need to get reference on the kmem_cache in memcg_kmem_get_cache()
> within the rcu lock where you get the memcg through css_tryget_online.

Yeah, a very good question.

I believe it's safe because css_tryget_online() guarantees that
the cgroup is online and won't go offline before css_free() in
slab_post_alloc_hook(). I do initialize kmem_cache's refcount to 1
and drop it on offlining, so it protects the online kmem_cache.

Thank you for looking into the patchset!