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@xxxxxxxxx> 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!
