Re: [PATCH v4 4/5] mm: memcontrol: use obj_cgroup APIs to charge kmem pages

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Mar 18, 2021 at 07:06:57PM +0800, Muchun Song wrote:
> Since Roman series "The new cgroup slab memory controller" applied. All
> slab objects are charged via the new APIs of obj_cgroup. The new APIs
> introduce a struct obj_cgroup to charge slab objects. It prevents
> long-living objects from pinning the original memory cgroup in the memory.
> But there are still some corner objects (e.g. allocations larger than
> order-1 page on SLUB) which are not charged via the new APIs. Those
> objects (include the pages which are allocated from buddy allocator
> directly) are charged as kmem pages which still hold a reference to
> the memory cgroup.
> 
> We want to reuse the obj_cgroup APIs to charge the kmem pages.
> If we do that, we should store an object cgroup pointer to
> page->memcg_data for the kmem pages.
> 
> Finally, page->memcg_data will have 3 different meanings.
> 
>   1) For the slab pages, page->memcg_data points to an object cgroups
>      vector.
> 
>   2) For the kmem pages (exclude the slab pages), page->memcg_data
>      points to an object cgroup.
> 
>   3) For the user pages (e.g. the LRU pages), page->memcg_data points
>      to a memory cgroup.
> 
> We do not change the behavior of page_memcg() and page_memcg_rcu().
> They are also suitable for LRU pages and kmem pages. Why?
> 
> Because memory allocations pinning memcgs for a long time - it exists
> at a larger scale and is causing recurring problems in the real world:
> page cache doesn't get reclaimed for a long time, or is used by the
> second, third, fourth, ... instance of the same job that was restarted
> into a new cgroup every time. Unreclaimable dying cgroups pile up,
> waste memory, and make page reclaim very inefficient.
> 
> We can convert LRU pages and most other raw memcg pins to the objcg
> direction to fix this problem, and then the page->memcg will always
> point to an object cgroup pointer. At that time, LRU pages and kmem
> pages will be treated the same. The implementation of page_memcg()
> will remove the kmem page check.
> 
> This patch aims to charge the kmem pages by using the new APIs of
> obj_cgroup. Finally, the page->memcg_data of the kmem page points to
> an object cgroup. We can use the __page_objcg() to get the object
> cgroup associated with a kmem page. Or we can use page_memcg()
> to get the memory cgroup associated with a kmem page, but caller must
> ensure that the returned memcg won't be released (e.g. acquire the
> rcu_read_lock or css_set_lock).
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Thanks for the updated version, looks good to me!

Acked-by: Johannes Weiner <hannes@cmpxchg.org>