mm: memcontrol: prepare objcg API for non-kmem usage

mm: memcontrol: prepare objcg API for non-kmem usage

Pagecache pages are charged at the allocation time and holding a reference
to the original memory cgroup until being reclaimed.  Depending on the
memory pressure, specific patterns of the page sharing between different
cgroups and the cgroup creation and destruction rates, a large number of
dying memory cgroups can be pinned by pagecache pages.  It makes the page
reclaim less efficient and wastes memory.

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.

Therefore, the infrastructure of objcg no longer only serves
CONFIG_MEMCG_KMEM.  In this patch, we move the infrastructure of the objcg
out of the scope of the CONFIG_MEMCG_KMEM so that the LRU pages can reuse
it to charge pages.

We know that the LRU pages are not accounted at the root level.  But the
page->memcg_data points to the root_mem_cgroup.  So the page->memcg_data
of the LRU pages always points to a valid pointer.  But the
root_mem_cgroup dose not have an object cgroup.  If we use obj_cgroup APIs
to charge the LRU pages, we should set the page->memcg_data to a root
object cgroup.  So we also allocate an object cgroup for the
root_mem_cgroup.

Link: https://lkml.kernel.org/r/20220621125658.64935-4-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>