mm: memcontrol: introduce obj_cgroup_{un}charge_pages

mm: memcontrol: introduce obj_cgroup_{un}charge_pages

We know that the unit of slab object charging is bytes, the unit of kmem
page charging is PAGE_SIZE.  If we want to reuse obj_cgroup APIs to
charge the kmem pages, we should pass PAGE_SIZE (as third parameter) to
obj_cgroup_charge().  Because the size is already PAGE_SIZE, we can skip
touch the objcg stock.  And obj_cgroup_{un}charge_pages() are introduced
to charge in units of page level.

In the latter patch, we also can reuse those two helpers to charge or
uncharge a number of kernel pages to a object cgroup.  This is just a
code movement without any functional changes.

Link: https://lkml.kernel.org/r/20210319163821.20704-3-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>