memcg: export struct mem_cgroup

memcg: export struct mem_cgroup

mem_cgroup structure is defined in mm/memcontrol.c currently which means
that the code outside of this file has to use external API even for
trivial access stuff.

This patch exports mm_struct with its dependencies and makes some of the
exported functions inlines.  This even helps to reduce the code size a bit
(make defconfig + CONFIG_MEMCG=y)

  text		data    bss     dec     	 hex 	filename
  12355346        1823792 1089536 15268674         e8fb42 vmlinux.before
  12354970        1823792 1089536 15268298         e8f9ca vmlinux.after

This is not much (370B) but better than nothing.

We also save a function call in some hot paths like callers of
mem_cgroup_count_vm_event which is used for accounting.

The patch doesn't introduce any functional changes.

[[email protected]: inline memcg_kmem_is_active]
[[email protected]: do not expose type outside of CONFIG_MEMCG]
[[email protected]: memcontrol.h needs eventfd.h for eventfd_ctx]
[[email protected]: export mem_cgroup_from_task() to modules]
Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: Vladimir Davydov <[email protected]>
Suggested-by: Johannes Weiner <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>