mm/memcg: unify swap and memsw page counters

mm/memcg: unify swap and memsw page counters

The swap page counter is v2 only while memsw is v1 only.  As v1 and v2
controllers cannot be active at the same time, there is no point to keep
both swap and memsw page counters in mem_cgroup.  The previous patch has
made sure that memsw page counter is updated and accessed only when in v1
code paths.  So it is now safe to alias the v1 memsw page counter to v2
swap page counter.  This saves 14 long's in the size of mem_cgroup.  This
is a saving of 112 bytes for 64-bit archs.

While at it, also document which page counters are used in v1 and/or v2.

Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Chris Down <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Yafang Shao <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>