mm, memcg: decouple e{low,min} state mutations from protection checks

mm, memcg: decouple e{low,min} state mutations from protection checks

mem_cgroup_protected currently is both used to set effective low and min
and return a mem_cgroup_protection based on the result.  As a user, this
can be a little unexpected: it appears to be a simple predicate function,
if not for the big warning in the comment above about the order in which
it must be executed.

This change makes it so that we separate the state mutations from the
actual protection checks, which makes it more obvious where we need to be
careful mutating internal state, and where we are simply checking and
don't need to worry about that.

[[email protected] - don't check protection on root memcgs]

Suggested-by: Johannes Weiner <[email protected]>
Signed-off-by: Chris Down <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Yafang Shao <[email protected]>
Link: http://lkml.kernel.org/r/ff3f915097fcee9f6d7041c084ef92d16aaeb56a.1594638158.git.chris@chrisdown.name
Signed-off-by: Linus Torvalds <[email protected]>