memcg: deprecate memory.force_empty knob

memcg: deprecate memory.force_empty knob

force_empty has been introduced primarily to drop memory before it gets
reparented on the group removal.  This alone doesn't sound fully justified
because reparented pages which are not in use can be reclaimed also later
when there is a memory pressure on the parent level.

Mark the knob CFTYPE_INSANE which tells the cgroup core that it shouldn't
create the knob with the experimental sane_behavior.  Other users will get
informed about the deprecation and asked to tell us more because I do not
expect most users will use sane_behavior cgroups mode very soon.

Anyway I expect that most users will be simply cgroup remove handlers
which do that since ever without having any good reason for it.

If somebody really cares because reparented pages, which would be dropped
otherwise, push out more important ones then we should fix the reparenting
code and put pages to the tail.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>