mm: enhance comment and bug check

mm: enhance comment and bug check

This patch updates comment and bug check.
It can be fold into [1].

[1] mm-revert-0def08e3-mm-mempolicyc-check-return-code-of-check_range.patch

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vasiliy Kulikov <segooon@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>