mm, vmscan: make compaction_ready() more accurate and readable

mm, vmscan: make compaction_ready() more accurate and readable

The compaction_ready() is used during direct reclaim for costly order
allocations to skip reclaim for zones where compaction should be attempted
instead.  It's combining the standard compaction_suitable() check with its
own watermark check based on high watermark with extra gap, and the result
is confusing at best.

This patch attempts to better structure and document the checks involved.
First, compaction_suitable() can determine that the allocation should
either succeed already, or that compaction doesn't have enough free pages
to proceed.  The third possibility is that compaction has enough free
pages, but we still decide to reclaim first - unless we are already above
the high watermark with gap.  This does not mean that the reclaim will
actually reach this watermark during single attempt, this is rather an
over-reclaim protection.  So document the code as such.  The check for
compaction_deferred() is removed completely, as it in fact had no proper
role here.

The result after this patch is mainly a less confusing code.  We also skip
some over-reclaim in cases where the allocation should already succed.

Link: http://lkml.kernel.org/r/20160810091226.6709-12-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>