mm: have order > 0 compaction start off where it left

mm: have order > 0 compaction start off where it left

This patch makes the comment for cc->wrapped longer, explaining what is
really going on.  It also incorporates the comment fix pointed out by
Minchan.

Additionally, Minchan found that, when no pages get isolated, high_pfn
could be a value that is much lower than desired, which might potentially
cause compaction to skip a range of pages.

Only assign zone->compact_cache_free_pfn if we actually isolated free
pages for compaction.

Split out the calculation to get the start of the last page block in a
zone into its own, commented function.

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Jim Schutt <jaschut@sandia.gov>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>