mm/compaction: ignore block suitable after check large free page

By reviewing code, I find that if the migrate target is a large free
page and we ignore suitable, it may splite large target free page into
smaller block which is not good for defrag.  So move the ignore block
suitable after check large free page.

As Vlastimil pointed out in RFC version that this patch is just based on
logical analyses which might be better for future-proofing the function
and it is most likely won't have any visible effect right now, for
direct compaction shouldn't have to be called if there's a
>=pageblock_order page already available.

Link: http://lkml.kernel.org/r/1489490743-5364-1-git-send-email-xieyisheng1@huawei.com
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: 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: Minchan Kim <minchan@kernel.org>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -992,9 +992,6 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
 static bool suitable_migration_target(struct compact_control *cc,
 							struct page *page)
 {
-	if (cc->ignore_block_suitable)
-		return true;
-
 	/* If the page is a large free page, then disallow migration */
 	if (PageBuddy(page)) {
 		/*
@@ -1006,6 +1003,9 @@ static bool suitable_migration_target(struct compact_control *cc,
 			return false;
 	}
 
+	if (cc->ignore_block_suitable)
+		return true;
+
 	/* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
 	if (migrate_async_suitable(get_pageblock_migratetype(page)))
 		return true;