mm: compaction: limit the suitable target page order to be less than cc->order

It can not improve the fragmentation if we isolate the target free pages
exceeding cc->order, especially when the cc->order is less than
pageblock_order.  For example, suppose the pageblock_order is MAX_ORDER
(size is 4M) and cc->order is 2M THP size, we should not isolate other 2M
free pages to be the migration target, which can not improve the
fragmentation.

Moreover this is also applicable for large folio compaction.

Link: https://lkml.kernel.org/r/afcd9377351c259df7a25a388a4a0d5862b986f4.1705928395.git.baolin.wang@linux.alibaba.comSigned-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/compaction.c

@@ -1365,12 +1365,14 @@ static bool suitable_migration_target(struct compact_control *cc,
 {
 	/* If the page is a large free page, then disallow migration */
 	if (PageBuddy(page)) {
+		int order = cc->order > 0 ? cc->order : pageblock_order;
+
 		/*
 		 * We are checking page_order without zone->lock taken. But
 		 * the only small danger is that we skip a potentially suitable
 		 * pageblock, so it's not worth to check order for valid range.
 		 */
-		if (buddy_order_unsafe(page) >= pageblock_order)
+		if (buddy_order_unsafe(page) >= order)
 			return false;
 	}