[PATCH] mm/compactoin: Fix edge case of fast_find_migrateblock()

[Wonhyuk Yang <vvghjk1234@gmail.com>]

In the fast_find_migrateblock(), It iterate freelist to find
proper pageblock. But there are two edge cases. First, if the page
we found is equal to cc->migrate_pfn, it is considered that we
didn't found suitable pageblock. Second, if the loop was terminated
because order is less than PAGE_ALLOC_COSTLY_ORDER, it could
be considered that we found suitable one.

Fixes: 70b44595eafe9 ("mm, compaction: use free lists to quickly locate
a migration source")

Signed-off-by: Wonhyuk Yang <vvghjk1234@gmail.com>
---
 mm/compaction.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index e5acb9714436..46f49e6b7d1a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1742,8 +1742,8 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 		distance >>= 2;
 	high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance);
 
-	for (order = cc->order - 1;
-	     order >= PAGE_ALLOC_COSTLY_ORDER && pfn == cc->migrate_pfn && nr_scanned < limit;
+	for (order = cc->order - 1, pfn = high_pfn;
+	     order >= PAGE_ALLOC_COSTLY_ORDER && pfn == high_pfn && nr_scanned < limit;
 	     order--) {
 		struct free_area *area = &cc->zone->free_area[order];
 		struct list_head *freelist;
@@ -1785,7 +1785,6 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 			}
 
 			if (nr_scanned >= limit) {
-				cc->fast_search_fail++;
 				move_freelist_tail(freelist, freepage);
 				break;
 			}
@@ -1799,9 +1798,10 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 	 * If fast scanning failed then use a cached entry for a page block
 	 * that had free pages as the basis for starting a linear scan.
 	 */
-	if (pfn == cc->migrate_pfn)
+	if (pfn == high_pfn) {
+		cc->fast_search_fail++;
 		pfn = reinit_migrate_pfn(cc);
-
+	}
 	return pfn;
 }
 
-- 
2.25.1