[PATCH v2] mm/compactoin: Fix misbehaviors of fast_find_migrateblock()

[Wonhyuk Yang <vvghjk1234@gmail.com>]

In the fast_find_migrateblock(), It iterate freelist to find
proper pageblock. But there are some misbehaviors.

 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. Third, if the skip bit is set on the page block
and go continue, it doesn't check the nr_scanned. Fourth, if
the page block's skip bit is set, it check that page block is
the last of list. But it is unnecessary.

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

Signed-off-by: Wonhyuk Yang <vvghjk1234@gmail.com>
---
Changes in v2:
  - Follow Vlastimil's suggestions, using bool varable.
  - Fix new misbehavior and remove unecessary .

v1: https://lore.kernel.org/linux-mm/20210123154320.24278-1-vvghjk1234@gmail.com
---
 mm/compaction.c | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index e5acb9714436..8e5cd9d20435 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1701,6 +1701,7 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 	unsigned long pfn = cc->migrate_pfn;
 	unsigned long high_pfn;
 	int order;
+	bool found_block = false;
 
 	/* Skip hints are relied on to avoid repeats on the fast search */
 	if (cc->ignore_skip_hint)
@@ -1743,7 +1744,7 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 	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;
+	     order >= PAGE_ALLOC_COSTLY_ORDER && !found_block && nr_scanned < limit;
 	     order--) {
 		struct free_area *area = &cc->zone->free_area[order];
 		struct list_head *freelist;
@@ -1758,7 +1759,11 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 		list_for_each_entry(freepage, freelist, lru) {
 			unsigned long free_pfn;
 
-			nr_scanned++;
+			if (nr_scanned++ >= limit) {
+				move_freelist_tail(freelist, freepage);
+				break;
+			}
+
 			free_pfn = page_to_pfn(freepage);
 			if (free_pfn < high_pfn) {
 				/*
@@ -1767,12 +1772,8 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 				 * the list assumes an entry is deleted, not
 				 * reordered.
 				 */
-				if (get_pageblock_skip(freepage)) {
-					if (list_is_last(freelist, &freepage->lru))
-						break;
-
+				if (get_pageblock_skip(freepage))
 					continue;
-				}
 
 				/* Reorder to so a future search skips recent pages */
 				move_freelist_tail(freelist, freepage);
@@ -1780,15 +1781,10 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 				update_fast_start_pfn(cc, free_pfn);
 				pfn = pageblock_start_pfn(free_pfn);
 				cc->fast_search_fail = 0;
+				found_block = true;
 				set_pageblock_skip(freepage);
 				break;
 			}
-
-			if (nr_scanned >= limit) {
-				cc->fast_search_fail++;
-				move_freelist_tail(freelist, freepage);
-				break;
-			}
 		}
 		spin_unlock_irqrestore(&cc->zone->lock, flags);
 	}
@@ -1799,9 +1795,10 @@ 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 (!found_block) {
+		cc->fast_search_fail++;
 		pfn = reinit_migrate_pfn(cc);
-
+	}
 	return pfn;
 }
 
-- 
2.25.1