[PATCH 3/6] mm: compaction: skip more fully scanned pageblock

[Baolin Wang]

In fast_isolate_around(), it assumes the pageblock is fully scanned if
cc->nr_freepages < cc->nr_migratepages after trying to isolate some free
pages, and will set skip flag to avoid scanning in future. However this
can miss setting the skip flag for a fully scanned pageblock (returned
'start_pfn' is equal to 'end_pfn') in the case where cc->nr_freepages
is larger than cc->nr_migratepages.

So using the returned 'start_pfn' from isolate_freepages_block() and
'end_pfn' to decide if a pageblock is fully scanned makes more sense.
It can also cover the case where cc->nr_freepages < cc->nr_migratepages,
which means the 'start_pfn' is usually equal to 'end_pfn' except some
uncommon fatal error occurs after non-strict mode isolation.

Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
 mm/compaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 3737c6591bfb..1e5183f39ca9 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1411,7 +1411,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn)
 	isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);
 
 	/* Skip this pageblock in the future as it's full or nearly full */
-	if (cc->nr_freepages < cc->nr_migratepages)
+	if (start_pfn == end_pfn)
 		set_pageblock_skip(page);
 
 	return;
-- 
2.27.0