[PATCH 05/10] mm: page_alloc: fix move_freepages_block() range error

[Johannes Weiner]

When a block is partially outside the zone of the cursor page, the
function cuts the range to the pivot page instead of the zone
start. This can leave large parts of the block behind, which
encourages incompatible page mixing down the line (ask for one type,
get another), and thus long-term fragmentation.

This triggers reliably on the first block in the DMA zone, whose
start_pfn is 1. The block is stolen, but everything before the pivot
page (which was often hundreds of pages) is left on the old list.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/page_alloc.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a1376a6fe7e4..7373329763e6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1645,9 +1645,15 @@ int move_freepages_block(struct zone *zone, struct page *page,
 	start_pfn = pageblock_start_pfn(pfn);
 	end_pfn = pageblock_end_pfn(pfn) - 1;
 
-	/* Do not cross zone boundaries */
+	/*
+	 * The caller only has the lock for @zone, don't touch ranges
+	 * that straddle into other zones. While we could move part of
+	 * the range that's inside the zone, this call is usually
+	 * accompanied by other operations such as migratetype updates
+	 * which also should be locked.
+	 */
 	if (!zone_spans_pfn(zone, start_pfn))
-		start_pfn = pfn;
+		return 0;
 	if (!zone_spans_pfn(zone, end_pfn))
 		return 0;
 
-- 
2.44.0