[PATCH v2] mm/page_isolation: do not isolate the max order page

[Muchun Song]

The max order page has no buddy page and never merge to other order.
So isolating and then freeing it is pointless. And if order == MAX_ORDER
- 1, then the buddy can actually be a !pfn_valid() in some corner case?
pfn_valid_within(buddy_pfn) that follows would only catch it on archs
with holes in zone. Then is_migrate_isolate_page(buddy) might access an
invalid buddy. So this is also a bug fix.

Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
Changes in v2:
 - Add Fixes tag in the commit log.

 mm/page_isolation.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index a254e1f370a3..bddf788f45bf 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	 */
 	if (PageBuddy(page)) {
 		order = buddy_order(page);
-		if (order >= pageblock_order) {
+		if (order >= pageblock_order && order < MAX_ORDER - 1) {
 			pfn = page_to_pfn(page);
 			buddy_pfn = __find_buddy_pfn(pfn, order);
 			buddy = page + (buddy_pfn - pfn);
-- 
2.11.0