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

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Message-Id: <20201203162237.21885-1-songmuchun@bytedance.com>
Date:   Fri,  4 Dec 2020 00:22:37 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH v2] mm/page_isolation: do not isolate the max order page

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@...edance.com>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
---
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