linux-kernel - [PATCH v2 2/2] mm/debug: Fix parameter passed to page_mapcount_is

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Message-ID: <20250321120222.1456770-3-gshan@redhat.com>
Date: Fri, 21 Mar 2025 22:02:22 +1000
From: Gavin Shan <gshan@...hat.com>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	willy@...radead.org,
	david@...hat.com,
	vbabka@...e.cz,
	linmiaohe@...wei.com,
	gehao@...inos.cn,
	shan.gavin@...il.com
Subject: [PATCH v2 2/2] mm/debug: Fix parameter passed to page_mapcount_is_type()

As the comments of page_mapcount_is_type() indicate, the parameter
passed to the function should be one more than page->_mapcount.
However, page->_mapcount is passed to the function by commit 4ffca5a96678
("mm: support only one page_type per page") where page_type_has_type()
is replaced by page_mapcount_is_type(), but the parameter isn't adjusted.

Fix the parameter for page_mapcount_is_type() to be (page->__mapcount
+ 1). Note that the issue doesn't cause any visible impacts due to the
safety gap introduced by PGTY_mapcount_underflow limit.

Fixes: 4ffca5a96678 ("mm: support only one page_type per page")
Signed-off-by: Gavin Shan <gshan@...hat.com>
Acked-by: David Hildenbrand <david@...hat.com>
---
 mm/debug.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index 8d2acf432385..b6bd9555ec7b 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -71,10 +71,10 @@ static void __dump_folio(struct folio *folio, struct page *page,
 		unsigned long pfn, unsigned long idx)
 {
 	struct address_space *mapping = folio_mapping(folio);
-	int mapcount = atomic_read(&page->_mapcount);
+	int mapcount = atomic_read(&page->_mapcount) + 1;
 	char *type = "";
 
-	mapcount = page_mapcount_is_type(mapcount) ? 0 : mapcount + 1;
+	mapcount = page_mapcount_is_type(mapcount) ? 0 : mapcount;
 	pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
 			folio_ref_count(folio), mapcount, mapping,
 			folio->index + idx, pfn);
-- 
2.48.1