linux-kernel - [PATCH v1 01/18] mm: allow for detecting underflows with page

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Message-ID: <20240409192301.907377-2-david@redhat.com>
Date: Tue,  9 Apr 2024 21:22:44 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	linux-doc@...r.kernel.org,
	cgroups@...r.kernel.org,
	linux-sh@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Peter Xu <peterx@...hat.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Yin Fengwei <fengwei.yin@...el.com>,
	Yang Shi <shy828301@...il.com>,
	Zi Yan <ziy@...dia.com>,
	Jonathan Corbet <corbet@....net>,
	Hugh Dickins <hughd@...gle.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Rich Felker <dalias@...c.org>,
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Naoya Horiguchi <naoya.horiguchi@....com>,
	Richard Chang <richardycc@...gle.com>
Subject: [PATCH v1 01/18] mm: allow for detecting underflows with page_mapcount() again

Commit 53277bcf126d ("mm: support page_mapcount() on page_has_type()
pages") made it impossible to detect mapcount underflows by treating
any negative raw mapcount value as a mapcount of 0.

We perform such underflow checks in zap_present_folio_ptes() and
zap_huge_pmd(), which would currently no longer trigger.

Let's check against PAGE_MAPCOUNT_RESERVE instead by using
page_type_has_type(), like page_has_type() would, so we can still catch
some underflows.

Fixes: 53277bcf126d ("mm: support page_mapcount() on page_has_type() pages")
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 include/linux/mm.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef34cf54c14f..0fb8a40f82dd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1229,11 +1229,10 @@ static inline void page_mapcount_reset(struct page *page)
  */
 static inline int page_mapcount(struct page *page)
 {
-	int mapcount = atomic_read(&page->_mapcount) + 1;
+	int mapcount = atomic_read(&page->_mapcount);
 
 	/* Handle page_has_type() pages */
-	if (mapcount < 0)
-		mapcount = 0;
+	mapcount = page_type_has_type(mapcount) ? 0 : mapcount + 1;
 	if (unlikely(PageCompound(page)))
 		mapcount += folio_entire_mapcount(page_folio(page));
 
-- 
2.44.0