linux-kernel - [PATCH v1 07/18] mm/memory: use folio_mapcount() in zap_present_folio

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Date: Tue,  9 Apr 2024 21:22:50 +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 07/18] mm/memory: use folio_mapcount() in zap_present_folio_ptes()

We want to limit the use of page_mapcount() to the places where it is
absolutely necessary. In zap_present_folio_ptes(), let's simply check
the folio mapcount(). If there is some issue, it will underflow at some
point either way when unmapping.

As indicated already in commit 10ebac4f95e7 ("mm/memory: optimize unmap/zap
with PTE-mapped THP"), we already documented "If we ever have a cheap
folio_mapcount(), we might just want to check for underflows there.".

There is no change for small folios. For large folios, we'll now catch
more underflows when batch-unmapping, because instead of only testing
the mapcount of the first subpage, we'll test if the folio mapcount
underflows.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/memory.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 78422d1c7381..178492efb4af 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1502,8 +1502,7 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
 	if (!delay_rmap) {
 		folio_remove_rmap_ptes(folio, page, nr, vma);
 
-		/* Only sanity-check the first page in a batch. */
-		if (unlikely(page_mapcount(page) < 0))
+		if (unlikely(folio_mapcount(folio) < 0))
 			print_bad_pte(vma, addr, ptent, page);
 	}
 
-- 
2.44.0