linux-kernel - [PATCH v3 1/4] mm/memory: introduce is_huge_zero_pfn() and use it in vm_normal_page

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Message-ID: <4940826e99f0c709a7cf7beb94f53288320aea5a.1752499009.git.luizcap@redhat.com>
Date: Mon, 14 Jul 2025 09:16:51 -0400
From: Luiz Capitulino <luizcap@...hat.com>
To: david@...hat.com,
	willy@...radead.org,
	akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	shivankg@....com,
	sj@...nel.org,
	harry.yoo@...cle.com
Subject: [PATCH v3 1/4] mm/memory: introduce is_huge_zero_pfn() and use it in vm_normal_page_pmd()

From: David Hildenbrand <david@...hat.com>

Let's avoid working with the PMD when not required. If
vm_normal_page_pmd() would be called on something that is not a present
pmd, it would already be a bug (pfn possibly garbage).

While at it, let's support passing in any pfn covered by the huge zero
folio by masking off PFN bits -- which should be rather cheap.

Signed-off-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Oscar Salvador <osalvador@...e.de>
Signed-off-by: Luiz Capitulino <luizcap@...hat.com>
---
 include/linux/huge_mm.h | 12 +++++++++++-
 mm/memory.c             |  2 +-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2f190c90192d..59e93fba15f4 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -486,9 +486,14 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
 	return READ_ONCE(huge_zero_folio) == folio;
 }
 
+static inline bool is_huge_zero_pfn(unsigned long pfn)
+{
+	return READ_ONCE(huge_zero_pfn) == (pfn & ~(HPAGE_PMD_NR - 1));
+}
+
 static inline bool is_huge_zero_pmd(pmd_t pmd)
 {
-	return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
+	return pmd_present(pmd) && is_huge_zero_pfn(pmd_pfn(pmd));
 }
 
 struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
@@ -636,6 +641,11 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
 	return false;
 }
 
+static inline bool is_huge_zero_pfn(unsigned long pfn)
+{
+	return false;
+}
+
 static inline bool is_huge_zero_pmd(pmd_t pmd)
 {
 	return false;
diff --git a/mm/memory.c b/mm/memory.c
index b0cda5aab398..3a765553bacb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -687,7 +687,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
 
 	if (pmd_devmap(pmd))
 		return NULL;
-	if (is_huge_zero_pmd(pmd))
+	if (is_huge_zero_pfn(pfn))
 		return NULL;
 	if (unlikely(pfn > highest_memmap_pfn))
 		return NULL;
-- 
2.50.0