linux-kernel - [PATCH v2 1/4] mm: don't special case !MMU for is_zero_pfn() and my_zero

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Message-ID: <20260209144058.2092871-2-rppt@kernel.org>
Date: Mon,  9 Feb 2026 16:40:54 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andreas Larsson <andreas@...sler.com>,
	Borislav Petkov <bp@...en8.de>,
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Subject: [PATCH v2 1/4] mm: don't special case !MMU for is_zero_pfn() and my_zero_pfn()

From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>

nommu architectures have empty_zero_page and define ZERO_PAGE() and
although they don't really use it to populate page tables, there is no
reason to hardwire !MMU implementation of is_zero_pfn() and my_zero_pfn()
to 0.

Drop #ifdef CONFIG_MMU around implementations of is_zero_pfn() and
my_zero_pfn() and remove !MMU version.

While on it, make zero_pfn __ro_after_init.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
---
 include/linux/pgtable.h | 14 +-------------
 mm/memory.c             | 13 -------------
 mm/mm_init.c            | 10 ++++++++++
 3 files changed, 11 insertions(+), 26 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 827dca25c0bc..08a88b0d56e5 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1882,7 +1882,6 @@ static inline void pfnmap_setup_cachemode_pfn(unsigned long pfn, pgprot_t *prot)
 	pfnmap_setup_cachemode(pfn, PAGE_SIZE, prot);
 }
 
-#ifdef CONFIG_MMU
 #ifdef __HAVE_COLOR_ZERO_PAGE
 static inline int is_zero_pfn(unsigned long pfn)
 {
@@ -1905,18 +1904,7 @@ static inline unsigned long my_zero_pfn(unsigned long addr)
 	extern unsigned long zero_pfn;
 	return zero_pfn;
 }
-#endif
-#else
-static inline int is_zero_pfn(unsigned long pfn)
-{
-	return 0;
-}
-
-static inline unsigned long my_zero_pfn(unsigned long addr)
-{
-	return 0;
-}
-#endif /* CONFIG_MMU */
+#endif /* __HAVE_COLOR_ZERO_PAGE */
 
 #ifdef CONFIG_MMU
 
diff --git a/mm/memory.c b/mm/memory.c
index 187f16b7e996..51d2018a387a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -162,21 +162,8 @@ static int __init disable_randmaps(char *s)
 }
 __setup("norandmaps", disable_randmaps);
 
-unsigned long zero_pfn __read_mostly;
-EXPORT_SYMBOL(zero_pfn);
-
 unsigned long highest_memmap_pfn __read_mostly;
 
-/*
- * CONFIG_MMU architectures set up ZERO_PAGE in their paging_init()
- */
-static int __init init_zero_pfn(void)
-{
-	zero_pfn = page_to_pfn(ZERO_PAGE(0));
-	return 0;
-}
-early_initcall(init_zero_pfn);
-
 void mm_trace_rss_stat(struct mm_struct *mm, int member)
 {
 	trace_rss_stat(mm, member);
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 1a29a719af58..dcf9eff34f83 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -53,6 +53,9 @@ EXPORT_SYMBOL(mem_map);
 void *high_memory;
 EXPORT_SYMBOL(high_memory);
 
+unsigned long zero_pfn __ro_after_init;
+EXPORT_SYMBOL(zero_pfn);
+
 #ifdef CONFIG_DEBUG_MEMORY_INIT
 int __meminitdata mminit_loglevel;
 
@@ -2667,6 +2670,13 @@ static void __init mem_init_print_info(void)
 		);
 }
 
+static int __init init_zero_pfn(void)
+{
+	zero_pfn = page_to_pfn(ZERO_PAGE(0));
+	return 0;
+}
+early_initcall(init_zero_pfn);
+
 void __init __weak arch_mm_preinit(void)
 {
 }
-- 
2.51.0