[PATCH v2 4/4] mm: cache struct page for empty_zero_page and return it from ZERO_PAGE()

Mike Rapoport
2026-02-09T14:42:04Z

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

For most architectures every invocation of ZERO_PAGE() does
virt_to_page(empty_zero_page). But empty_zero_page is in BSS and it is
enough to get its struct page once at initialization time and then use
it whenever a zero page should be accessed.

Add yet another __zero_page variable that will be initialized as
virt_to_page(empty_zero_page) for most architectures in a weak
arch_setup_zero_pages() function.

For architectures that use colored zero pages (MIPS and s390) rename their
setup_zero_pages() to arch_setup_zero_pages() and make it global rather
than static.

For architectures that cannot use virt_to_page() for BSS (arm64 and
sparc64) add override of arch_setup_zero_pages().

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
arch/arm64/include/asm/pgtable.h | 6 ------
arch/arm64/mm/init.c | 5 +++++
arch/mips/mm/init.c | 11 +----------
arch/s390/mm/init.c | 4 +---
arch/sparc/include/asm/pgtable_64.h | 3 ---
arch/sparc/mm/init_64.c | 17 +++++++----------
include/linux/pgtable.h | 11 ++++++++---
mm/mm_init.c | 21 +++++++++++++++++----
8 files changed, 39 insertions(+), 39 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 63da07398a30..2c1ec7cc8612 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -106,12 +106,6 @@ static inline void arch_leave_lazy_mmu_mode(void)
#define flush_tlb_fix_spurious_fault_pmd(vma, address, pmdp) \
local_flush_tlb_page_nonotify(vma, address)

-/*
- * ZERO_PAGE is a global shared page that is always zero: used
- * for zero-mapped memory areas etc..
- */
-#define ZERO_PAGE(vaddr) phys_to_page(__pa_symbol(empty_zero_page))
-
#define pte_ERROR(e) \
pr_err("%s:%d: bad pte %016llx.\n", __FILE__, __LINE__, pte_val(e))

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 96711b8578fd..417ec7efe569 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -328,6 +328,11 @@ void __init bootmem_init(void)
memblock_dump_all();
}

+void __init arch_setup_zero_pages(void)
+{
+ __zero_page = phys_to_page(__pa_symbol(empty_zero_page));
+}
+
void __init arch_mm_preinit(void)
{
unsigned int flags = SWIOTLB_VERBOSE;
diff --git a/arch/mips/mm/init.c b/arch/mips/mm/init.c
index 4f6449ad02ca..55b25e85122a 100644
--- a/arch/mips/mm/init.c
+++ b/arch/mips/mm/init.c
@@ -56,10 +56,7 @@ unsigned long empty_zero_page, zero_page_mask;
EXPORT_SYMBOL_GPL(empty_zero_page);
EXPORT_SYMBOL(zero_page_mask);

-/*
- * Not static inline because used by IP27 special magic initialization code
- */
-static void __init setup_zero_pages(void)
+void __init arch_setup_zero_pages(void)
{
unsigned int order;

@@ -450,7 +447,6 @@ void __init arch_mm_preinit(void)
BUILD_BUG_ON(IS_ENABLED(CONFIG_32BIT) && (PFN_PTE_SHIFT > PAGE_SHIFT));

maar_init();
- setup_zero_pages(); /* Setup zeroed pages. */
highmem_init();

#ifdef CONFIG_64BIT
@@ -461,11 +457,6 @@ void __init arch_mm_preinit(void)
0x80000000 - 4, KCORE_TEXT);
#endif
}
-#else /* CONFIG_NUMA */
-void __init arch_mm_preinit(void)
-{
- setup_zero_pages(); /* This comes from node 0 */
-}
#endif /* !CONFIG_NUMA */

void free_init_pages(const char *what, unsigned long begin, unsigned long end)
diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
index 3c20475cbee2..1f72efc2a579 100644
--- a/arch/s390/mm/init.c
+++ b/arch/s390/mm/init.c
@@ -69,7 +69,7 @@ unsigned long empty_zero_page, zero_page_mask;
EXPORT_SYMBOL(empty_zero_page);
EXPORT_SYMBOL(zero_page_mask);

-static void __init setup_zero_pages(void)
+void __init arch_setup_zero_pages(void)
{
unsigned long total_pages = memblock_estimated_nr_free_pages();
unsigned int order;
@@ -159,8 +159,6 @@ void __init arch_mm_preinit(void)
cpumask_set_cpu(0, mm_cpumask(&init_mm));

pv_init();
-
- setup_zero_pages(); /* Setup zeroed pages. */
}

unsigned long memory_block_size_bytes(void)
diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
index 615f460c50af..74ede706fb32 100644
--- a/arch/sparc/include/asm/pgtable_64.h
+++ b/arch/sparc/include/asm/pgtable_64.h
@@ -210,9 +210,6 @@ extern unsigned long _PAGE_CACHE;
extern unsigned long pg_iobits;
extern unsigned long _PAGE_ALL_SZ_BITS;

-extern struct page *mem_map_zero;
-#define ZERO_PAGE(vaddr) (mem_map_zero)
-
/* PFNs are real physical page numbers. However, mem_map only begins to record
* per-page information starting at pfn_base. This is to handle systems where
* the first physical page in the machine is at some huge physical address,
diff --git a/arch/sparc/mm/init_64.c b/arch/sparc/mm/init_64.c
index 0cc8de2fea90..707c1df67d79 100644
--- a/arch/sparc/mm/init_64.c
+++ b/arch/sparc/mm/init_64.c
@@ -177,9 +177,6 @@ extern unsigned long sparc_ramdisk_image64;
extern unsigned int sparc_ramdisk_image;
extern unsigned int sparc_ramdisk_size;

-struct page *mem_map_zero __read_mostly;
-EXPORT_SYMBOL(mem_map_zero);
-
unsigned int sparc64_highest_unlocked_tlb_ent __read_mostly;

unsigned long sparc64_kern_pri_context __read_mostly;
@@ -2496,11 +2493,17 @@ static void __init register_page_bootmem_info(void)
register_page_bootmem_info_node(NODE_DATA(i));
#endif
}
-void __init mem_init(void)
+
+void __init arch_setup_zero_pages(void)
{
phys_addr_t zero_page_pa = kern_base +
((unsigned long)&empty_zero_page[0] - KERNBASE);

+ __zero_page = phys_to_page(zero_page_pa);
+}
+
+void __init mem_init(void)
+{
/*
* Must be done after boot memory is put on freelist, because here we
* might set fields in deferred struct pages that have not yet been
@@ -2509,12 +2512,6 @@ void __init mem_init(void)
*/
register_page_bootmem_info();

- /*
- * Set up the zero page, mark it reserved, so that page count
- * is not manipulated when freeing the page from user ptes.
- */
- mem_map_zero = pfn_to_page(PHYS_PFN(zero_page_pa));
-
if (tlb_type == cheetah || tlb_type == cheetah_plus)
cheetah_ecache_flush_init();
}
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 9ba1f03fca54..722df2149d58 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1894,6 +1894,8 @@ static inline void pfnmap_setup_cachemode_pfn(unsigned long pfn, pgprot_t *prot)
* For architectures that don't __HAVE_COLOR_ZERO_PAGE the zero page lives in
* empty_zero_page in BSS.
*/
+void arch_setup_zero_pages(void);
+
extern unsigned long zero_page_pfn;

#ifdef __HAVE_COLOR_ZERO_PAGE
@@ -1918,10 +1920,13 @@ static inline unsigned long zero_pfn(unsigned long addr)
}

extern uint8_t empty_zero_page[PAGE_SIZE];
+extern struct page *__zero_page;

-#ifndef ZERO_PAGE
-#define ZERO_PAGE(vaddr) ((void)(vaddr),virt_to_page(empty_zero_page))
-#endif
+static inline struct page *_zero_page(unsigned long addr)
+{
+ return __zero_page;
+}
+#define ZERO_PAGE(vaddr) _zero_page(vaddr)

#endif /* __HAVE_COLOR_ZERO_PAGE */

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 1eac634ece1a..b08608c1b71d 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -59,7 +59,10 @@ EXPORT_SYMBOL(zero_page_pfn);
#ifndef __HAVE_COLOR_ZERO_PAGE
uint8_t empty_zero_page[PAGE_SIZE] __page_aligned_bss;
EXPORT_SYMBOL(empty_zero_page);
-#endif
+
+struct page *__zero_page __ro_after_init;
+EXPORT_SYMBOL(__zero_page);
+#endif /* __HAVE_COLOR_ZERO_PAGE */

#ifdef CONFIG_DEBUG_MEMORY_INIT
int __meminitdata mminit_loglevel;
@@ -2675,12 +2678,21 @@ static void __init mem_init_print_info(void)
);
}

-static int __init init_zero_page_pfn(void)
+#ifndef __HAVE_COLOR_ZERO_PAGE
+/*
+ * architectures that __HAVE_COLOR_ZERO_PAGE must define this function
+ */
+void __init __weak arch_setup_zero_pages(void)
+{
+ __zero_page = virt_to_page(empty_zero_page);
+}
+#endif
+
+static void __init init_zero_page_pfn(void)
{
+ arch_setup_zero_pages();
zero_page_pfn = page_to_pfn(ZERO_PAGE(0));
- return 0;
}
-early_initcall(init_zero_page_pfn);

void __init __weak arch_mm_preinit(void)
{
@@ -2704,6 +2716,7 @@ void __init mm_core_init_early(void)
void __init mm_core_init(void)
{
arch_mm_preinit();
+ init_zero_page_pfn();

/* Initializations relying on SMP setup */
BUILD_BUG_ON(MAX_ZONELISTS > 2);
--
2.51.0