[PATCH V3 4/4] arm64/mm: Use pgdp_get() for PGD accesses

From: Anshuman Khandual <hidden>
Date: 2026-09-24 06:23:03
Also in: linux-mm, lkml
Subsystem: arm64 port (aarch64 architecture), kasan, the rest · Maintainers: Catalin Marinas, Will Deacon, Andrey Ryabinin, Linus Torvalds

Replace READ_ONCE() with pgdp_get() for PGD accesses in preparation for
supporting both D64 and D128 translation table formats.

READ_ONCE() cannot currently be used for 128-bit page table entries on
arm64 because it does not provide the required 128-bit single-copy
atomicity, causing builds to fail for accesses wider than 64 bits.

Although LDP/STP provide the required atomicity when FEAT_LSE is
available (as required by FEAT_D128), extending READ_ONCE() to support
128-bit accesses is undesirable. READ_ONCE() is a general-purpose API,
so doing so could encourage other 128-bit users that would either fail
to build in configurations without D128 support or, if D128 becomes a
runtime option, silently permit tearing on systems without the required
hardware support.

Instead, standardize PGD accesses on the existing page-table helpers.
These can be overridden on arm64 to provide 128-bit single-copy
atomicity when required.

For now override pgdp_get() which is same as the generic memory default
to avoid build dependency problem for callers in header <asm/pgtable.h>.

No functional change intended.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: kasan-dev@googlegroups.com
Signed-off-by: Anshuman Khandual <redacted>
---
Changes in V3

- Added platform override for pgdp_get()
- Replaced READ_ONCE() in p4d_offset_phys() and p4d_offset()

 arch/arm64/include/asm/pgtable.h | 10 ++++++++--
 arch/arm64/mm/fault.c            |  2 +-
 arch/arm64/mm/hugetlbpage.c      |  2 +-
 arch/arm64/mm/kasan_init.c       |  6 +++---
 arch/arm64/mm/mmu.c              |  6 +++---
 arch/arm64/mm/pageattr.c         |  2 +-
 arch/arm64/mm/trans_pgd.c        |  4 ++--
 7 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index f235f68a7518..05d273f276b8 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h

@@ -102,6 +102,12 @@ static inline p4d_t p4dp_get(p4d_t *p4dp)
 	return READ_ONCE(*p4dp);
 }
 
+#define pgdp_get pgdp_get
+static inline pgd_t pgdp_get(pgd_t *pgdp)
+{
+	return READ_ONCE(*pgdp);
+}
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE

@@ -1102,7 +1108,7 @@ static inline phys_addr_t p4d_offset_phys(pgd_t *pgdp, unsigned long addr)
 {
 	VM_WARN_ON_ONCE(!pgtable_l5_enabled());
 
-	return pgd_page_paddr(READ_ONCE(*pgdp)) + p4d_index(addr) * sizeof(p4d_t);
+	return pgd_page_paddr(pgdp_get(pgdp)) + p4d_index(addr) * sizeof(p4d_t);
 }
 
 static inline

@@ -1116,7 +1122,7 @@ p4d_t *p4d_offset_lockless(pgd_t *pgdp, pgd_t pgd, unsigned long addr)
 
 static inline p4d_t *p4d_offset(pgd_t *pgdp, unsigned long addr)
 {
-	return p4d_offset_lockless(pgdp, READ_ONCE(*pgdp), addr);
+	return p4d_offset_lockless(pgdp, pgdp_get(pgdp), addr);
 }
 
 static inline p4d_t *p4d_set_fixmap(unsigned long addr)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index a6afd8929a10..43f25418275a 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c

@@ -160,7 +160,7 @@ static void show_pte(unsigned long addr)
 	guard(irqsave)();
 
 	pgdp = pgd_offset(mm, addr);
-	pgd = READ_ONCE(*pgdp);
+	pgd = pgdp_get(pgdp);
 	ptval_to_str(pxd_str, pgd_val(pgd));
 	pr_alert("[%016lx] pgd=%s", addr, pxd_str);

diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index 5771bf49e1fe..e05e5b41ae50 100644
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c

@@ -284,7 +284,7 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
 	pmd_t *pmdp, pmd;
 
 	pgdp = pgd_offset(mm, addr);
-	if (!pgd_present(READ_ONCE(*pgdp)))
+	if (!pgd_present(pgdp_get(pgdp)))
 		return NULL;
 
 	p4dp = p4d_offset(pgdp, addr);

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index 4dd8c1186471..f61f5bc09467 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c

@@ -102,7 +102,7 @@ static pud_t *__init kasan_pud_offset(p4d_t *p4dp, unsigned long addr, int node,
 static p4d_t *__init kasan_p4d_offset(pgd_t *pgdp, unsigned long addr, int node,
 				      bool early)
 {
-	if (pgd_none(READ_ONCE(*pgdp))) {
+	if (pgd_none(pgdp_get(pgdp))) {
 		phys_addr_t p4d_phys = early ?
 				__pa_symbol(kasan_early_shadow_p4d)
 					: kasan_alloc_zeroed_page(node);

@@ -256,7 +256,7 @@ static int __init root_level_idx(u64 addr)
 static void __init clone_next_level(u64 addr, pgd_t *tmp_pg_dir, pud_t *pud)
 {
 	int idx = root_level_idx(addr);
-	pgd_t pgd = READ_ONCE(swapper_pg_dir[idx]);
+	pgd_t pgd = pgdp_get(swapper_pg_dir + idx);
 	pud_t *pudp = (pud_t *)__phys_to_kimg(__pgd_to_phys(pgd));
 
 	memcpy(pud, pudp, PAGE_SIZE);

@@ -280,7 +280,7 @@ static int __init next_level_idx(u64 addr)
  */
 static void __init clear_next_level(int pgd_idx, int start, int end)
 {
-	pgd_t pgd = READ_ONCE(swapper_pg_dir[pgd_idx]);
+	pgd_t pgd = pgdp_get(swapper_pg_dir + pgd_idx);
 	pud_t *pudp = (pud_t *)__phys_to_kimg(__pgd_to_phys(pgd));
 
 	memset(&pudp[start], 0, (end - start) * sizeof(pud_t));

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 83709d987e7c..76d8d320aeed 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -434,7 +434,7 @@ static int alloc_init_p4d(pgd_t *pgdp, unsigned long addr, unsigned long end,
 {
 	int ret;
 	unsigned long next;
-	pgd_t pgd = READ_ONCE(*pgdp);
+	pgd_t pgd = pgdp_get(pgdp);
 	p4d_t *p4dp;
 
 	if (pgd_none(pgd)) {

@@ -1649,7 +1649,7 @@ static void unmap_hotplug_range(unsigned long addr, unsigned long end,
 	do {
 		next = pgd_addr_end(addr, end);
 		pgdp = pgd_offset_k(addr);
-		pgd = READ_ONCE(*pgdp);
+		pgd = pgdp_get(pgdp);
 		if (pgd_none(pgd))
 			continue;

@@ -1827,7 +1827,7 @@ static void free_empty_tables(unsigned long addr, unsigned long end,
 	do {
 		next = pgd_addr_end(addr, end);
 		pgdp = pgd_offset_k(addr);
-		pgd = READ_ONCE(*pgdp);
+		pgd = pgdp_get(pgdp);
 		if (pgd_none(pgd))
 			continue;

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 826856ef64a8..dfa1356daac2 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c

@@ -399,7 +399,7 @@ bool kernel_page_present(struct page *page)
 	unsigned long addr = (unsigned long)page_address(page);
 
 	pgdp = pgd_offset_k(addr);
-	if (pgd_none(READ_ONCE(*pgdp)))
+	if (pgd_none(pgdp_get(pgdp)))
 		return false;
 
 	p4dp = p4d_offset(pgdp, addr);

diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
index 7afe2beca4ba..06470d690f9f 100644
--- a/arch/arm64/mm/trans_pgd.c
+++ b/arch/arm64/mm/trans_pgd.c

@@ -134,7 +134,7 @@ static int copy_p4d(struct trans_pgd_info *info, pgd_t *dst_pgdp,
 	unsigned long next;
 	unsigned long addr = start;
 
-	if (pgd_none(READ_ONCE(*dst_pgdp))) {
+	if (pgd_none(pgdp_get(dst_pgdp))) {
 		dst_p4dp = trans_alloc(info);
 		if (!dst_p4dp)
 			return -ENOMEM;

@@ -164,7 +164,7 @@ static int copy_page_tables(struct trans_pgd_info *info, pgd_t *dst_pgdp,
 	dst_pgdp = pgd_offset_pgd(dst_pgdp, start);
 	do {
 		next = pgd_addr_end(addr, end);
-		if (pgd_none(READ_ONCE(*src_pgdp)))
+		if (pgd_none(pgdp_get(src_pgdp)))
 			continue;
 		if (copy_p4d(info, dst_pgdp, src_pgdp, addr, next))
 			return -ENOMEM;

-- 
2.43.0

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