[PATCH V2 1/4] arm64/mm: Use pmdp_get() for PMD accesses
Ryan Roberts
ryan.roberts at arm.com
Wed Sep 23 01:32:17 PDT 2026
On 22/09/2026 07:16, Anshuman Khandual wrote:
> Replace READ_ONCE() with pmdp_get() for PMD 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 PMD accesses on the existing page-table helpers.
> These can be overridden on arm64 to provide 128-bit single-copy
> atomicity when required. No functional change intended.
I notice you have an unconverted READ_ONCE(*pmdp) in
pmdp_test_and_clear_young(). Is that intentional?
Thanks,
Ryan
>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Ryan Roberts <ryan.roberts at arm.com>
> Cc: Mark Rtland <mark.rtland at arm.com>
> Cc: linx-arm-kernel at lists.infradead.org
> Cc: linx-kernel at vger.kernel.org
> Cc: kasan-dev at googlegrops.com
> Reviewed-by: Ryan Roberts <ryan.roberts at arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
> ---
> arch/arm64/include/asm/pgtable.h | 3 ++-
> arch/arm64/mm/fault.c | 2 +-
> arch/arm64/mm/fixmap.c | 2 +-
> arch/arm64/mm/hugetlbpage.c | 2 +-
> arch/arm64/mm/kasan_init.c | 4 ++--
> arch/arm64/mm/mmu.c | 22 +++++++++++-----------
> arch/arm64/mm/pageattr.c | 2 +-
> arch/arm64/mm/trans_pgd.c | 2 +-
> 8 files changed, 20 insertions(+), 19 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 763c5a411d64..b953ea0ba3ed 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -811,7 +811,8 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
> }
>
> /* Find an entry in the third-level page table. */
> -#define pte_offset_phys(dir,addr) (pmd_page_paddr(READ_ONCE(*(dir))) + pte_index(addr) * sizeof(pte_t))
> +#define pte_offset_phys(dir, addr) (pmd_page_paddr(pmdp_get(dir)) + \
> + pte_index(addr) * sizeof(pte_t))
>
> #define pte_set_fixmap(addr) ((pte_t *)set_fixmap_offset(FIX_PTE, addr))
> #define pte_set_fixmap_offset(pmd, addr) pte_set_fixmap(pte_offset_phys(pmd, addr))
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 2cecf6ba6df7..2757ee0c4300 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -188,7 +188,7 @@ static void show_pte(unsigned long addr)
> break;
>
> pmdp = pmd_offset_lockless(pudp, pud, addr);
> - pmd = READ_ONCE(*pmdp);
> + pmd = pmdp_get(pmdp);
> ptval_to_str(pxd_str, pmd_val(pmd));
> pr_cont(", pmd=%s", pxd_str);
> if (pmd_none(pmd) || pmd_bad(pmd))
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index f66a0016dd02..3cdac8021d4f 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -42,7 +42,7 @@ static inline pte_t *fixmap_pte(unsigned long addr)
>
> static void __init early_fixmap_init_pte(pmd_t *pmdp, unsigned long addr)
> {
> - pmd_t pmd = READ_ONCE(*pmdp);
> + pmd_t pmd = pmdp_get(pmdp);
> pte_t *ptep;
>
> if (pmd_none(pmd)) {
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 8e799c1fe0aa..cdaa4500faf9 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -304,7 +304,7 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> addr &= CONT_PMD_MASK;
>
> pmdp = pmd_offset(pudp, addr);
> - pmd = READ_ONCE(*pmdp);
> + pmd = pmdp_get(pmdp);
> if (!(sz == PMD_SIZE || sz == CONT_PMD_SIZE) &&
> pmd_none(pmd))
> return NULL;
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index 45fbdce684c8..7ca833c5de5e 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -62,7 +62,7 @@ static phys_addr_t __init kasan_alloc_raw_page(int node)
> static pte_t *__init kasan_pte_offset(pmd_t *pmdp, unsigned long addr, int node,
> bool early)
> {
> - if (pmd_none(READ_ONCE(*pmdp))) {
> + if (pmd_none(pmdp_get(pmdp))) {
> phys_addr_t pte_phys = early ?
> __pa_symbol(kasan_early_shadow_pte)
> : kasan_alloc_zeroed_page(node);
> @@ -138,7 +138,7 @@ static void __init kasan_pmd_populate(pud_t *pudp, unsigned long addr,
> do {
> next = pmd_addr_end(addr, end);
> kasan_pte_populate(pmdp, addr, next, node, early);
> - } while (pmdp++, addr = next, addr != end && pmd_none(READ_ONCE(*pmdp)));
> + } while (pmdp++, addr = next, addr != end && pmd_none(pmdp_get(pmdp)));
> }
>
> static void __init kasan_pud_populate(p4d_t *p4dp, unsigned long addr,
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index e589fb00d1e3..95621913679e 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -200,7 +200,7 @@ static int alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
> int flags)
> {
> unsigned long next;
> - pmd_t pmd = READ_ONCE(*pmdp);
> + pmd_t pmd = pmdp_get(pmdp);
> pte_t *ptep;
>
> BUG_ON(pmd_leaf(pmd));
> @@ -257,7 +257,7 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
> unsigned long next;
>
> do {
> - pmd_t old_pmd = READ_ONCE(*pmdp);
> + pmd_t old_pmd = pmdp_get(pmdp);
>
> next = pmd_addr_end(addr, end);
>
> @@ -272,7 +272,7 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
> * only allow updates to the permission attributes.
> */
> BUG_ON(!pgattr_change_is_safe(pmd_val(old_pmd),
> - READ_ONCE(pmd_val(*pmdp))));
> + pmd_val(pmdp_get(pmdp))));
> } else {
> int ret;
>
> @@ -282,7 +282,7 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
> return ret;
>
> VM_WARN_ON_ONCE(pmd_val(old_pmd) != 0 &&
> - pmd_val(old_pmd) != READ_ONCE(pmd_val(*pmdp)));
> + pmd_val(old_pmd) != pmd_val(pmdp_get(pmdp)));
> }
> phys += next - addr;
> } while (pmdp++, addr = next, addr != end);
> @@ -293,7 +293,7 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
> static bool pmd_range_has_valid_noncont(pmd_t *pmdp)
> {
> for (int i = 0; i < CONT_PMDS; i++) {
> - pte_t pte = pmd_pte(READ_ONCE(pmdp[i]));
> + pte_t pte = pmd_pte(pmdp_get(pmdp + i));
>
> if (pte_valid(pte) && !pte_cont(pte))
> return true;
> @@ -1553,7 +1553,7 @@ static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
> do {
> next = pmd_addr_end(addr, end);
> pmdp = pmd_offset(pudp, addr);
> - pmd = READ_ONCE(*pmdp);
> + pmd = pmdp_get(pmdp);
> if (pmd_none(pmd))
> continue;
>
> @@ -1708,7 +1708,7 @@ static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
> do {
> next = pmd_addr_end(addr, end);
> pmdp = pmd_offset(pudp, addr);
> - pmd = READ_ONCE(*pmdp);
> + pmd = pmdp_get(pmdp);
> if (pmd_none(pmd))
> continue;
>
> @@ -1729,7 +1729,7 @@ static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
> */
> pmdp = pmd_offset(pudp, 0UL);
> for (i = 0; i < PTRS_PER_PMD; i++) {
> - if (!pmd_none(READ_ONCE(pmdp[i])))
> + if (!pmd_none(pmdp_get(pmdp + i)))
> return;
> }
>
> @@ -1881,7 +1881,7 @@ int pmd_set_huge(pmd_t *pmdp, phys_addr_t phys, pgprot_t prot)
> pmd_t new_pmd = pfn_pmd(__phys_to_pfn(phys), mk_pmd_sect_prot(prot));
>
> /* Only allow permission changes for now */
> - if (!pgattr_change_is_safe(READ_ONCE(pmd_val(*pmdp)),
> + if (!pgattr_change_is_safe(pmd_val(pmdp_get(pmdp)),
> pmd_val(new_pmd)))
> return 0;
>
> @@ -1906,7 +1906,7 @@ int pud_clear_huge(pud_t *pudp)
>
> int pmd_clear_huge(pmd_t *pmdp)
> {
> - if (!pmd_leaf(READ_ONCE(*pmdp)))
> + if (!pmd_leaf(pmdp_get(pmdp)))
> return 0;
> pmd_clear(pmdp);
> return 1;
> @@ -1917,7 +1917,7 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
> pte_t *table;
> pmd_t pmd;
>
> - pmd = READ_ONCE(*pmdp);
> + pmd = pmdp_get(pmdp);
>
> if (!pmd_table(pmd)) {
> VM_WARN_ON(1);
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index bbe98ac9ad8c..0ca07bd5ded9 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -414,7 +414,7 @@ bool kernel_page_present(struct page *page)
> return pud_valid(pud);
>
> pmdp = pmd_offset(pudp, addr);
> - pmd = READ_ONCE(*pmdp);
> + pmd = pmdp_get(pmdp);
> if (pmd_none(pmd))
> return false;
> if (pmd_leaf(pmd))
> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> index cca9706a875c..b27b2d2c20c3 100644
> --- a/arch/arm64/mm/trans_pgd.c
> +++ b/arch/arm64/mm/trans_pgd.c
> @@ -74,7 +74,7 @@ static int copy_pmd(struct trans_pgd_info *info, pud_t *dst_pudp,
>
> src_pmdp = pmd_offset(src_pudp, start);
> do {
> - pmd_t pmd = READ_ONCE(*src_pmdp);
> + pmd_t pmd = pmdp_get(src_pmdp);
>
> next = pmd_addr_end(addr, end);
> if (pmd_none(pmd))
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