[PATCH V2 3/4] arm64/mm: Use p4dp_get() for P4D accesses
Ryan Roberts
ryan.roberts at arm.com
Wed Sep 23 01:30:26 PDT 2026
On 22/09/2026 07:16, Anshuman Khandual wrote:
> Replace READ_ONCE() with p4dp_get() for P4D 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 P4D 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 still have uncoverted READ_ONCE(*p4dp) in:
- pud_offset_phys()
- pud_offset()
Is that intentional? If so, perhaps we need a comment explain why it's not using
the helper?
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 Rutland <mark.rutland at arm.com>
> Cc: linux-arm-kernel at lists.infradead.org
> Cc: linux-kernel at vger.kernel.org
> Cc: kasan-dev at googlegroups.com
> Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
> ---
> 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 | 12 ++++++------
> arch/arm64/mm/pageattr.c | 2 +-
> arch/arm64/mm/trans_pgd.c | 4 ++--
> 7 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 435e2e14c070..a6afd8929a10 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -174,7 +174,7 @@ static void show_pte(unsigned long addr)
> break;
>
> p4dp = p4d_offset_lockless(pgdp, pgd, addr);
> - p4d = READ_ONCE(*p4dp);
> + p4d = p4dp_get(p4dp);
> ptval_to_str(pxd_str, p4d_val(p4d));
> pr_cont(", p4d=%s", pxd_str);
> if (p4d_none(p4d) || p4d_bad(p4d))
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index d9a870836faf..2a0fe0b0f771 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -74,7 +74,7 @@ static void __init early_fixmap_init_pmd(pud_t *pudp, unsigned long addr,
> static void __init early_fixmap_init_pud(p4d_t *p4dp, unsigned long addr,
> unsigned long end)
> {
> - p4d_t p4d = READ_ONCE(*p4dp);
> + p4d_t p4d = p4dp_get(p4dp);
> pud_t *pudp;
>
> if (CONFIG_PGTABLE_LEVELS > 3 && !p4d_none(p4d) &&
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index c9ad5e75b073..5771bf49e1fe 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -288,7 +288,7 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> return NULL;
>
> p4dp = p4d_offset(pgdp, addr);
> - if (!p4d_present(READ_ONCE(*p4dp)))
> + if (!p4d_present(p4dp_get(p4dp)))
> return NULL;
>
> pudp = pud_offset(p4dp, addr);
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index aad29bcc7622..4dd8c1186471 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -89,7 +89,7 @@ static pmd_t *__init kasan_pmd_offset(pud_t *pudp, unsigned long addr, int node,
> static pud_t *__init kasan_pud_offset(p4d_t *p4dp, unsigned long addr, int node,
> bool early)
> {
> - if (p4d_none(READ_ONCE(*p4dp))) {
> + if (p4d_none(p4dp_get(p4dp))) {
> phys_addr_t pud_phys = early ?
> __pa_symbol(kasan_early_shadow_pud)
> : kasan_alloc_zeroed_page(node);
> @@ -162,7 +162,7 @@ static void __init kasan_p4d_populate(pgd_t *pgdp, unsigned long addr,
> do {
> next = p4d_addr_end(addr, end);
> kasan_pud_populate(p4dp, addr, next, node, early);
> - } while (p4dp++, addr = next, addr != end && p4d_none(READ_ONCE(*p4dp)));
> + } while (p4dp++, addr = next, addr != end && p4d_none(p4dp_get(p4dp)));
> }
>
> static void __init kasan_pgd_populate(unsigned long addr, unsigned long end,
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 22efaee79293..83709d987e7c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -367,7 +367,7 @@ static int alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
> {
> int ret = 0;
> unsigned long next;
> - p4d_t p4d = READ_ONCE(*p4dp);
> + p4d_t p4d = p4dp_get(p4dp);
> pud_t *pudp;
>
> if (p4d_none(p4d)) {
> @@ -457,7 +457,7 @@ static int alloc_init_p4d(pgd_t *pgdp, unsigned long addr, unsigned long end,
> }
>
> do {
> - p4d_t old_p4d = READ_ONCE(*p4dp);
> + p4d_t old_p4d = p4dp_get(p4dp);
>
> next = p4d_addr_end(addr, end);
>
> @@ -467,7 +467,7 @@ static int alloc_init_p4d(pgd_t *pgdp, unsigned long addr, unsigned long end,
> goto out;
>
> VM_WARN_ON_ONCE(p4d_val(old_p4d) != 0 &&
> - p4d_val(old_p4d) != READ_ONCE(p4d_val(*p4dp)));
> + p4d_val(old_p4d) != (p4d_val(p4dp_get(p4dp))));
>
> phys += next - addr;
> } while (p4dp++, addr = next, addr != end);
> @@ -1622,7 +1622,7 @@ static void unmap_hotplug_p4d_range(pgd_t *pgdp, unsigned long addr,
> do {
> next = p4d_addr_end(addr, end);
> p4dp = p4d_offset(pgdp, addr);
> - p4d = READ_ONCE(*p4dp);
> + p4d = p4dp_get(p4dp);
> if (p4d_none(p4d))
> continue;
>
> @@ -1788,7 +1788,7 @@ static void free_empty_p4d_table(pgd_t *pgdp, unsigned long addr,
> do {
> next = p4d_addr_end(addr, end);
> p4dp = p4d_offset(pgdp, addr);
> - p4d = READ_ONCE(*p4dp);
> + p4d = p4dp_get(p4dp);
> if (p4d_none(p4d))
> continue;
>
> @@ -1809,7 +1809,7 @@ static void free_empty_p4d_table(pgd_t *pgdp, unsigned long addr,
> */
> p4dp = p4d_offset(pgdp, 0UL);
> for (i = 0; i < PTRS_PER_P4D; i++) {
> - if (!p4d_none(READ_ONCE(p4dp[i])))
> + if (!p4d_none(p4dp_get(p4dp + i)))
> return;
> }
>
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 07b2fa4de57f..826856ef64a8 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -403,7 +403,7 @@ bool kernel_page_present(struct page *page)
> return false;
>
> p4dp = p4d_offset(pgdp, addr);
> - if (p4d_none(READ_ONCE(*p4dp)))
> + if (p4d_none(p4dp_get(p4dp)))
> return false;
>
> pudp = pud_offset(p4dp, addr);
> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> index d119119455f1..7afe2beca4ba 100644
> --- a/arch/arm64/mm/trans_pgd.c
> +++ b/arch/arm64/mm/trans_pgd.c
> @@ -99,7 +99,7 @@ static int copy_pud(struct trans_pgd_info *info, p4d_t *dst_p4dp,
> unsigned long next;
> unsigned long addr = start;
>
> - if (p4d_none(READ_ONCE(*dst_p4dp))) {
> + if (p4d_none(p4dp_get(dst_p4dp))) {
> dst_pudp = trans_alloc(info);
> if (!dst_pudp)
> return -ENOMEM;
> @@ -145,7 +145,7 @@ static int copy_p4d(struct trans_pgd_info *info, pgd_t *dst_pgdp,
> src_p4dp = p4d_offset(src_pgdp, start);
> do {
> next = p4d_addr_end(addr, end);
> - if (p4d_none(READ_ONCE(*src_p4dp)))
> + if (p4d_none(p4dp_get(src_p4dp)))
> continue;
> if (copy_pud(info, dst_p4dp, src_p4dp, addr, next))
> return -ENOMEM;
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