[PATCH V2 4/4] arm64/mm: Use pgdp_get() for PGD accesses
Ryan Roberts
ryan.roberts at arm.com
Wed Sep 23 01:31:28 PDT 2026
On 22/09/2026 07:16, Anshuman Khandual wrote:
> 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. No functional change intended.
I notice you have unconverted READ_ONCE(*pgdp) in:
- p4d_offset_phys()
- p4d_offset()
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 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/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 ++--
> 6 files changed, 11 insertions(+), 11 deletions(-)
>
> 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;
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