[PATCH 1/4] arm64/mm: Convert READ_ONCE() as pmdp_get() while accessing PMD
Anshuman Khandual
anshuman.khandual at arm.com
Mon Sep 21 05:44:01 PDT 2026
On Mon, Sep 21, 2026 at 11:49:28AM +0100, Ryan Roberts wrote:
> On 03/09/2026 07:06, Anshuman Khandual wrote:
> > Convert all READ_ONCE() based PMD accesses as pmdp_get() instead which will
> > support both D64 and D128 translation regime going forward. That is because
> > READ_ONCE() would need 128 bit single copy atomic guarantees, while reading
> > 128 bit page table entries which is currently not supported on arm64. Build
> > fails for READ_ONCE() while accessing beyond 64 bits.
> >
> > Load Pair/Store Pair (ldp/stp) are only single copy atomic if FEAT_LSE1
> > is supported (which is required when FEAT_D128 is supported). Currently 128
> > bit pgtables is a compile time decision - so we could have chosen to extend
> > READ_ONCE()/WRITE_ONCE() to allow 128 bit for this configuration. But then
> > it's a general purpose API and we were concerned that other users might
> > eventually creep in that expect 128 and then fail to compile in the other
> > configs.
> >
> > But worse, we are considering eventually making D128 a boot time option, at
> > which point we'd have to make READ_ONCE() always allow 128 bit at compile
> > time but then it might silently tear at runtime.
> >
> > So our preference is to standardize on these existing helpers, which we can
> > override in arm64 to give the 128 bit single copy guarantee when required.
>
> Perhaps something like this conveys the justification a bit more clearly?:
>
> ---8<---
> arm64/mm: Use pmdp_get() for PMD accesses
>
> 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.
> ---8<---
Sure will update, assume similar format being applicable all patches here.
>
> With that:
>
> Reviewed-by: Ryan Roberts <ryan.roberts at arm.com>
>
>
> >
> > 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
> > 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 61fbfab3e06e..51f55498d43d 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -805,7 +805,8 @@ 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 c75bab3c2f4b..cb25cb130e83 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(pudp, addr);
> > - pmd = READ_ONCE(*pmdp);
> > + pmd = pmdp_get(pmdp);
> > ptval_to_str(pmd_str, pmd_val(pmd));
> > pr_cont(", pmd=%s", pmd_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 b5de4095d650..32daaa547d09 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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