[PATCH V2 3/4] arm64/mm: Use p4dp_get() for P4D accesses
Anshuman Khandual
anshuman.khandual at arm.com
Wed Sep 23 21:23:51 PDT 2026
On Wed, Sep 23, 2026 at 09:30:26AM +0100, Ryan Roberts wrote:
> 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?
Platform overrride for p4dp_get() should solve the problem.
>
> 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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