From: Ryan Roberts <ryan.roberts@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
linux-arm-kernel@lists.infradead.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Lorenzo Stoakes <ljs@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
David Hildenbrand <david@kernel.org>,
Mike Rapoport <rppt@kernel.org>,
Linu Cherian <linu.cherian@arm.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
kasan-dev@googlegroups.com
Subject: Re: [PATCH V3 2/4] arm64/mm: Use pudp_get() for PUD accesses
Date: Thu, 24 Sep 2026 13:10:17 +0100 [thread overview]
Message-ID: <41151166-ebdd-4c7f-b1a9-95625afd74d1@arm.com> (raw)
In-Reply-To: <20260924062214.1635078-3-anshuman.khandual@arm.com>
On 24/09/2026 07:22, Anshuman Khandual wrote:
> Replace READ_ONCE() with pudp_get() for PUD 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 PUD accesses on the existing page-table helpers. These
> can be overridden on arm64 to provide 128-bit single-copy atomicity when
> required.
>
> For now override pudp_get() which is same as the generic memory default in
> order to be consistent with similar helpers.
>
> No functional change intended.
>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: kasan-dev@googlegroups.com
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> Changes in V3
>
> - Added platform override for pudp_get()
>
> arch/arm64/include/asm/pgtable.h | 9 ++++++++-
> arch/arm64/mm/fault.c | 2 +-
> arch/arm64/mm/fixmap.c | 2 +-
> arch/arm64/mm/hugetlbpage.c | 4 ++--
> arch/arm64/mm/kasan_init.c | 4 ++--
> arch/arm64/mm/mmu.c | 20 ++++++++++----------
> arch/arm64/mm/pageattr.c | 2 +-
> arch/arm64/mm/trans_pgd.c | 4 ++--
> 8 files changed, 27 insertions(+), 20 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 9ad2d03e86ad..b46930f844a2 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -90,6 +90,12 @@ static inline pmd_t pmdp_get(pmd_t *pmdp)
> return READ_ONCE(*pmdp);
> }
>
> +#define pudp_get pudp_get
> +static inline pud_t pudp_get(pud_t *pudp)
> +{
> + return READ_ONCE(*pudp);
> +}
> +
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE
>
> @@ -879,7 +885,8 @@ static inline pmd_t *pud_pgtable(pud_t pud)
> }
>
> /* Find an entry in the second-level page table. */
> -#define pmd_offset_phys(dir, addr) (pud_page_paddr(READ_ONCE(*(dir))) + pmd_index(addr) * sizeof(pmd_t))
> +#define pmd_offset_phys(dir, addr) (pud_page_paddr(pudp_get(dir)) + \
> + pmd_index(addr) * sizeof(pmd_t))
>
> #define pmd_set_fixmap(addr) ((pmd_t *)set_fixmap_offset(FIX_PMD, addr))
> #define pmd_set_fixmap_offset(pud, addr) pmd_set_fixmap(pmd_offset_phys(pud, addr))
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 2757ee0c4300..435e2e14c070 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -181,7 +181,7 @@ static void show_pte(unsigned long addr)
> break;
>
> pudp = pud_offset_lockless(p4dp, p4d, addr);
> - pud = READ_ONCE(*pudp);
> + pud = pudp_get(pudp);
> ptval_to_str(pxd_str, pud_val(pud));
> pr_cont(", pud=%s", pxd_str);
> if (pud_none(pud) || pud_bad(pud))
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index 3cdac8021d4f..d9a870836faf 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -56,7 +56,7 @@ static void __init early_fixmap_init_pmd(pud_t *pudp, unsigned long addr,
> unsigned long end)
> {
> unsigned long next;
> - pud_t pud = READ_ONCE(*pudp);
> + pud_t pud = pudp_get(pudp);
> pmd_t *pmdp;
>
> if (pud_none(pud))
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index cdaa4500faf9..c9ad5e75b073 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -262,7 +262,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
> WARN_ON(addr & (sz - 1));
> ptep = pte_alloc_huge(mm, pmdp, addr);
> } else if (sz == PMD_SIZE) {
> - if (want_pmd_share(vma, addr) && pud_none(READ_ONCE(*pudp)))
> + if (want_pmd_share(vma, addr) && pud_none(pudp_get(pudp)))
> ptep = huge_pmd_share(mm, vma, addr, pudp);
> else
> ptep = (pte_t *)pmd_alloc(mm, pudp, addr);
> @@ -292,7 +292,7 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> return NULL;
>
> pudp = pud_offset(p4dp, addr);
> - pud = READ_ONCE(*pudp);
> + pud = pudp_get(pudp);
> if (sz != PUD_SIZE && pud_none(pud))
> return NULL;
> /* hugepage or swap? */
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index 7ca833c5de5e..aad29bcc7622 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -76,7 +76,7 @@ static pte_t *__init kasan_pte_offset(pmd_t *pmdp, unsigned long addr, int node,
> static pmd_t *__init kasan_pmd_offset(pud_t *pudp, unsigned long addr, int node,
> bool early)
> {
> - if (pud_none(READ_ONCE(*pudp))) {
> + if (pud_none(pudp_get(pudp))) {
> phys_addr_t pmd_phys = early ?
> __pa_symbol(kasan_early_shadow_pmd)
> : kasan_alloc_zeroed_page(node);
> @@ -150,7 +150,7 @@ static void __init kasan_pud_populate(p4d_t *p4dp, unsigned long addr,
> do {
> next = pud_addr_end(addr, end);
> kasan_pmd_populate(pudp, addr, next, node, early);
> - } while (pudp++, addr = next, addr != end && pud_none(READ_ONCE(*pudp)));
> + } while (pudp++, addr = next, addr != end && pud_none(pudp_get(pudp)));
> }
>
> static void __init kasan_p4d_populate(pgd_t *pgdp, unsigned long addr,
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 95621913679e..22efaee79293 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -309,7 +309,7 @@ static int alloc_init_cont_pmd(pud_t *pudp, unsigned long addr,
> {
> int ret;
> unsigned long next;
> - pud_t pud = READ_ONCE(*pudp);
> + pud_t pud = pudp_get(pudp);
> pmd_t *pmdp;
>
> /*
> @@ -390,7 +390,7 @@ static int alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
> }
>
> do {
> - pud_t old_pud = READ_ONCE(*pudp);
> + pud_t old_pud = pudp_get(pudp);
>
> next = pud_addr_end(addr, end);
>
> @@ -408,7 +408,7 @@ static int alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
> * only allow updates to the permission attributes.
> */
> BUG_ON(!pgattr_change_is_safe(pud_val(old_pud),
> - READ_ONCE(pud_val(*pudp))));
> + pud_val(pudp_get(pudp))));
> } else {
> ret = alloc_init_cont_pmd(pudp, addr, next, phys, prot,
> pgtable_alloc, flags);
> @@ -416,7 +416,7 @@ static int alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
> goto out;
>
> VM_WARN_ON_ONCE(pud_val(old_pud) != 0 &&
> - pud_val(old_pud) != READ_ONCE(pud_val(*pudp)));
> + pud_val(old_pud) != pud_val(pudp_get(pudp)));
> }
> phys += next - addr;
> } while (pudp++, addr = next, addr != end);
> @@ -1591,7 +1591,7 @@ static void unmap_hotplug_pud_range(p4d_t *p4dp, unsigned long addr,
> do {
> next = pud_addr_end(addr, end);
> pudp = pud_offset(p4dp, addr);
> - pud = READ_ONCE(*pudp);
> + pud = pudp_get(pudp);
> if (pud_none(pud))
> continue;
>
> @@ -1748,7 +1748,7 @@ static void free_empty_pud_table(p4d_t *p4dp, unsigned long addr,
> do {
> next = pud_addr_end(addr, end);
> pudp = pud_offset(p4dp, addr);
> - pud = READ_ONCE(*pudp);
> + pud = pudp_get(pudp);
> if (pud_none(pud))
> continue;
>
> @@ -1769,7 +1769,7 @@ static void free_empty_pud_table(p4d_t *p4dp, unsigned long addr,
> */
> pudp = pud_offset(p4dp, 0UL);
> for (i = 0; i < PTRS_PER_PUD; i++) {
> - if (!pud_none(READ_ONCE(pudp[i])))
> + if (!pud_none(pudp_get(pudp + i)))
> return;
> }
>
> @@ -1867,7 +1867,7 @@ int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot)
> pud_t new_pud = pfn_pud(__phys_to_pfn(phys), mk_pud_sect_prot(prot));
>
> /* Only allow permission changes for now */
> - if (!pgattr_change_is_safe(READ_ONCE(pud_val(*pudp)),
> + if (!pgattr_change_is_safe(pud_val(pudp_get(pudp)),
> pud_val(new_pud)))
> return 0;
>
> @@ -1898,7 +1898,7 @@ void p4d_clear_huge(p4d_t *p4dp)
>
> int pud_clear_huge(pud_t *pudp)
> {
> - if (!pud_leaf(READ_ONCE(*pudp)))
> + if (!pud_leaf(pudp_get(pudp)))
> return 0;
> pud_clear(pudp);
> return 1;
> @@ -1938,7 +1938,7 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
> pud_t pud;
> unsigned long next, end;
>
> - pud = READ_ONCE(*pudp);
> + pud = pudp_get(pudp);
>
> if (!pud_table(pud)) {
> VM_WARN_ON(1);
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 0ca07bd5ded9..07b2fa4de57f 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -407,7 +407,7 @@ bool kernel_page_present(struct page *page)
> return false;
>
> pudp = pud_offset(p4dp, addr);
> - pud = READ_ONCE(*pudp);
> + pud = pudp_get(pudp);
> if (pud_none(pud))
> return false;
> if (pud_leaf(pud))
> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> index b27b2d2c20c3..d119119455f1 100644
> --- a/arch/arm64/mm/trans_pgd.c
> +++ b/arch/arm64/mm/trans_pgd.c
> @@ -64,7 +64,7 @@ static int copy_pmd(struct trans_pgd_info *info, pud_t *dst_pudp,
> unsigned long next;
> unsigned long addr = start;
>
> - if (pud_none(READ_ONCE(*dst_pudp))) {
> + if (pud_none(pudp_get(dst_pudp))) {
> dst_pmdp = trans_alloc(info);
> if (!dst_pmdp)
> return -ENOMEM;
> @@ -109,7 +109,7 @@ static int copy_pud(struct trans_pgd_info *info, p4d_t *dst_p4dp,
>
> src_pudp = pud_offset(src_p4dp, start);
> do {
> - pud_t pud = READ_ONCE(*src_pudp);
> + pud_t pud = pudp_get(src_pudp);
>
> next = pud_addr_end(addr, end);
> if (pud_none(pud))
next prev parent reply other threads:[~2026-09-24 12:10 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-24 6:22 [PATCH V3 0/4] arm64/mm: Convert pgtable READ_ONCE() as pxdp_get() Anshuman Khandual
2026-09-24 6:22 ` [PATCH V3 1/4] arm64/mm: Use pmdp_get() for PMD accesses Anshuman Khandual
2026-09-24 12:09 ` Ryan Roberts
2026-09-24 6:22 ` [PATCH V3 2/4] arm64/mm: Use pudp_get() for PUD accesses Anshuman Khandual
2026-09-24 12:10 ` Ryan Roberts [this message]
2026-09-24 6:22 ` [PATCH V3 3/4] arm64/mm: Use p4dp_get() for P4D accesses Anshuman Khandual
2026-09-24 12:10 ` Ryan Roberts
2026-09-24 6:22 ` [PATCH V3 4/4] arm64/mm: Use pgdp_get() for PGD accesses Anshuman Khandual
2026-09-24 12:10 ` Ryan Roberts
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=41151166-ebdd-4c7f-b1a9-95625afd74d1@arm.com \
--to=ryan.roberts@arm.com \
--cc=akpm@linux-foundation.org \
--cc=anshuman.khandual@arm.com \
--cc=catalin.marinas@arm.com \
--cc=david@kernel.org \
--cc=kasan-dev@googlegroups.com \
--cc=linu.cherian@arm.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=ljs@kernel.org \
--cc=mark.rutland@arm.com \
--cc=rppt@kernel.org \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox
all inboxes | Powered by JetHome®