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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=erV4TueHzMTKU7EV61dmBcEuKYzA1op4hwcZB7VEFgaJWmKI5FE7ugnqJLu5ERObj Q50JhfvJ6lkvid/bPPod28UwjH6vdEo+d+vjjz43+VycruIntOw4hXFlZ850Oh67Co N6iKAnN/H18buKKpap1r1rQzAeicQUyvZ1ACH5b4= Message-ID: Date: Mon, 21 Sep 2026 11:49:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] arm64/mm: Convert READ_ONCE() as pmdp_get() while accessing PMD To: Anshuman Khandual , linux-arm-kernel@lists.infradead.org Cc: Catalin Marinas , Will Deacon , Mark Rutland , Lorenzo Stoakes , Andrew Morton , David Hildenbrand , Mike Rapoport , Linu Cherian , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mark Rtland , linx-arm-kernel@lists.infradead.org, linx-kernel@vger.kernel.org, kasan-dev@googlegrops.com References: <20260903060657.2717457-1-anshuman.khandual@arm.com> <20260903060657.2717457-2-anshuman.khandual@arm.com> From: Ryan Roberts Content-Language: en-GB In-Reply-To: <20260903060657.2717457-2-anshuman.khandual@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 603CA180002 X-Stat-Signature: qs3tny7d9mfamhuescg8rtyia597jr34 X-Rspam-User: X-HE-Tag: 1789987774-32910 X-HE-Meta: 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 HyxxgkKk l3mlVFcemQfUKuYiku+U/RgS/v8UKpoySGVw5DarGFPAgUCZRsUZOwYwtXyMM3OJiy9J56UPoGiJgIgyVXVK6Cc80fqwBkc+rE/epI4cBQJY8w2zKvo+HHuERLOVvNv9XlFuX8rPthcOZUZ0vHcJeGjv4DdT7UbQ5syVTYZsdR4huBOC9xTT/q20/ne58BoaQL6JNvvI6GJNMssqV/DAbcGRrCthoqk9Xjr5NnsnTkuGOQl1CXCnSlLlpEWlr+yEPdBUHujDa/ud1oY7Bn30/g1coXRldYQxn9FHvrTjXc9DCR9LAuuWCCUMoyNgRXiHazmC+vQsuHvCMXXyKQPKdd3wUX+ws7oeEvNfu24xX8X42VOApC/DZ+4bqq8GTJRiGvwXkLyvOsm7VJtaPip+g1veHyX1495GMq25Yvja5TSl5b70a8eRPbRuU1htn54UasNXNsBb6mm0MVvcxFemfxi72hRb04agiqf88cqdI6y6P6y4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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<--- With that: Reviewed-by: Ryan Roberts > > Cc: Catalin Marinas > Cc: Will Deacon > Cc: Ryan Roberts > Cc: Mark Rtland > Cc: linx-arm-kernel@lists.infradead.org > Cc: linx-kernel@vger.kernel.org > Cc: kasan-dev@googlegrops.com > Signed-off-by: Anshuman Khandual > --- > 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))