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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=t495K1qyq4gbgcnSG0SSkLCtbRbfyt9DTrRQyvaBKXRgWElv+2ae2A1bD2fxdJf0/ 681XPXlRf6M3F7YyDs3WLLANT8GrMr/gQfEi0VwjuSAghLqAccfqN6AXJrpj312UWZ 4T0/KjaUOq23Jc2B3il6L43TsxM6OJYJ08/OJIMk= Message-ID: Date: Thu, 24 Sep 2026 13:10:35 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 3/4] arm64/mm: Use p4dp_get() for P4D accesses 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, kasan-dev@googlegroups.com References: <20260924062214.1635078-1-anshuman.khandual@arm.com> <20260924062214.1635078-4-anshuman.khandual@arm.com> From: Ryan Roberts Content-Language: en-GB In-Reply-To: <20260924062214.1635078-4-anshuman.khandual@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 0E1F81C000C X-Stat-Signature: 4nfh1oynz3g97cz9xjx6zknn4qj5p14m X-Rspam-User: X-HE-Tag: 1790251839-156647 X-HE-Meta: U2FsdGVkX1/3Im6KNHrxO+pldBZuj/FQ8dNm2mJMFEbcutzsP8DZAVTvwx0u3/Opm7Mdiz2mWz5YmGwSaEAc/+tT/Sl5l1NzXouS3ns1mV0GRLjTOaspbWKMr7Uux3VMJOmsU6A4mRCJLOhxXyXzku4rHWfYPkgB9xRipeBmdUAFzVgH2RoPxMNUzBPSVglpNMrZ0pvt451drCF5yTD+0rwmpdOQ78ZUTLSbQoIqpG8WUDUu6cDqNvSP6LqvYPLSA4H0mdtq5HgDJZqS56ccRLpH8Le7RwikOznPQivYtsG6/GDC0gDaeEa8gGFLWq4FZNPmGCL2tH73YI4hnxZVG6Dzkhzz221ltQweojIIeZr2muQI+3nyIAqLZvf0jUP270ZEMmG+9tbnlcihuTbdd3mb1CjyRgjiUamQAOR8q/Sbi5hW7l5N/8nGzmWqL5TQF5tOK0R0ui7uYmq3RbM+fLdKx6vLLgCa6VCclFY/TwjjU6mT24nGPq5PUmBfynwqtGZW3+m61it8WUCLAflQrvq8Od6d52wDF+pZ9oim8uHDNKZ9UynX01PnS52YKgWutV+bVd0uzeI6ENxbDn5HO4XDJRunMhUJM0GHMHigNZxmGXt8b83GDYs/LZCbHeUyD5ZCRu5MZSWWBT5dThj14wc1Pd9ipGlSckCJIWRu7vOBiShZi5hNFGd/MLFgo/cGQJo+8QJGDo4g/RoOS2G1C+lw66Ok2eFG4IroPTTv7ouxd50LebcjuHYSlJTIQzvZH3FERj03SW2KPsRyWyq3Svaas31QJowTxmODj3hsEDbhmzPZ4SNgxOTjB76ZCZCpN+iQ8kHyz9bgXKhDQVTstrMaL7WbFmtTCH/ijM43vfhetw5GmQ5HLUEAuSMtdeERx5lixPv/HgjaizKFHf5SH7CFyGM+hvbS8n505EhRzCCAT+DQaZQQ98iKQ6pAgf/3EEFzGaGiwX8OjQBC16y gXB4TCyf q7kRjtAHE7SZBEvp6Ko4MzOQ2DV/cfBSPn9SOHgwtlKdGpvgP9MaI6/p6x077TrjEPs3cscHNCGEqZn/85W+s+1sPLAuu5GcWIT+VDilXv//ygVW6Z8uu1Y3nqPQ9b8gtPOyWQWPdqn3W6REHjT1/29NbKPlAzgEgt3O5271xxNcGnFfIMv9caLTO9k56FlBYXLL6Fg2XYg99gX4u7hMp0t/LbHtmeULkKuyCVDEsu0Ewm02GJquBYFqwh2FjEQe5jxGWnu4/N9RjUb0mX8Ag4Mb/ThKxMFBUHBX4ZN928xfp6QASbFE1hiFOR5NPOPcrcIo1C2OLOwZV2i8HjtpcSpfGQatWtF2QIBRIHgt0WtOzZNCIoHAFXLAxuR0BZh1PPG5SJJJyBIzk7U8RsLW99d21QrwzxIErtpTN44d3cdrCPvjtqvpbya3vq0V4Yp6ovZkESVCDexrkhvIwRgpvoX4L6mTdEtxFGEHtvncEFKIJo3M= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 24/09/2026 07:22, 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. > > For now override p4dp_get() which is same as the generic memory default > to avoid build dependency problem for callers in header . > > No functional change intended. > > Cc: Catalin Marinas > Cc: Will Deacon > Cc: Ryan Roberts > Cc: Mark Rutland > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-kernel@vger.kernel.org > Cc: kasan-dev@googlegroups.com > Signed-off-by: Anshuman Khandual Reviewed-by: Ryan Roberts > --- > Changes in V3 > > - Added platform override for p4dp_get() > - Replaced READ_ONCE() in pud_offset_phys() and pud_offset() > > arch/arm64/include/asm/pgtable.h | 10 ++++++++-- > 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 ++-- > 8 files changed, 22 insertions(+), 16 deletions(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index b46930f844a2..f235f68a7518 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -96,6 +96,12 @@ static inline pud_t pudp_get(pud_t *pudp) > return READ_ONCE(*pudp); > } > > +#define p4dp_get p4dp_get > +static inline p4d_t p4dp_get(p4d_t *p4dp) > +{ > + return READ_ONCE(*p4dp); > +} > + > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE > > @@ -976,7 +982,7 @@ static inline phys_addr_t pud_offset_phys(p4d_t *p4dp, unsigned long addr) > { > VM_WARN_ON_ONCE(!pgtable_l4_enabled()); > > - return p4d_page_paddr(READ_ONCE(*p4dp)) + pud_index(addr) * sizeof(pud_t); > + return p4d_page_paddr(p4dp_get(p4dp)) + pud_index(addr) * sizeof(pud_t); > } > > static inline > @@ -990,7 +996,7 @@ pud_t *pud_offset_lockless(p4d_t *p4dp, p4d_t p4d, unsigned long addr) > > static inline pud_t *pud_offset(p4d_t *p4dp, unsigned long addr) > { > - return pud_offset_lockless(p4dp, READ_ONCE(*p4dp), addr); > + return pud_offset_lockless(p4dp, p4dp_get(p4dp), addr); > } > #define pud_offset pud_offset > > 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;