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Received: from foss.arm.com ([217.140.110.172]) by desiato.infradead.org with esmtp (Exim 4.99.2 #2 (Red Hat Linux)) id 1x9iI0-0000000FweJ-0Mf1 for linux-arm-kernel@lists.infradead.org; Thu, 24 Sep 2026 12:11:02 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4B88C152B; Thu, 24 Sep 2026 05:10:54 -0700 (PDT) Received: from [10.57.83.110] (unknown [10.57.83.110]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 592DA3F86C; Thu, 24 Sep 2026 05:10:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1790251857; bh=IgtQ7NG3+ltNOk7/3bBV0TizU/mZJ41BEdTzaOl3LaQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=fVueGZa1I/lX4r15+916hs6NmRSDKrWZR6N4zsyNwcmBGnEr7G+vVnyiLovJQEz4/ Sr3z02kzeKphesgRh4uF3U/WGgbLannb3g6KrCHJUu+dr50lL+cvaZDOlBigyiftFV sAhDDsEP16LQbxhuU1UpU1ewgOxX04Eg/s1QVvCg= Message-ID: <070d08d9-f778-4780-878a-2c2c4cb4f160@arm.com> Date: Thu, 24 Sep 2026 13:10:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 4/4] arm64/mm: Use pgdp_get() for PGD 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> <20260924062214.1635078-5-anshuman.khandual@arm.com> From: Ryan Roberts Content-Language: en-GB In-Reply-To: <20260924062214.1635078-5-anshuman.khandual@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260924_131100_574313_33C4526B X-CRM114-Status: GOOD ( 25.79 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 24/09/2026 07:22, Anshuman Khandual wrote: > Replace READ_ONCE() with pgdp_get() for PGD 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 PGD 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 pgdp_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 pgdp_get() > - Replaced READ_ONCE() in p4d_offset_phys() and p4d_offset() > > arch/arm64/include/asm/pgtable.h | 10 ++++++++-- > arch/arm64/mm/fault.c | 2 +- > arch/arm64/mm/hugetlbpage.c | 2 +- > arch/arm64/mm/kasan_init.c | 6 +++--- > arch/arm64/mm/mmu.c | 6 +++--- > arch/arm64/mm/pageattr.c | 2 +- > arch/arm64/mm/trans_pgd.c | 4 ++-- > 7 files changed, 19 insertions(+), 13 deletions(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index f235f68a7518..05d273f276b8 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -102,6 +102,12 @@ static inline p4d_t p4dp_get(p4d_t *p4dp) > return READ_ONCE(*p4dp); > } > > +#define pgdp_get pgdp_get > +static inline pgd_t pgdp_get(pgd_t *pgdp) > +{ > + return READ_ONCE(*pgdp); > +} > + > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE > > @@ -1102,7 +1108,7 @@ static inline phys_addr_t p4d_offset_phys(pgd_t *pgdp, unsigned long addr) > { > VM_WARN_ON_ONCE(!pgtable_l5_enabled()); > > - return pgd_page_paddr(READ_ONCE(*pgdp)) + p4d_index(addr) * sizeof(p4d_t); > + return pgd_page_paddr(pgdp_get(pgdp)) + p4d_index(addr) * sizeof(p4d_t); > } > > static inline > @@ -1116,7 +1122,7 @@ p4d_t *p4d_offset_lockless(pgd_t *pgdp, pgd_t pgd, unsigned long addr) > > static inline p4d_t *p4d_offset(pgd_t *pgdp, unsigned long addr) > { > - return p4d_offset_lockless(pgdp, READ_ONCE(*pgdp), addr); > + return p4d_offset_lockless(pgdp, pgdp_get(pgdp), addr); > } > > static inline p4d_t *p4d_set_fixmap(unsigned long addr) > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c > index a6afd8929a10..43f25418275a 100644 > --- a/arch/arm64/mm/fault.c > +++ b/arch/arm64/mm/fault.c > @@ -160,7 +160,7 @@ static void show_pte(unsigned long addr) > guard(irqsave)(); > > pgdp = pgd_offset(mm, addr); > - pgd = READ_ONCE(*pgdp); > + pgd = pgdp_get(pgdp); > ptval_to_str(pxd_str, pgd_val(pgd)); > pr_alert("[%016lx] pgd=%s", addr, pxd_str); > > diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c > index 5771bf49e1fe..e05e5b41ae50 100644 > --- a/arch/arm64/mm/hugetlbpage.c > +++ b/arch/arm64/mm/hugetlbpage.c > @@ -284,7 +284,7 @@ pte_t *huge_pte_offset(struct mm_struct *mm, > pmd_t *pmdp, pmd; > > pgdp = pgd_offset(mm, addr); > - if (!pgd_present(READ_ONCE(*pgdp))) > + if (!pgd_present(pgdp_get(pgdp))) > return NULL; > > p4dp = p4d_offset(pgdp, addr); > diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c > index 4dd8c1186471..f61f5bc09467 100644 > --- a/arch/arm64/mm/kasan_init.c > +++ b/arch/arm64/mm/kasan_init.c > @@ -102,7 +102,7 @@ static pud_t *__init kasan_pud_offset(p4d_t *p4dp, unsigned long addr, int node, > static p4d_t *__init kasan_p4d_offset(pgd_t *pgdp, unsigned long addr, int node, > bool early) > { > - if (pgd_none(READ_ONCE(*pgdp))) { > + if (pgd_none(pgdp_get(pgdp))) { > phys_addr_t p4d_phys = early ? > __pa_symbol(kasan_early_shadow_p4d) > : kasan_alloc_zeroed_page(node); > @@ -256,7 +256,7 @@ static int __init root_level_idx(u64 addr) > static void __init clone_next_level(u64 addr, pgd_t *tmp_pg_dir, pud_t *pud) > { > int idx = root_level_idx(addr); > - pgd_t pgd = READ_ONCE(swapper_pg_dir[idx]); > + pgd_t pgd = pgdp_get(swapper_pg_dir + idx); > pud_t *pudp = (pud_t *)__phys_to_kimg(__pgd_to_phys(pgd)); > > memcpy(pud, pudp, PAGE_SIZE); > @@ -280,7 +280,7 @@ static int __init next_level_idx(u64 addr) > */ > static void __init clear_next_level(int pgd_idx, int start, int end) > { > - pgd_t pgd = READ_ONCE(swapper_pg_dir[pgd_idx]); > + pgd_t pgd = pgdp_get(swapper_pg_dir + pgd_idx); > pud_t *pudp = (pud_t *)__phys_to_kimg(__pgd_to_phys(pgd)); > > memset(&pudp[start], 0, (end - start) * sizeof(pud_t)); > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 83709d987e7c..76d8d320aeed 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -434,7 +434,7 @@ static int alloc_init_p4d(pgd_t *pgdp, unsigned long addr, unsigned long end, > { > int ret; > unsigned long next; > - pgd_t pgd = READ_ONCE(*pgdp); > + pgd_t pgd = pgdp_get(pgdp); > p4d_t *p4dp; > > if (pgd_none(pgd)) { > @@ -1649,7 +1649,7 @@ static void unmap_hotplug_range(unsigned long addr, unsigned long end, > do { > next = pgd_addr_end(addr, end); > pgdp = pgd_offset_k(addr); > - pgd = READ_ONCE(*pgdp); > + pgd = pgdp_get(pgdp); > if (pgd_none(pgd)) > continue; > > @@ -1827,7 +1827,7 @@ static void free_empty_tables(unsigned long addr, unsigned long end, > do { > next = pgd_addr_end(addr, end); > pgdp = pgd_offset_k(addr); > - pgd = READ_ONCE(*pgdp); > + pgd = pgdp_get(pgdp); > if (pgd_none(pgd)) > continue; > > diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c > index 826856ef64a8..dfa1356daac2 100644 > --- a/arch/arm64/mm/pageattr.c > +++ b/arch/arm64/mm/pageattr.c > @@ -399,7 +399,7 @@ bool kernel_page_present(struct page *page) > unsigned long addr = (unsigned long)page_address(page); > > pgdp = pgd_offset_k(addr); > - if (pgd_none(READ_ONCE(*pgdp))) > + if (pgd_none(pgdp_get(pgdp))) > return false; > > p4dp = p4d_offset(pgdp, addr); > diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c > index 7afe2beca4ba..06470d690f9f 100644 > --- a/arch/arm64/mm/trans_pgd.c > +++ b/arch/arm64/mm/trans_pgd.c > @@ -134,7 +134,7 @@ static int copy_p4d(struct trans_pgd_info *info, pgd_t *dst_pgdp, > unsigned long next; > unsigned long addr = start; > > - if (pgd_none(READ_ONCE(*dst_pgdp))) { > + if (pgd_none(pgdp_get(dst_pgdp))) { > dst_p4dp = trans_alloc(info); > if (!dst_p4dp) > return -ENOMEM; > @@ -164,7 +164,7 @@ static int copy_page_tables(struct trans_pgd_info *info, pgd_t *dst_pgdp, > dst_pgdp = pgd_offset_pgd(dst_pgdp, start); > do { > next = pgd_addr_end(addr, end); > - if (pgd_none(READ_ONCE(*src_pgdp))) > + if (pgd_none(pgdp_get(src_pgdp))) > continue; > if (copy_p4d(info, dst_pgdp, src_pgdp, addr, next)) > return -ENOMEM;