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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ILKJZuuKIEuzmuevKqKO4tOS30td5XpGCkCczw00msIJ1M68Pm2su8zMpXlopRKtq uO92e8JU8rn3U2M+muQPsaL6hrrjejHKeBhqQGZnHztXAU4EKE14nCDGLtMFyqT3Sf nAHVmD4U/JKqIVNq3KlgE1+TBT5qqNmtP3stWqns= Message-ID: Date: Wed, 23 Sep 2026 09:31:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2 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: <20260922061638.3967467-1-anshuman.khandual@arm.com> <20260922061638.3967467-5-anshuman.khandual@arm.com> From: Ryan Roberts Content-Language: en-GB In-Reply-To: <20260922061638.3967467-5-anshuman.khandual@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: duxcobqirxc5b6hdfq3smdksffk9939d X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 8FFFDC000E X-HE-Tag: 1790152292-756182 X-HE-Meta: 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 GC2wKWGU CVfdPyocRnvYVUJcAjHd4NZ2dGD1v1vSVcuHrF3DGW2vzGMVt6gjhcWetSAdV2fBEXGaj2JLPbaIEq77p4xHq/JhSEPXL0V+8M5+zUqiNFZePOKtoJBwMSZh1PI24/Ov4efWnQEpxXVf26f8zIh/S8gHjbEcadFTtYqjIhA58tVNY8H+qkXp/5KsQD6AGQZFdyHhQG9wRlTvvILlcNy8r5BfovGHY3D750sbJjXI8EW2Env/k+XuWEVKk5+sNKq8OTgMndlciQJPAqo0MGwNgRW6kV1HRGxdx44QZTxxKSTBQHx4jSH3UZa7vf6+yRzc7ELg0GuSB7mvbwaxAHz05pRqK60HWtKp8C7eYf0FyjzSqr56K1qClWAuQMRLdpuaa57thBOtq+FpCzMVGkcWxE6pJHloLlFXIm80fwQWvtb9j6aSKx7QTTZrZyE7cYo9/RwTfZAqNmVvus6FnETxAG55qeOX3sHWEowqrZ4G91sHpZ1k= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 22/09/2026 07:16, 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. No functional change intended. I notice you have unconverted READ_ONCE(*pgdp) in: - p4d_offset_phys() - p4d_offset() Is that intentional? Thanks, Ryan > > 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 > --- > 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 ++-- > 6 files changed, 11 insertions(+), 11 deletions(-) > > 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;