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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=nF7al3z+DZBg+8dub1pTw8FtztOSgeR7ezbMdaeQ5LFQzTP/C37w01jTHe1I43jHD vNLmEuv2MdLvnuEzA7jG3n2GM2rMZ4v6LRUA4Ml8zBGUdRio/9nN0TS0jYrMzZ0iP7 1t17BjusAYyUNC/TSoZrGmORAsTqHX9uScNIy1hk= Message-ID: Date: Wed, 23 Sep 2026 09:30:26 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2 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: <20260922061638.3967467-1-anshuman.khandual@arm.com> <20260922061638.3967467-4-anshuman.khandual@arm.com> From: Ryan Roberts Content-Language: en-GB In-Reply-To: <20260922061638.3967467-4-anshuman.khandual@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: u9oo9176m6rywmmewutnqc13igctjnan X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 79F2B14000E X-HE-Tag: 1790152230-834365 X-HE-Meta: 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 CiiG5ig4 M/21MMRefBWZ5nqQpqHkzWe4Vy493jJlT71bk17B5Jg2ufq9PYnAB8lQ5KDjg4IKgfK/bLWbr2Vf8ffs/txcGcSuSPW+dfQwshcZA9Ct8C+B+UcHF+lX/BiC4TSoWVgZ11Oc0OdGYcTYdnwv+GgHJtqoy/Xk/BHt5zkCPMCELENA3R5W8943/cgSvfzy2KsFkYn5ricpjsxplG9SwjFFHBDrqrzl1Av9BifA+7b2HfeZpR3eFWAmo064dN9ni5eOuVpCmvrbwXruoFzHbO0/4HX4Dk6RiGD5qMaCSeDbgAUvTcAxcAZA5CXZSedFo4CrcTqIOsZE+DdKvaWZYkPKXcXWzZ+oEwF51+zyQHMal9CQWSHVCWVA0v0eR6eI+M0sVuyitGG9tjxDv7PFurw3aGNqENQgllYkIScGpmTzxavKVD9tYTtzBM9qpiSDAt73ZFZNsOTvYOrPlkcvRW7tAP+3dRez7Ji2M3vnr8e0dWZyANfc= 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 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. No functional change intended. I notice you still have uncoverted READ_ONCE(*p4dp) in: - pud_offset_phys() - pud_offset() Is that intentional? If so, perhaps we need a comment explain why it's not using the helper? 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/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 ++-- > 7 files changed, 14 insertions(+), 14 deletions(-) > > 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;