From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id A538E47F2D3; Wed, 2 Sep 2026 11:56:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788350215; cv=none; b=bjUYPMf8Y2qbcqsVtK6iL0QM/G0sy+ImeoXr5n33h5dLA4vZ36BywsSjqW5dqFpOgUmKGsDDAm7QQaXo3DYW1kWrclY0BL0c+W6u6/sL9IXV/3FEoCGHQYYAHZ4p+gbzGk/LQFIMZLRzqf0xXKnigmMPYd5jl7fTsJWJDwcIac4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788350215; c=relaxed/simple; bh=YuonSXf6Kv6A3r5/HpLRgNYFjXuTFjIeFymIciQQgIg=; h=From:Subject:Date:Message-Id:MIME-Version:Content-Type:To:Cc; b=L19EykvhA83d2gR3GggvI7Rj/MYzNRwLv3MQNzIOqKO8W5BbUPPLYCloPRJy9fJE5RteOStAV/LZfH8ETbwJAQbBATzSK0FXUcUsS6mo82psUCDL2bFodv4K2btYX/hPk8mDsVE1yuZO0Y2kb4yLURMC+BMlwsBJmXJQK7uY+5M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=Xxjg4fJp; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="Xxjg4fJp" 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 3BD681682; Wed, 2 Sep 2026 04:56:32 -0700 (PDT) Received: from e129823.arm.com (e129823.arm.com [10.2.213.3]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 92A023F85F; Wed, 2 Sep 2026 04:56:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1788350196; bh=YuonSXf6Kv6A3r5/HpLRgNYFjXuTFjIeFymIciQQgIg=; h=From:Subject:Date:To:Cc:From; b=Xxjg4fJpRJ8F5G+7cbkqdkGJjWatdFv/Ov5DeTu8h9BJf2J2coeOwj/INJ5P+pVED 2qhO3ib5wRtEVRSDMIHofLwkFvhmcK5eq5jD/Ko+dLLqeQDW4AYHJ9BKS/3Cdbode0 ctD+ffUCqGOB6s28nyAPSM2ecQMkC8q3HpHlE+U0= From: Yeoreum Yun Subject: [PATCH RFC v3 00/21] mm: change behavior of pXdp_get()/pXd_page() in compile-time folded pgtable Date: Wed, 02 Sep 2026 12:56:02 +0100 Message-Id: <20260902-dummy_ptxp3-v3-0-5d8f5b17c25c@arm.com> Precedence: bulk X-Mailing-List: linux-mips@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-B4-Tracking: v=1; b=H4sIANMOmGoC/23PwWrDMAwG4FcpPtdFthLb2Wkw2APsOkpxZGX1I U1wutBS8u4zgXYb7lES3y/pJiZOkSfxsrmJxHOc4nDKBW43go7+9MUyhlwLDdqA1VqG776/Hsb zZUSJtlLBOiILnchiTNzFy5r2KT7e38Q+N49xOg/pum6Y9Tp6GjZrCTI0XLU+oGLwrz71Oxr6N WXGv9L9l5hlSwbazmtmDA+5vR+OqhTOEzahck7VVAgHVSmsCQ0COAvclUI9EUz5nxqwDh5Koet SKLSGlGkc0u+O/bIsP/ddlCSrAQAA To: Russell King , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , Catalin Marinas , Will Deacon , Arnd Bergmann , Andrew Morton , Kairui Song , Qi Zheng , Shakeel Butt , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Johannes Weiner , David Hildenbrand , Michal Hocko , Lorenzo Stoakes , Tianrui Zhao , Bibo Mao , Anup Patel , Atish Patra , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonas Bonn , Stefan Kristiansson , Stafford Horne Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-openrisc@vger.kernel.org, Muhammad Usama Anjum , Yeoreum Yun X-Mailer: b4 0.13.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=9281; i=yeoreum.yun@arm.com; h=from:subject:message-id; bh=YuonSXf6Kv6A3r5/HpLRgNYFjXuTFjIeFymIciQQgIg=; b=owEB7QES/pANAwAKAW3Vw9FaxTEzAcsmYgBqmA7hF91UJH19ihMYiSFd6MmKb/oB8b00GDLAW UHYaZejF4qJAbMEAAEKAB0WIQQtg+CS3QUzuFh1pJ1t1cPRWsUxMwUCapgO4QAKCRBt1cPRWsUx M8ccDACIIBPnKxLvshr/NbigK+M7Y5yJaArlLvis3vTY8LkJ0KrOcOnQS1OGkruG3z3mRodyiOj UPC4AAl4piHxpr2RrAQjZROJkZfzhVXwn0ZGJ6fJ8rlzNJlQBNLK/GA4q1PNekyb6M7nbCDwQLC 7YXNX07Mnv77V6qWHh+59365sNkoBwrdewd4Mz0hadHuAw0ys0cQNTCz9esxfWt73LzAOow/J3N SgJCM1Fuk4JEJkzl/AfS+/+NOAWdAn+y4CctwAsFbbLE5bWlzSpgqzBXfH3rXUQRBlBdT5MTNYQ xocI1UnUpikoZpck5Ie1JvlkArsSXc9+l7dedk/sUdyOt+vx4aXKYUgSSmx3AvEBZeOWmiSUX+r AImN2O7YeU3VwFPYsnqvD6FhJT+N5rCQrBHsnW1uhBJGgBFkxU9RcFsS3awOUoIGkCe5lbp2e8J hPeBnjqqsQdLL0SXOVaYPATlLVN1J781gwFBqhUoZRBDwhMecb8EXWZ5ydQjzSGZ3n4sw= X-Developer-Key: i=yeoreum.yun@arm.com; a=openpgp; fpr=2D83E092DD0533B85875A49D6DD5C3D15AC53133 Using ptep_get() and its counterparts in common code is suboptimal on kernel configurations with generic compile-time folded page tables. By default, ptep_get() and its friends expands to READ_ONCE(), forcing the compiler to emit a load even when the value is not used afterwards. This issue was recently reported by Christophe Leroy [1] for ppc32 preventing futher code conversion to ptep_get()/pmdp_get()/... helper and the same behavior can also be observed on arm64 when built with 2- or 3-level page tables e.g) perf_get_page_size() in arm64 with CONFIG_PGTABLE_LEVEL=3: 00000000000052a0 : ... 52dc: d53b4234 mrs x20, DAIF 52e0: d50343df msr DAIFSet, #0x3 ... 52fc: d35e9a69 ubfx x9, x19, #30, #9 /* pud_offset_lockless() */ 5300: f9403508 ldr x8, [x8, #0x68] 5304: f869790a ldr x10, [x8, x9, lsl #3] /* pudp_get() */ 5308: f90007ea str x10, [sp, #0x8] 530c: f8697908 ldr x8, [x8, x9, lsl #3] /* pudp_get() */ ... 5360: 90000009 adrp x9, 0x5000 5364: 92746908 and x8, x8, #0x7ffffff000 5368: d3557675 ubfx x21, x19, #21, #9 /* pmd_offset_lockless() */ ... 5394: f8757ac8 ldr x8, [x22, x21, lsl #3] /* pmdp_get() */ Though PGTABLE_LEVEL=3, since the pudp_get() still remain with READ_ONCE(), there's redundant load for the pud which is folded. To prevent generating suboptimal code, make pXdp_get() return a dummy entry for compile-time folded page tables, make the helpers such as pXd_offset()/pXd_offset_lockless(), set_pXd() validate dummy entries at compile time to catch the wrong usage and prohibit calls to pXd_page() in pgtable-nopXd.h. This series does not change the behaviour of existing code that directly manipulates folded page-table levels using set_pgd(), pgd_page_vaddr(), and related helpers. Those helpers continue to behave as before. The new restrictions only apply to code that adopts the pXdp_get()-based access model for compile-time folded page tables. As the pXdp_get() can return *dummy* entry, some of code using the stack value where saves the pXdp_get() could be a problematic: 1. Passing address of stack value where saves the pXdp_get() result to pXd_offset() for example: pud_t *pudp, pud; pmd_t *pmdp; pud = pudp_get(pudp, address); pmdp = pmd_offset(&pud, pud, address); (e.g. host_pfn_mapping_level() in loongarch). 2. Using the pXdp_get() result to use as argument of pXd_val() and to check prot without checking pgtable is folded. for example, x86's effective_prot(). 3. Using set_pXd() with pXdp_get() will set problematic dummy entry in folded page table like: set_pXd(pxdp, pXdp_get(pxdp_k)); 4. Using pgd_page_vaddr() to get the first-level pgtable. passing dummy pxdp_get() for pgd_page_vaddr() will return wrong address. Therefore, make pgd_page_vaddr() and pXd_pgtable() to trigger the error for improper usage with folded dummy entry in the generic compile-time folded pgtable. Thanksfully, above cases are rare since (1) most of usage using pXd_offset() with result of upper pXd_offset(), (2) it's extreamely rare to use pXd_val() for non-leaf entry in the kernel, (3) is to handle the vmalloc_fault or set the first level of page table and (4) to setup early page table and etc. Therefore, properly handle this uncommon and problematic pattern, and document the current design of compile-time folded page tables. This patch is based on mm-unstable. Future work =========== - print_bad_page_map() and show_pte() still prints dummy values instead of printing the same content for all generic compile-time folded page tables. We might want to skip printing dummy values later. - We currently catch abuse of dummy values on the stack at compile-time by relying on constant propagation by the compiler. Usama's work [3] on using distinct types for sw vs. hw PTEs could help here as well." - Clean up vmalloc fault handling by synchronizing the vmalloc entry on 32-bit architectures. This code is almost identical across architectures. - Unfortunately, the current design of compile-time folded page tables appears to be internally consistent but confusing. For example, when CONFIG_PGTABLE_LEVELS is 2, p4d, pud, and pmd are expected to be folded into pgd. However, the architecture code uses set_pmd() to update the top-level page-table entry, even though it includes pgtable-nopmd.h. In the future, it would be good to eliminate this source of confusion, possibly by treating all folded upper levels consistently as dummy wrappers around the highest real page-table level: NOPGD --> +------+ P4D | ptr0 |-------> +------+ PUD +------+ | ptr0 |-------> +-----+ | ptr1 |- | ptr | -------> ... | ptr2 | \ | ptr | | ptr3 | \ ... ... \ \ PUD +----> +-----+ | ptr | -------> ... | ptr | ... Patch History ============= from v2 to v3: - repasre commit message - move ptdump_pgtable_first_level() into pgalloc.h - skip the huge pud operation when CONFIG_X86_DIRECT_GBPAGES is disabled - docuemtns compile-time folded page table - drop the applied patch. - https://lore.kernel.org/all/20260722-dummy_ptxp3-v2-0-d9e4bad31e0a@arm.com/ from v1 to v2: - Restore slient fallback to next pXd in set_pXd() and pXd_pgtable() and add check whether they're called with dummy entry. - Add some comment for returning first entry of pgd in arm with 2 pgtable-level - https://lore.kernel.org/all/20260713135614.1618183-1-yeoreum.yun@arm.com/ Link: [1] https://lore.kernel.org/all/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org/ Link: [2] https://lore.kernel.org/all/20251113014656.2605447-1-samuel.holland@sifive.com/ Link: [3] https://lore.kernel.org/r/74182e50-b54f-4d2d-a27f-3a59a538d6bc@arm.com --- David Hildenbrand (Arm) (13): ARM: mm: make nommu pgd_t a scalar ARM: mm: make 2-level pgd_t a scalar ARM: mm: remove custom pgdp_get() LoongArch: mm: define pud_leaf() only when PUD exists MIPS: mm: define pud_leaf() only when PUD exists mm/pgtable: define (pgd|p4d|pud)_leaf() for folded page tables mm/pgtable: define (pgd|p4d|pud)_offset_lockless() for folded page tables openrisc/pgtable: drop __pmd_offset() mm/pgtable: optimize pmdp_get() and friends for folded pagetable levels mm/pgtable: catch abuse of folded dummy pgd_t/p4d_t/pud_t mm/pgtable: disallow calling (pgd|p4d|pud)_page, pgd_page_vaddr() and (p4d|pud)_pgtable with dummy mm/pgtable: disallow calling folded set_pgd/set_p4d/set_pud with dummy Documentation: mm: clarify behaviour of compile-time folded page tables Yeoreum Yun (8): mm: vmscan: remove stack copy address of pud/pmd pass in walk_pud/pmd_range() loongarch: kvm: remove stack copy address of pXd in pXd_offset() riscv: kvm: remove stack copy address of pXd in pXd_offset() riscv: mm: use proper set_pXd() for generic compile-time folded patable in vmalloc_fault() mm/pgtable: redefine PGTABLE_LEVEL enum with ascend order from PGD x86: mm: use pgtable_level enum in effective_prot_pXd() x86: mm: carve out the generic compile-time folded pgtable case in effective_prot() x86: mm: skip collapse_pud_page() when CONFIG_X86_DIRECT_GBPAGES disabled Documentation/mm/page_tables.rst | 84 +++++++++++++++++++++++++---- arch/arm/include/asm/page-nommu.h | 4 +- arch/arm/include/asm/pgtable-2level-types.h | 20 +++++-- arch/arm/include/asm/pgtable.h | 2 - arch/arm64/include/asm/pgtable.h | 23 -------- arch/loongarch/include/asm/pgtable.h | 2 + arch/loongarch/kvm/mmu.c | 20 ++++--- arch/mips/include/asm/pgtable.h | 2 + arch/openrisc/include/asm/pgtable.h | 3 -- arch/riscv/kvm/mmu.c | 20 ++++--- arch/riscv/mm/fault.c | 52 +++++++++++------- arch/x86/mm/dump_pagetables.c | 18 ++++--- arch/x86/mm/pat/set_memory.c | 2 +- include/asm-generic/pgtable-nop4d.h | 54 +++++++++++++++---- include/asm-generic/pgtable-nopmd.h | 54 +++++++++++++++---- include/asm-generic/pgtable-nopud.h | 56 +++++++++++++++---- include/linux/pgtable.h | 69 +++++++++++++++++------- mm/vmscan.c | 4 +- 18 files changed, 352 insertions(+), 137 deletions(-) --- base-commit: e3b5239afe1b8f0194db7436b17c33e94c1988c4 change-id: 20260722-dummy_ptxp3-3741d78cc70f Best regards, -- Sincerely, Yeoreum Yun