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 D369D7080D; Fri, 2 Oct 2026 15:19:40 +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=1790954382; cv=none; b=fyZbFbJFRYhXkHuTbB37kCJoIE25lnoaRM51N/biD+VqlyMt1/dUI4Ga+VZyHcpz55wjjQjnztE0Qs/Z1MuiuruBGoDRv10YWGk5DSjRT87IdTvA2tHKZ0Vg/kV8T+zB8r0jEhIJWJiV2NjD9viu8QHedRfpZAaX/88WSbCDD2A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790954382; c=relaxed/simple; bh=SSl2JQPa1sjMP27v12qlQtHRT4X5Eb5ZVPfcjcc+ylY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=A+VzQ2eo5/Jb/PH+xuFks5AcCVlsinj3PAxQZ0fiNTOcOp1qZw27BZ+oliY0x4CQ5z6ZxDIKa2u/3L6X3dyUriMOkJXmnIZLePbmR70EhrCNkGvQwZ1EWS5wlrWzUrDQIqcPETAWIypMwseos/1NbgDfgUO/rFZtyupNxnCjeSQ= 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=IlyejI7q; 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="IlyejI7q" 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 032FA1476; Fri, 2 Oct 2026 08:19:37 -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 641853F85F; Fri, 2 Oct 2026 08:19:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1790954380; bh=SSl2JQPa1sjMP27v12qlQtHRT4X5Eb5ZVPfcjcc+ylY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=IlyejI7qb0gs7j139i/OXFX7YSa7K8RDIzOWTpRuOE9JAmghBbLW79oG+rU9uR0nM /TUyql12KMCKrjEBq3NTEwvaca43sTKTauxz9jPkFBtfHxO2tjJ+5SNvDYkgZd4Cbt yJb4XX+muSik5WMBpZk3TvG/LO7AvQ/V672lsWiA= Date: Fri, 2 Oct 2026 16:19:32 +0100 From: Yeoreum Yun To: Muhammad Usama Anjum Cc: Yeoreum Yun , 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, Sohil Mehta , 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 Subject: Re: [PATCH 00/21] mm: change behavior of pXdp_get()/pXd_page() in compile-time folded pgtable Message-ID: References: <20260921-dummy_ptxp3-v1-0-cd40cf68242e@arm.com> <81b1fec1-3b5e-43ed-a5b3-1a73c80201d8@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <81b1fec1-3b5e-43ed-a5b3-1a73c80201d8@arm.com> On Mon, Sep 21, 2026 at 10:11:04PM +0100, Muhammad Usama Anjum wrote: > On 21/09/2026 11:55 am, Yeoreum Yun wrote: > > 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 | > > ... > > > > 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 > > I applied all 21 patches to the declared base and built and booted the > kernels before and after on x86_64 and arm64 using virtme-ng. > > Tested-by: Muhammad Usama Anjum Thanks for your testing ;) [...] -- Sincerely, Yeoreum Yun