From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2BDB1352030; Mon, 13 Jul 2026 19:49:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783972197; cv=none; b=oJTvBqPpQKuC6+Q+WXkafQ3N1IJfLTEwHUIjFYrCMEuHDnbZ3/hgnuF7smFz6V8abjHi+1aIv8wLujblgoMwxledfAQE/2eZ04L05m7RjstS+eO//XDwkIVsyIKVhEKnBU71klJmJbt/LAD9wtvHiOO4ib5KNYSVe8x9Gb65w5k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783972197; c=relaxed/simple; bh=Eq7bGOWesNmzzI8pBeX6tyFZR/4dHCTHgvAbjNlqpmU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZWNVtWsqoNvoJF0vehRc7CAdrDfVwVTcUMUng06DO/0uVUBG4a/CFjWNzEFl9d1nZw8cSz0XnXuuCe+zIlsF7Xvh6kCOaUA9LD4aKSJ9Ao8wxXhN4rdIs4x1yn5Wy2+NagIjfaWXRr8t/wuBEckumZMgXLvHlBdsHUk+PFaogks= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LvSI84Kp; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LvSI84Kp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 88C921F000E9; Mon, 13 Jul 2026 19:49:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783972195; bh=IZe9zYBUgL5Fku0tOf4jGfW4HRaI0/J5FQVtBy0Ve6I=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LvSI84KpyEW4PF6a7LNDng1GF+rwXXsJKi7/6liu+LAH2SvyPUp3Y3dazy85hfiJ2 8wmORaiWbxnT9QD4Gktp63fmyZHYGlJwN0LNqE9aW/GK3kqC9X/vUDxMT2Dh2t1JkM YdU53xX2zJldz/W8YkvqFYp3XZ6jIqIACguCJ9Paeh/RuCinRwhFichfxdD0IQw8AI uzF+0XEdW14nQUGG2j3iuaQeox3i/AnIlQp+tfHaGhsUjH1ROQ6M8kUvkdg6XHz0xG 0JTaMX5GR3Vnw/gkw8JisANxpqabirohlF5apD06wIS/ws1h48ypbW2+WJUaYP+JQn 0DyzG/pm3fC1Q== Message-ID: <3ea30f8a-bb29-4bf5-8400-1c4840d46a88@kernel.org> Date: Mon, 13 Jul 2026 21:49:34 +0200 Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 10/34] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot() To: Dave Hansen , 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, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-mm@kvack.org, kasan-dev@googlegroups.com, linux-csky@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org Cc: linux@armlinux.org.uk, akpm@linux-foundation.org, ankur.a.arora@oracle.com, rppt@kernel.org, linmag7@gmail.com, chleroy@kernel.org, klarasmodin@gmail.com, chenhuacai@kernel.org, kernel@xen0n.name, kas@kernel.org, zhangtianyang@loongson.cn, wangyuli@aosc.io, tsbogend@alpha.franken.de, ljs@kernel.org, jgg@ziepe.ca, catalin.marinas@arm.com, will@kernel.org, arnd@arndb.de, ryan.roberts@arm.com, pasha.tatashin@soleen.com, rmclure@linux.ibm.com, baolin.wang@linux.alibaba.com, tj@kernel.org, kevin.brodsky@arm.com, anup@brainfault.org, atish.patra@linux.dev, pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, hannes@cmpxchg.org, mhocko@kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, kasong@tencent.com, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, ryabinin.a.a@gmail.com, glider@google.com, andreyknvl@gmail.com, dvyukov@google.com, vincenzo.frascino@arm.com, anshuman.khandual@arm.com, yang@os.amperecomputing.com, chaitanyas.prakash@arm.com, ardb@kernel.org, guoren@kernel.org, yang.li85200@gmail.com, viro@zeniv.linux.org.uk, dinguyen@kernel.org, schuster.simon@siemens-energy.com, wangruikang@iscas.ac.cn, junhui.liu@pigmoral.tech, muchun.song@linux.dev, vishal.moola@gmail.com, namcao@linutronix.de, pavel@kernel.org, djbw@kernel.org, yu-cheng.yu@intel.com, baolu.lu@linux.intel.com, Jonathan.Cameron@huawei.com, coxu@redhat.com, andreas@gaisler.com, liam@infradead.org, vbabka@kernel.org, surenb@google.com, mhocko@suse.com, geert@linux-m68k.org, shorne@gmail.com, jonas@southpole.se, stefan.kristiansson@saunalahti.fi References: <20260713135614.1618183-1-yeoreum.yun@arm.com> <20260713135614.1618183-11-yeoreum.yun@arm.com> <710b7eb0-8e0c-4f07-991c-2285c77e1beb@intel.com> <6007625e-c3f9-4ad6-99a8-61396bccbcec@kernel.org> <32d459d1-ad19-4baf-bbb1-0565458001d2@intel.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/13/26 19:57, Dave Hansen wrote: > On 7/13/26 08:02, David Hildenbrand (Arm) wrote: >> It's always been the latter: >> >> ...folding => PUD => PMD => PTE > > This is kinda arguing semantics, and there are different ways to look at > it. But I'd argue it couldn't *possibly* have always been this way. > Linux hasn't always had a PUD or a PMD for that matter. The i386 only > had two levels. PAE support was added later. > > I think the pmd_t came with Alpha support (before my time) and pud_t > came later IIRC. I'm pretty sure Linux didn't have a pud_t when I first > started hacking on it circa 2001. Yes, I'm sure there is some horrible, horrible history to how we came to what we have today :) > > Second (and again this is my subjective opinion) a pgd_t* has always > been the "top" or "base" of the page tables. This is why there's an > mm->pgd and not an mm->pud. We've also talked *forever* about things > like a "special pgd" for the LDT remap. Or having two pgd pages for KPTI. Agreed. We always called it pgd, which is just an alias to p4d/pud etc really. > > My point is that the PGD hasn't itself ever been folded. It's the top > hardware level of the page tables and always has been. With the history in mind, yes. Looking at the current state it's more complicated. Because pgd_present() are really just dummy functions. And this series proposes to let also pgdp_get() be a dummy function and disallow something like set_pgd(pgdp_get()) Which is just asking for trouble. Similar with using pgd_page(pgdp_get()) on We could get away with catching that case similar to how we catch pgd_offset() abuse, but I'd rather try get rid of the wrapper functions set_pgd() / pgd_page() entirely. But if we feel like that is too much, we can look into that direction. > > I really do think this series is changing things, not keeping them the same. Looking at it from that angle, I tend do agree: it further moves into the direction that actually the p4d_present() carries value, and the previous pgd_present() is just a dummy. I can see that some more work is required to untangle the mess we already have. I don't know why pgd_present() was made a dummy function and not p4d_present(), but I'm sure there is a good reason to it. Changing that now ... omg. -- Cheers, David