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 A72EB1CAA6D; Tue, 21 Jul 2026 18:21:41 +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=1784658102; cv=none; b=D90zTebqM16TMrojtslntT0nIvFHcPPF9/2L25P45wsWx+jiA8FNUhhxYNYOKZdr4s7OrKh5wAEszBc1PWzdEPNJLR6wKeVg+zc6878J8cvK3iHr8YDUNRy3MToBfJRJA973DfXLnRmDQAWX+gBf1T0K8HV6lYxifrQAfTocO3c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784658102; c=relaxed/simple; bh=wPeF523VPrsePN5don7EbC4JB3FiUjP6cFlZdJkEZIE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IgXNhtfLnM7fk+pnGYnJMrZffhcLsfY4I1eJZsLzFV5SKebJAWxw/DzJ/r6DLWawce4M3IByF36xE94NBuBZyoxmux1GYQcxHGceXyVeF3QEqh+XfKkQ8zEZz4jDPSJHSuU1cswAiO1IACkesRY6eQX4EgbvrkcYgHFg2/tA9U4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NBS1CkQn; 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="NBS1CkQn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3578A1F000E9; Tue, 21 Jul 2026 18:21:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784658101; bh=WED5aVE31e9UmcJgmLBdBgW0L+ihGmbx3NEOMbcoNj8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=NBS1CkQn+apjU9r6kMfeBbrfcScZVkMbtbg0YyRdbXNT+gb4TpmpHJl+AOBiN1KVA HwrHXpXfZNMr9D8vd+zfaoUpFJBcF3TlSQ4xpe/XUE/Jd2I1zSJ1+gJB/NQRHdQkiU euAQM8EjX27K5x3alp4YE9DIy6wlXv0AmkFFEPPyENPX9nkSAGX/WlmcvYG0UgaDrD oHj+KMHEfIuSx1gyjONCpBlMYtBxQ9nwdZ8hoFq5/ABYRveZE9N0ZtbiFzc9j8NwYx HKBs6XvN+IqTFBSymLn8MS6F3gx+mXhj0GSEL22UekMGMVca95FdR81fu7ZBMfmcdJ 368TJQQ7O2ErQ== Message-ID: <78087721-6598-44f7-b89d-1fd98dd94b6b@kernel.org> Date: Tue, 21 Jul 2026 20:21:03 +0200 Precedence: bulk X-Mailing-List: linux-mips@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: Yeoreum Yun Cc: Dave Hansen , 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, 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: <710b7eb0-8e0c-4f07-991c-2285c77e1beb@intel.com> <6007625e-c3f9-4ad6-99a8-61396bccbcec@kernel.org> <32d459d1-ad19-4baf-bbb1-0565458001d2@intel.com> <3ea30f8a-bb29-4bf5-8400-1c4840d46a88@kernel.org> <7e84b200-25eb-43a6-b5e2-5f27f2d82a77@intel.com> <31988089-095a-4eed-b5e2-c677c70f79f6@kernel.org> <14e250db-1641-4085-8d13-02f819657d5f@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 7/21/26 19:45, Yeoreum Yun wrote: > Hi, > >>> >>> Yes, exactly. :) >>> >>> And I agree that it is something we should be optimizing. >>> >>> >>> I'd be curious about that as well. I mean, looking at the generated code >>> it's clear that it is suboptimal. >>> >>> >>> The solution space I see: >>> >>> >>> (1) Pass the result from e.g., pgdp_get() into p4d_get(), so it can just return >>> the value with folded p4ds. >>> >>> That requires extreme amounts of churn in core-mm AFAIKS, so I don't see that as feasible. >>> >>> >>> (2) Rewrite folding code to make p4d_present() be a dummy instead of >>> pgd_present(), and make p4dp_get() return a dummy value. >>> >>> ... a lot of churn across architectures. I'm sure we'll learn soon why it was done >>> ike the way it is today in the first place. Something interesting to look at, >>> certainly, but a bit of a stretch just to optimize reads. >>> >>> >>> (3) Make folded pgdp_get() use an ordinary read instead of a READ_ONCE / dummy. >>> >>> I don't like that, because we couldn't catch easily if the value is then >>> actually used some old/new code. >>> >>> If pgd_present() etc are supposed to ignore the value, I'd rather have a mechanism >>> that enforces that the values are actually ignore in pgd_offset() etc as well. >>> >>> >>> (4) What we do in this series, but instead of forbidding set_pgd(), make it only >>> complain when we are passing in a dummy value. >>> >>> The expectation is that it would avoid touching most architecture code in patch >>> 12 -> 27 and still prevent mistakes in the future. >>> >>> @Yeoreum can you play with that and see what the end result would be? > > If there's no comment anymore for (4), Could I post next version on tomorrow? Works for me. As discussed off-list, we should look into de-duplicating some of the arch vmalloc_fault fixup code, but that will be a separate effort. -- Cheers, David