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 A65584519B7; Wed, 5 Aug 2026 12:07:18 +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=1785931639; cv=none; b=ZQHsR5sI80B/DqCbyzFe9ypLnTYUNY7iKoMydjx9OxzR52YYQxjIFLvX7VJh75Ksm0IedCKNEjVsIDMpXVpYhWkDvOvUGO4jNWV1kSoOyWEe6YGLhnh/2zoHBpxHR/oUEMT0UsBo3Pt9eVpWDgBTyqdDiNP6zVTN9BHiKDRK7dg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785931639; c=relaxed/simple; bh=YRcdc4mrunCunHDqvf/shzKd2lJSpl7S04iRZ8h15Oo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=paIX2Ht6Q2Ukt4JV/EUayZmIsVq+UooYZmjMl2GjpTfgYxNljxLsU07+OViFEVfZbpVnVvXGhtQMC85F20zVMg2rGjY3iXdEpvw87fYny+Eq3CDCCsVsKSvJ3O9QLw+a4YgakE0PGPbgDp9k3WPh77q1mxSO3jLZe4h4KnGFLwQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G08xuQ9v; 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="G08xuQ9v" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2A82F1F000E9; Wed, 5 Aug 2026 12:07:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785931638; bh=aGl+2n0y8opMkIO+tIk0uV2YGP/4CHQ7NMUa3eVt6C8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=G08xuQ9vztT8MWAY4GWQjNu0SA3oW0WrAzxB+WUMH6xTvVM4MnCCvAzMX4Y2bXu9/ wMQarK/Q0WzmH8cJs2dytIclXaHZiQrRUQW2XaJHZZyivkVl4Hy24GBiNCN796Df+y 2CzrddHZ/z5jmR+FqtxzKW00oGiX2uY3jNIgcnVBHu+JGJoYG6tUcNsk5lKbqM/zi4 nvu4A4Z/uwi6godbKpH5dHpUVRyOLRICUQPbrtp8FehB8uI3y++d5t0FWhLzy/JTGh PJ9ozl262SzjHQNrxDhcJRvOBQJo4aA/7EyfOJtrMCFELMOATMaHl0coqmtItf6c/P aGqVAA3RBHoPw== Message-ID: <33fc9d31-2866-4315-9510-0004deb93994@kernel.org> Date: Wed, 5 Aug 2026 14:07:01 +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: [PATCH 03/22] mm: introduce MMF_KERNEL flag and set it for init_mm To: "Christophe Leroy (CS GROUP)" , Kevin Brodsky , linux-mm@kvack.org Cc: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Pasha Tatashin , Russell King , Catalin Marinas , Will Deacon , Ryan Roberts , linux-arm-kernel@lists.infradead.org, Huacai Chen , loongarch@lists.linux.dev, "James E.J. Bottomley" , Helge Deller , linux-parisc@vger.kernel.org, Madhavan Srinivasan , Michael Ellerman , linuxppc-dev@lists.ozlabs.org, Paul Walmsley , Palmer Dabbelt , Albert Ou , linux-riscv@lists.infradead.org, Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Gerald Schaefer , linux-s390@vger.kernel.org, "David S. Miller" , Andreas Larsson , sparclinux@vger.kernel.org, Richard Weinberger , Anton Ivanov , Johannes Berg , linux-um@lists.infradead.org, Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Ning Sun , x86@kernel.org, tboot-devel@lists.sourceforge.net, Ard Biesheuvel , Ilias Apalodimas , linux-efi@vger.kernel.org, Vishal Moola , Alistair Popple , "Matthew Wilcox (Oracle)" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org References: <20260714-remove_pgtable_cdtor-v1-0-44be8a7685d7@arm.com> <20260714-remove_pgtable_cdtor-v1-3-44be8a7685d7@arm.com> <01f07ea2-8bdd-4450-8a49-548f8b646cfd@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <01f07ea2-8bdd-4450-8a49-548f8b646cfd@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 8/3/26 15:59, Christophe Leroy (CS GROUP) wrote: > > > Le 14/07/2026 à 16:03, Kevin Brodsky a écrit : >> mm code often needs to know whether some mm represents a kernel or >> user address space. This is currently done by comparing the mm >> pointer with &init_mm; besides not being particularly elegant, this >> ignores the fact that other mm's (e.g. efi_mm) may also represent >> parts of the kernel address space. >> >> Introduce a new mm flag MMF_KERNEL and set it for init_mm. >> Subsequent patches will use this flag to replace comparisons with >> &init_mm. No functional change is introduced for now. > > Did you consider performance impact ? This test is usually done in quite > critical memory handling functions. > > init_mm is known at link time. Before your patch 08/22 there is just a > comparison of mm (r3) with a constant (loaded in r10): > > c0014048 : > c0014048:    3d 40 c1 09     lis     r10,-16119 > c001404c:    39 4a 03 98     addi    r10,r10,920 > c0014050:    7c 03 50 00     cmpw    r3,r10 > c0014054:    4d 82 00 20     beqlr > ... > > After patch 08/22 we have, it first checks that mm is not 0, then it loads the > word located at mm+528 then AND it with 0x1. This load might be costly. > > c0014048 : > c0014048:    2c 03 00 00     cmpwi   r3,0 > c001404c:    7c 85 23 78     mr      r5,r4 > c0014050:    41 82 00 10     beq     c0014060 > c0014054:    81 23 02 10     lwz     r9,528(r3) > c0014058:    71 29 00 01     andi.   r9,r9,1 > c001405c:    4c 82 00 20     bnelr Is that a real problem, though? What we could do is to just keep the == &init_mm check internally on configs where we know that there is only a single such MM context. Just a thought. -- Cheers, David