From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id BBC21C98307 for ; Wed, 23 Sep 2026 09:08:39 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CA4E06B008C; Wed, 23 Sep 2026 05:08:38 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C80BC6B0093; Wed, 23 Sep 2026 05:08:38 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B6B4F6B0095; Wed, 23 Sep 2026 05:08:38 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 8E05B6B008C for ; Wed, 23 Sep 2026 05:08:38 -0400 (EDT) Received: from smtpin20.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 262741A0720 for ; Wed, 23 Sep 2026 09:08:38 +0000 (UTC) X-FDA: 85244451516.20.8A0D273 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf20.hostedemail.com (Postfix) with ESMTP id 553A81C0002 for ; Wed, 23 Sep 2026 09:08:36 +0000 (UTC) Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=LJ0LQp4w; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1790154516; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=mUB3dtDJtbKg0h/89Rij/dkAu8zr108kl0u7Zd/GSTs=; b=w68q1iF1gFfAdQ0VhF0kRugujmz0ZCNb8Y6jblNLZo7AsmvH8dVpZ7ErpThyDgtqZR6OFB OsY//IDZTC4UGFQJvKUx6Pla1VG1N7Cd0FsenUDX42G6DXQxGiC9O6NAjtIqcRCp1SSG4t zZqhmj4t0hB5UxaMaS74UapAQCtQ4Y8= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=LJ0LQp4w; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1790154516; b=GetpsR7qGO/IzNA8ry/j1c8S/mFrlr2t6UPsUf70aqmIjg3an8vyfpaf0X1mRMBefJG+hO TxCfgooxPH/5wODq+wDfcL9iPJOkU0hAvBvlAhw5pIMmmJsiElZXijcydgYkHwp7rObeP1 dXiIncNJqlg30hcAtwTJqznDDNfVGeo= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 7985E413F8; Wed, 23 Sep 2026 09:08:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 86B0C1F000FF; Wed, 23 Sep 2026 09:08:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790154515; bh=mUB3dtDJtbKg0h/89Rij/dkAu8zr108kl0u7Zd/GSTs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LJ0LQp4wN89aKqlJd06DaCcBQBpV7ry3y27/DBdd+/AC1PsXGbcwRIqlFJVAegiTa oR80pxJR3nlg7MU1ZcoK9TcWgnV1c3oIFnMmd9gtUuezuS07DbQEuhV5MFNUiAR2oC r3wp94UXPgLNXIDb0SCYlTGTZeyrPFyy93WceP9DYN6IKram+1g7xxxQ4UkjiIJbHQ yeETtvktdPZlGl9TDjTuk/WpBV0WqKdpeG9gv5Y2AXMESWyJDgYxX6GCCnS38Pciys 4Y4X9gsrWo0YOnRthqVe1Aw62X3ry+euUqxmIrYpnQAp96ns8D3Bn9jHtcFEkfU9HH Sy3YYGPfnp+AQ== Message-ID: <92b52e07-39e0-4df0-8b8d-ffa18ccc8820@kernel.org> Date: Wed, 23 Sep 2026 11:08:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 12/12] mm: change the contract for free_pgtables(), update docs To: "Lorenzo Stoakes (ARM)" , Andrew Morton , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Guo Ren , Brian Cain , Geert Uytterhoeven , Dinh Nguyen , Simon Schuster , Jonas Bonn , Stefan Kristiansson , Stafford Horne , Rich Felker , John Paul Adrian Glaubitz , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Russell King , Vineet Gupta , Michal Simek , Chris Zankel , Max Filippov , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , "David S. Miller" , Andreas Larsson , Richard Henderson , Matt Turner , Magnus Lindholm , Catalin Marinas , Mark Rutland , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , "James E.J. Bottomley" , Helge Deller , Madhavan Srinivasan , Michael Ellerman , "Christophe Leroy (CS GROUP)" , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Richard Weinberger , Anton Ivanov , Johannes Berg , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Arnd Bergmann , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jason Gunthorpe , John Hubbard , Peter Xu , Yoshinori Sato , Shakeel Butt , Jonathan Corbet , Randy Dunlap Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org, linux-sh@vger.kernel.org, linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-snps-arc@lists.infradead.org, linux-arch@vger.kernel.org, sparclinux@vger.kernel.org, linux-alpha@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-um@lists.infradead.org, Hugh Dickins , Qi Zheng , linux-doc@vger.kernel.org References: <20260922-rcu-pagetable-freeing-v4-0-fe1ad1f1e303@kernel.org> <20260922-rcu-pagetable-freeing-v4-12-fe1ad1f1e303@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: <20260922-rcu-pagetable-freeing-v4-12-fe1ad1f1e303@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: amtqopuwta31dhfgk4o6hxqbqd4skowo X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 553A81C0002 X-HE-Tag: 1790154516-821651 X-HE-Meta: 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 kn4GAhZz pdH4ur3PgwQx4DF41OdWD5l8saeHcttMQJXzMy4pyFhPcAGVc43/H9cVHR+cQU0YDyqYV1TPkz0gDcPOujc7otHnADn4R96Q/RFojAKqJM+7xDcnqet3q47fFd7Q0QE+OfxymiMqqPj09Ky657j1jUyGqieEUmUo9qJfBBly0T9Hhljbfr8DknYNk9HO/dwNLI+xzg5qZ49SogWm9duxm3fJT26u0FDokqPvXz26LLr0fqsVsw/N3via5HV280xZYpixQclzzf+HwPCmeQv+YZ+H5XFx18j8WBXjhnxouD3eV0Cpi4AlitgVQ8i03OXTMoNst/1arxFk6ScUrEaI0kySzI1y3xF3HVqpmDAJMivPdkhFoHEDu+l5CbPZ/G+wsABT/NF7MZ0+nDG8ZeYDXUTvO5w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/22/26 17:35, Lorenzo Stoakes (ARM) wrote: > Now that page tables are freed after an RCU grace period, it is safe for > read-only page table walkers to walk page table ranges that are being > concurrently torn down, provided the mm is kept alive via mmgrab(). > > It is however unsafe for writers to do so, as they must obtain an > appropriate lock to do so safely. > > Update the pte_offset_map_lock()'s comment block to reflect this. > > Similarly update the process addresses documentation. > > Acked-by: Kiryl Shutsemau (Meta) > Signed-off-by: Lorenzo Stoakes (ARM) > --- > Documentation/mm/process_addrs.rst | 6 ++++++ > mm/pgtable-generic.c | 15 +++++++++++---- > 2 files changed, 17 insertions(+), 4 deletions(-) > > diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst > index a7296f251799..b1f4f44d75eb 100644 > --- a/Documentation/mm/process_addrs.rst > +++ b/Documentation/mm/process_addrs.rst > @@ -537,6 +537,12 @@ We establish basic locking rules when interacting with page tables: > * When changing a page table entry the page table lock for that page table > **must** be held, except if you can safely assume nobody can access the page > tables concurrently (such as on invocation of :c:func:`!free_pgtables`). > +* Page tables may be *walked* under RCU alone, as page tables are freed only > + after an RCU grace period has elapsed. However, any entry found must be > + revalidated after the page table lock is taken (such as the > + :c:func:`!pmd_same` recheck performed by :c:func:`!pte_offset_map_lock`) > + before it is acted upon. Changing an entry requires the page table > + lock and one of the locks that excludes teardown (mmap or VMA lock). While we can walk the page tables, I assume there are some limits to what we can actually do with leaf entries. E.g., Doing a careless pte->folio lookup might be dangerous. I wonder if we want to hint at that here: that walking under RCU (traversing page tables) is something different than actually operating on the leaf entries. Apart from that LGTM. -- Cheers, David