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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 6BA9BC98307 for ; Wed, 23 Sep 2026 11:56:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=gI2ApmNHR5nvfQEPZwMmj5bem4eNho0/TBrRZbck1DQ=; b=ugx1y3TnaUbvnT M/oPeqT+humdMCMnKrxBzN2a07N8f0UKwxz3KEG6Z9SXgvBv0lAVAwCgoFQwoege7psmhwim/fZ3y mVivl7dTNgDz3kSPyup7IWG+Lqrnhojzc7Aidw+i+LzuzQqLgZ2+TeGUQQfa+UPGZlHatkfh6KEME zaLQsnuJ3CiPVGU7bHhI+5SnEOeWVPEl1vuzrC9j2ZFvMSZQW1vtzfbk3bwFfJbV7YoxYxJg4d/sZ 3uQd+xh3/byZyvr+Y3xRBzkX6u8nLCFjvwZ3sYknOscpsLVD0gueQwABLbvyVhUQvtM1OkN1QN2k1 ihAHTba0/9m/sSqR+t5w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x9LZt-000000089TE-3zRo; Wed, 23 Sep 2026 11:55:57 +0000 Received: from tor.source.kernel.org ([172.105.4.254]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x9LZs-000000089Sc-2Fd3; Wed, 23 Sep 2026 11:55:56 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 1E9936021A; Wed, 23 Sep 2026 11:55:55 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id AEE8B1F000FF; Wed, 23 Sep 2026 11:55:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790164554; bh=QFgWbkSrnnzB4diBo+Fp62kuEHCA64AGOW6NYy35L6o=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=gYz1Pt5PUhNtjbLXn1RLrw2dFysefT++UNiHs/SY0zWy5shyg/oEU6E0yjY37gDeZ 9rsf10akyHx7s8T7g8XCeeIKm6JD/TBTo+rbL9FidMwyb4MQL2taRuxWv6XNsJ2Tt4 N784KzTEgcnkVGUYZhttWOE8oa6UAPy77g1bCEwXeUUTBuPyaUy79r/OgIPnXhbNZN GAd8MzN+orb0DLZflODRpffDhPgXrHiWn71IMcOoF/KLaOTeF7y9kFNYkCXycUbuH8 FWnYsVp3707DiRyWGtxaVY+tnF+ffZNWg/NkRfDfo3LM1TPERhDg1+mA0b1Bx5ZIl+ J0ITAsTfBBCkA== Date: Wed, 23 Sep 2026 12:55:31 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: 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 , 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 Subject: Re: [PATCH v4 12/12] mm: change the contract for free_pgtables(), update docs Message-ID: References: <20260922-rcu-pagetable-freeing-v4-0-fe1ad1f1e303@kernel.org> <20260922-rcu-pagetable-freeing-v4-12-fe1ad1f1e303@kernel.org> <92b52e07-39e0-4df0-8b8d-ffa18ccc8820@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <92b52e07-39e0-4df0-8b8d-ffa18ccc8820@kernel.org> X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Wed, Sep 23, 2026 at 11:08:11AM +0200, David Hildenbrand (Arm) wrote: > 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. Yes. > > 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. Yeah I can add a new entry in the list perhaps like: * When traversing page tables under RCU alone it is important to take care when operating upon leaf entries - if the value is operated upon (for instance getting the folio associated with a PTE) an appropriate lock must be taken to prevent concurrent modification. ? > > Apart from that LGTM. > > -- > Cheers, > > David -- Cheers, Lorenzo _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv