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 B9F8B47ACF4; Wed, 23 Sep 2026 09:08:35 +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=1790154517; cv=none; b=gAEmRKL0I3hXUtT+QFEVXBuhnx9VS2AxbKUacg09ysOKGQJZjKxM5VQs5R9NhsydbqqqOwBFi4AjBd5+CQn5we1AiwL0gZQtj9S5Aqj6R/ZqlafEWCNzTAwj/e396iRZ17BSShRzymutun4gU715pRwsSXXjbf6JrFqbS2EQzus= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790154517; c=relaxed/simple; bh=VMLvSIPNgoAspYK60dnNSjfjgckC+KZxRNVC36JgKEE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QO/+idqOWaTzY13r5qAI7HMyK8s1IR5zMZFa6Z7+/2H+FaegMvrUj2mAqxPUkTQfvVPUu3XVriF0YbDx28PFfWjLdk3Ev3TAYq/3WzfM7p3liyv7psMT3Rr+r0u0Cu+LM/ggLyM+c3j7lT5/mGMy92dbsrgGaG21M5S4+hHXk5A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LJ0LQp4w; 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="LJ0LQp4w" 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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