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 9C2BE582B8B; Wed, 9 Sep 2026 16:43:11 +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=1788972193; cv=none; b=pOC7xacY+Mtz8iGRknzUSq0fjGn90abmNQjz7A/8PsasikyXeE2LkcovhpUPVmjoB6Zk7+/NRTqNpc4ROp9us8U/vJ5DIhJFfGXl7f+F4ZEaSrKBxgAndlAsE6dqt1D2pFBzHNTU2p8icqJd15/PikdbBdEtRgOCNn1YC5VQzRs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788972193; c=relaxed/simple; bh=mjMEDB4wGR/2HGZp+iWAWmZt9I8i7GrCCpgQk6Cnjwk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=oUV9SPD3ajjoIptOEvVV0iF4xtc3lIY/g8ibNYQdV+tKIAiDMmjlLCVf/W8rD9qF86EF8lm/R31mz9gTfTb6840U5T7go2BT+sTOcwsKy499AdrCqLk+/QgxwL43ZnimyaozzUpYf80OrQ4mJ5u8Lbu++e7/ta8jETDiz7r/yT8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gIPVuofi; 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="gIPVuofi" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0F4721F000FF; Wed, 9 Sep 2026 16:42:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788972191; bh=+bb9T4qzNzmlycp9ThZUfYsXOaxsYgWc9M6tavwahvE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=gIPVuofiiuXzwRKr3oimigWerGVK3163gi1cAcgzOdWIHvv7vHRHaOOm70oBmvuof gM+DrCV50r5q2fX37AG+T2Z9/95C3wKDpEICIKuj714cWc9kpK2a643U857hnxKrt6 F+VhD/Iigk9dREUeU4ysQJtISisTduJzSnmSk21qc6hooSphBl5H9CJ0/x7dZKu/Q6 u+L9ir5TwzY1RhFgoDiNZuSP0ijZXhIOTL8sytWHlzA6zcGHNHpORsIUOjNpgYzeac YkdgjhwiwM1Yl/vDXjEBIxQFkvn0uBOf3YaFLZFvbyOIKYs37Ib6WZ5TYOCA3N7iFE 8WN0ksIUU49KQ== Date: Wed, 9 Sep 2026 17:42:45 +0100 From: "Lorenzo Stoakes (ARM)" To: Kiryl Shutsemau Cc: Andrew Morton , David Hildenbrand , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Guo Ren , Brian Cain , Geert Uytterhoeven , Dinh Nguyen , Simon Schuster , Jonas Bonn , Stefan Kristiansson , Stafford Horne , Yoshinori Sato , 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 , 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 Subject: Re: [PATCH v2 12/12] mm: change the contract for free_pgtables(), update docs Message-ID: References: <20260908-rcu-pagetable-freeing-v2-0-1f60b64e878e@kernel.org> <20260908-rcu-pagetable-freeing-v2-12-1f60b64e878e@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Sep 09, 2026 at 10:24:21AM +0100, Kiryl Shutsemau wrote: > On Tue, Sep 08, 2026 at 01:32:21PM +0100, 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. > > > > 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..1e65b139f355 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 always requires the page table > > + lock. > > This says the PTL plus a recheck is enough to act on the entry. It is > not: free_pte_range() clears the PMD without the PTL, so pmd_same() can > pass and the table is torn down right after. That is the case we settled > on for the pte_offset_map_lock() comment, and the two now disagree. > > Something like "Changing an entry requires the page table lock and one > of the locks that excludes teardown (mmap or VMA lock)" would match the > comment. Ack will change! -- Cheers, Lorenzo