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 48F5247988A; Wed, 23 Sep 2026 09:04:49 +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=1790154290; cv=none; b=MnlEK7FVYFGqLtEaP27tPe/KFiVSTkq4hQQktVsc/8sXKqtxVcxbhzdAGeCoVXLwpD+FORRCFLU9tghaSyFIMVT6EUPSKiGWQZokXpAvcJqlTi57P4EUV+515odzQEPHQgdCWT9RZ4Be3cNKnVZXaiwHQRMoZn19cNHB7K3krcE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790154290; c=relaxed/simple; bh=gEadWWeVwsP72AdTcrIu/BrymxUSb9bfTf85AP35pSs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JrCFdq3QY0ccmner9Hf4ZF3fu44YbLIvLiFuCC1uPPf6AnqlIAYlM4ouyIKHfcyRm6lh/Hsdy/1obTTzqPwsmKucrOhD2my8ZSbswn1AbDraIDdAVi5+N+5b3+aqsJBdEbezeZcBslm3X1uyZ/5wAfzOEqWnWA/FaZwo7mcet5Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kh1UONb7; 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="kh1UONb7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DB6821F000FF; Wed, 23 Sep 2026 09:04:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790154288; bh=nLHxCy9N9i62IPUkO2MSj7TD6JUKjGO86GHnnJbc2pQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kh1UONb749avQ2GxPrEt2KoDCYqbkJnuEA3WJQAQ8JrfHuUaTqmRRSt+oFpaQ7lG1 ILnPGbWd/UazinfGWFBjRdNkvn/tu+pWKz4Lq2pV4pOq8TzUuzRHULxonAZBgbJk7H 6CtvWjimQdjfbvI2Qy0fLwPnggwdlj0TkjEbxzEvV7wBXSai+0OqQFH5kgVoCZ0KpY U6KeqOnEa1DpFksg3Q9CpkhGA/BUNk2ePpUHAvUu+CsQpwJHlIjrHYn8a7BMPOZT39 ftQVWCDrBTlTq6hiuY4SXKY/CE93NMwJj2wtTo1UUe0tma4NBXjXH8U25SuKms+7GR ct8e/Zv52t1KA== Message-ID: <72d1548a-a4ee-4832-b2ab-4bc82f1933f8@kernel.org> Date: Wed, 23 Sep 2026 11:04:17 +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 11/12] mm: make userland page table freeing RCU-safe 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-v4-11-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-v4-11-fe1ad1f1e303@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/22/26 17:35, Lorenzo Stoakes (ARM) wrote: The patch subject confused me at first: that's not what the patch is doing, no? Previous patches did that, and now you're just removing the leftovers? Maybe I'm missing something that this patch does along the lines of teaching the code to be RCU-safe. > Now every architecture has been converted to support > CONFIG_MMU_GATHER_RCU_TABLE_FREE, this configuration option no longer makes > any sense to keep around. > > Therefore remove it, and remove all the dead code that existed for > !CONFIG_MMU_GATHER_RCU_TABLE_FREE architectures previously. > > Additionally, CONFIG_MMU_GATHER_TABLE_FREE is no longer necessary, as all > architectures instead use CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE when a custom > __tlb_remove_table() is required, so remove this too. > > A number of architectures only enabled CONFIG_MMU_GATHER_RCU_TABLE_FREE if > CONFIG_MMU was set, however the mmu_gather logic only actually does > something meaningful if CONFIG_MMU is set (mmu_gather.c is only compiled in > this case, for instance). > > As a result, there's no need to gate any of this logic on CONFIG_MMU > explicitly. > > CONFIG_PT_RECLAIM however does have a strict dependency on CONFIG_MMU, so > make this dependency explicit. > > Additionally, correct comments to remove references to non-RCU page table > gathering and make it clear that this is not 'semi-RCU', nor has it been > since commit 1fb3d8c20bfa ("mm/mmu_gather: replace IPI with > synchronize_rcu() when batch allocation fails"). > > With this change in place the kernel policy is now that userspace page > tables are freed after an RCU grace period, and thus it is now safe to > unconditionally perform page table walks under RCU, safe in the knowledge > that page tables will not be freed underneath the walker. > > This is all that is guaranteed, however, so naturally it is still incumbent > upon page table walkers to ensure that the page table entries are as > expected. > > Acked-by: Kiryl Shutsemau (Meta) > Signed-off-by: Lorenzo Stoakes (ARM) [...] > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -1465,7 +1465,7 @@ config HAVE_ARCH_TLB_REMOVE_TABLE > > config PT_RECLAIM > def_bool y > - depends on MMU_GATHER_RCU_TABLE_FREE && !HAVE_ARCH_TLB_REMOVE_TABLE > + depends on MMU && !HAVE_ARCH_TLB_REMOVE_TABLE > help > Try to reclaim empty user page table pages in paths other than munmap > and exit_mmap path. $ git grep PT_RECLAIM mm/Kconfig:config PT_RECLAIM mm/memory.c: if (!IS_ENABLED(CONFIG_PT_RECLAIM)) mm/mmu_gather.c:#ifdef CONFIG_PT_RECLAIM mm/mmu_gather.c:#endif /* CONFIG_PT_RECLAIM */ whereby all files are MMU-only. Makes sense. Very nice Acked-by: David Hildenbrand (Arm) -- Cheers, David