From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 7DA2235F612; Thu, 26 Mar 2026 08:42:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774514529; cv=none; b=Bi/F9as3ktXADocIdIQB64QUmEBiISN/V/M01X68N6VH8HONiPSPFve6ZRskvvnQiEsY4tvfKfn91iQ4HSC4yamXWMFSppp9263xLgw1J7eX/a1eTj2a7EBlsS6EQJnfLtE9lTR41t+KnGexG99Zn5aM1Qidk2g2C/cGq68JsMY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774514529; c=relaxed/simple; bh=4MXTZi02qDWDZ/7PsLgwqab9shZCnf3DgC2ixokvzH0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tTewgR8dPemvfCTAy2dK72PYKEB9qy4f1M7dx0EFfQnx2zbYwGKqbcAVYqLhIkOyNgxvBgODf6rpXvaVSgbi48MPV/8vlWnsYj3qaJu+lgDzUtbH9DFaE65viFQ3fDVG2HTr0CesyHqTKf/iq86FGhLyX+xBSNgX3XgTd0nF7E4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Zf5TImpo; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Zf5TImpo" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2F063C2BC87; Thu, 26 Mar 2026 08:42:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774514529; bh=4MXTZi02qDWDZ/7PsLgwqab9shZCnf3DgC2ixokvzH0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Zf5TImpomYw1infcDhd6sl6UsbdPtwCS3Ee6bIMuJgdbpciFcDhAXTBM8SenbNVkU mbjFKHsHtbKzUM0cUrUS0VgvxH98yDpS1/9tv3IrVRJM+617P1ufRtC1RhtRw1XSQt 79Dq7fPojYix+HOp8ak4xDwk6IUlmxOJR22pwa5gz5+9s0OuVkb6Zi4CNkYQzen6w2 eQhb7ZkDP8rVCeu8QWp+IAK9wq+LhPGO5kdxNtVKXF4T3mLwWR+6lsnPeDm8Mrp3o4 HXqQB8cV3KrIWhnwNFqHn0m4gEKHcs7ROdpxYT0+BQYUgaxgUXygC9/gY+ANxBhw69 /AgGEESrWp9LA== Message-ID: Date: Thu, 26 Mar 2026 09:42:02 +0100 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/pagewalk: fix race between concurrent split and refault To: Andrew Morton , mboone@akamai.com Cc: Max Boone via B4 Relay , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, stable@vger.kernel.org References: <20260325-pagewalk-check-pmd-refault-v2-1-707bff33bc60@akamai.com> <20260325175006.1c3cae2ee50dd491a153226e@linux-foundation.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: <20260325175006.1c3cae2ee50dd491a153226e@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/26/26 01:50, Andrew Morton wrote: > On Wed, 25 Mar 2026 10:59:16 +0100 Max Boone via B4 Relay wrote: > >> The splitting of a PUD entry in walk_pud_range() can race with >> a concurrent thread refaulting the PUD leaf entry causing it to >> try walking a PMD range that has disappeared. >> >> An example and reproduction of this is to try reading numa_maps of >> a process while VFIO-PCI is setting up DMA (specifically the >> vfio_pin_pages_remote call) on a large BAR for that process. >> >> This will trigger a kernel BUG: >> vfio-pci 0000:03:00.0: enabling device (0000 -> 0002) >> BUG: unable to handle page fault for address: ffffa23980000000 >> PGD 0 P4D 0 >> Oops: Oops: 0000 [#1] SMP NOPTI > > Thanks, updated. > > AI review has a couple of questions: > https://sashiko.dev/#/patchset/20260317-pagewalk-check-pmd-refault-v1-1-f699a010f2b3%40akamai.com > > It flagged the same things against the v1 patch - maybe nobody checked? > "could a concurrent thread collapse the PUD into a huge leaf right before pmd_offset() is called?" No. Collapsing while holding mmap lock etc is impossible. That's what the comment says, if there is a PUD table, the PUD table can't go away. Not to mention that a thing like "PUD collapse" does not exist. "Should pmd_offset() be passed the address of the snapshot (&pudval) instead?" No. "Can this loop infinitely on unsplittable PUD leaves? ... For device memory mapped as large PUD leaves, split_huge_pud() does nothing and the entry remains a leaf." split_huge_pud() -> __split_huge_pud() checks "pud_trans_huge()". pud_trans_huge() is mostly just a check for "is this a pud leaf". RiscV checks pud_leaf(), x86 just the _PAGE_PSE bit. PPC64 with radix the _PAGE_PTE bit. So this will match any PUD leaves, and the code will split (here clear the PTE) them. As long as pud_trans_huge() is properly implemented by an architecture making use of PUD mappings. PUD support is guarded by CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD. It's really just the three architectures above that support it. (non-present entries might not be handled properly yet, but we don't really support non-present entries on the pud level, so not a concern) Great waste of 15min of my time ;) -- Cheers, David