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 E90BC387357; Tue, 24 Mar 2026 08:05:47 +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=1774339548; cv=none; b=rDw+hJ0w86auIsBqmE5DHJh0Y1qaDrb6F5mlxaxt1UZvA1JiULR1sNYxrqdcMi1i/fY7u3Mczp+R3iWcWah34D1jixRNaTZDVAIBl4Tiq6gtTKoe9RlXp1eTDbbftCajmPGbPcjnH6NuwikDa4rFLR81FAXw+I/ZczY4BfDxtzw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774339548; c=relaxed/simple; bh=R3tUPav66vuh2+XNlYS0TIlLO+2gZChki+fScnkn0+o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=SncQNSKrq/Tc7X24Q0HMrdysZLMuJ/0DKaFu+/+RpfZ5l2FN3M1Mq4MPInl9ZjSa6OXhmqUxexORDXtK4Qo4No5BRJ8xULv/UoeRweIHOwV2x+9SZkvFM8HTpjXWFErrh/s2bjQxAIFAFU8E1aNv9LUyNL5aMyPkeVOUXmjVX54= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=goE3OWbq; 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="goE3OWbq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F38E7C19424; Tue, 24 Mar 2026 08:05:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774339547; bh=R3tUPav66vuh2+XNlYS0TIlLO+2gZChki+fScnkn0+o=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=goE3OWbq+wiuW7jn3ojwJ2aTTY3bepFl3ITL6wOa9ecfISbctDqFx0fDwqDYbZMgI mY8GssZHDoARjvmjQjN04YED+wZsyAXGuYgCAXTrt1K8/QbAhX2xbw0G54tDycbYoJ HT2LSHIQplpmbAPpJ5ya+feBbIN/2z/TAsysqFDwL8NQSG/RHRIlkMUfL8BjUVwgQT GQqIQDYV4u0uk6y+AQ7MBUZxFygEklypAhv0pKEOLnRJZoD9Hayno5z3plsaLQE0uh lue6CMPXDV9Q34VMIYC8z6Ijwc0wOEZ1CpmC0d+nM4rbffwjlGPDjQTO6osa6J04wp kNMg/i54lGekA== Message-ID: <3ddd7629-7282-4ce9-9de5-9de201ea395d@kernel.org> Date: Tue, 24 Mar 2026 09:05:41 +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] mm/memory: fix PMD/PUD checks in follow_pfnmap_start() To: "Vlastimil Babka (SUSE)" , linux-kernel@vger.kernel.org Cc: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Peter Xu , linux-mm@kvack.org, Alex Williamson , Max Boone , stable@vger.kernel.org References: <20260323-follow_pfnmap_fix-v1-1-5b0ec10872b3@kernel.org> <47346263-ee56-4a78-b55d-7f28c617c673@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: <47346263-ee56-4a78-b55d-7f28c617c673@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/24/26 08:33, Vlastimil Babka (SUSE) wrote: > On 3/23/26 21:20, David Hildenbrand (Arm) wrote: >> follow_pfnmap_start() suffers from two problems: >> >> (1) We are not re-fetching the pmd/pud after taking the PTL >> >> Therefore, we are not properly stabilizing what the lock lock actually >> protects. If there is concurrent zapping, we would indicate to the >> caller that we found an entry, however, that entry might already have >> been invalidated, or contain a different PFN after taking the lock. >> >> Properly use pmdp_get() / pudp_get() after taking the lock. >> >> (2) pmd_leaf() / pud_leaf() are not well defined on non-present entries >> >> pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries. >> >> There is no real guarantee that pmd_leaf()/pud_leaf() returns something >> reasonable on non-present entries. Most architectures indeed either >> perform a present check or make it work by smart use of flags. >> >> However, for example loongarch checks the _PAGE_HUGE flag in pmd_leaf(), >> and always sets the _PAGE_HUGE flag in __swp_entry_to_pmd(). Whereby >> pmd_trans_huge() explicitly checks pmd_present(), pmd_leaf() does not >> do that. >> >> Let's check pmd_present()/pud_present() before assuming "the is a >> present PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page >> table handling code that traverses user page tables does. >> >> Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP, >> (1) is likely more relevant than (2). It is questionable how often (1) >> would actually trigger, but let's CC stable to be sure. >> >> This was found by code inspection. >> >> Fixes: 6da8e9634bb7 ("mm: new follow_pfnmap API") >> Cc: stable@vger.kernel.org >> Signed-off-by: David Hildenbrand (Arm) > > Acked-by: Vlastimil Babka (SUSE) > > Should we also convert pgd_none() and p4d_none() checks to X_present() checks? For these, we don't really support non-present entries yet (excluding hugetlb, hmmm).. I have some more cleanups laying around here, I'll throw that in as well, thanks! -- Cheers, David