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 37FB2581225 for ; Wed, 9 Sep 2026 16:25:04 +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=1788971106; cv=none; b=VvaWuBza2f1R3DtYT6d8yuasQd+ZxUSN6ZCvh4wsznxzHpg5Sboq8eyRhIBszRf5vM89QaGfwpsZbPKDtt56bx3eW7gGCJG08KpMx3UGPY7g/F7krDpozqBILm/zei7j/DqV/kWLCDRNuplxNhLpsegUkYIAnbyeZ++FjRWOsTE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788971106; c=relaxed/simple; bh=n0p4U8mz3IoO3d7w6sJyCWNxihi7V1IJRiupV4z+Y1s=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uEUwCFgTpoXr8m6IDz/RGPgfTqpz49UB9EXCs2C9kEXI9ZYjZaMjVhSKtZCaIuL7mPp0eNGl8ytVBgnYDW57zeQDJAh7VYITLuWGLf0XfRG/OszcFtLz0+d7vVoohjsxKzELbqAqS4ZwrIfplvSyJEUEAFX3anlim4vuIuqVXiE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dCBJngrM; 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="dCBJngrM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E25011F000FF; Wed, 9 Sep 2026 16:24:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788971104; bh=VYLM04L7ssSmfltKij2eIwMOU1AvYN8AP+bKfG+5ZS8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=dCBJngrMDn8g9WBFEeEyvC1hW0hy881w0YKkillVQwwMV/FuxXNT0vH3Ks0Qqdeep fG6mirwDTCEuK9l+sN2QIaHbClbNksbi+bzbxN1WV3zg0ScGvcn+6M1JUKGWKAdGCU HPTuT9Bw6DcE+NZE8C1h4DbtbIn1gk4SrmmPNTB/NALlJsApX/KnW2g3Uhqhy+kyTm 6se/bnP5NEIxIq7+jfspCaeaS+M7vW7ZOYnLdkrJuiHkEJh3RnMFyhlSVYQu3kmBRg /7Y2mVGCqk4JMlDXvR4/RfHZ+aSoDqzZtinRv6GM6d4r6HAxiTZlY7qGzD2qGvW0mR aE57NbE9jeSdQ== Message-ID: <3a261c3e-4c26-41b3-b55f-5800ae1081f3@kernel.org> Date: Wed, 9 Sep 2026 18:24:52 +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] mm: implement and use vma_is_faulted(), silence KCSAN To: "Lorenzo Stoakes (ARM)" , Andrew Morton , Suren Baghdasaryan , "Liam R. Howlett" , Vlastimil Babka , Shakeel Butt , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Mike Rapoport , Michal Hocko , Xu Xin , Chengming Zhou , Jann Horn , Pedro Falcato , Rik van Riel , Harry Yoo , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park , Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Guilherme Giacomo Simoes References: <20260909-vma-is-faulted-v1-1-3a701f48984a@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: <20260909-vma-is-faulted-v1-1-3a701f48984a@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/9/26 18:14, Lorenzo Stoakes (ARM) wrote: > Provide a function to abstract the common task of checking whether > a VMA is faulted in or not. > > A VMA or mmap lock must be held when calling this function. For an attached > VMA the transitions between unfaulted/faulted state are: > > Transition | VMA/mmap Lock state > ------------------------|----------------------------------------------- > unfaulted to faulted | write lock OR read lock + mm->page_table_lock > faulted to unfaulted | write lock > > So vma_is_faulted() never provides a false positive (the lock precludes > it), but if only a read lock is held, a negative result must be re-checked > with mm->page_table_lock held. > > Detached VMAs cannot be concurrently manipulated as they are removed from > the maple tree so require no guarantees. > > Use data_race() to silence KCSAN about non-existent data races between > concurrent vma->anon_vma read/write on optimistic fault tests. > > Also while here, const-ify vma_is_attached(), vma_assert_stabilised() and > dependants. > > Finally, update the core VMA merge/split, rmap, mremap, KSM and fault > preparation callers which test vma->anon_vma directly to use > vma_is_faulted() instead. > > Note that the lockless read in reusable_anon_vma() is doing more than > checking whether the VMA is faulted - it is returning the anon_vma to be > used on fault, so this check is not altered. > > There is one odd one out - file_backed_vma_is_retractable() - which holds > neither a VMA nor mmap lock and is stabilised by the file rmap lock only. > > Therefore just add a comment to explain why the direct vma->anon_vma check > is required. > > Reported-by: Guilherme Giacomo Simoes > Closes: https://lore.kernel.org/all/20260829100034.423064-1-trintaeoitogc@gmail.com/ > Closes: https://lore.kernel.org/all/20260909115723.528501-1-trintaeoitogc@gmail.com/ > Signed-off-by: Lorenzo Stoakes (ARM) > --- Is vma_is_faulted() really the right thing to use when wanting to say that we (likely) faulted in an anon page? I think that's highly confusing, considering just MAP_SHARED mappings where that will never be true. -- Cheers, David