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 8EDA438C2C0 for ; Wed, 9 Sep 2026 17:33:09 +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=1788975190; cv=none; b=hgCbxIU3Y1PnE9RzWT1h41ijPmCeDVSrhWa9CKyAZ/TmBK04bE+umxW02mfa44XZh2KePK5mO8D0JF8cmMCVrWhUXGj2IsrTzxLPaHFjBYRvQWFoZevWh00zlBWpoTYE3CLa10OiLa1Y4FlZj27gL5j3g+QGTq62qzojHe3HF7o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788975190; c=relaxed/simple; bh=SURE3r0SmnxKIsk3mBFbCEYHpUR7xImyMM/JfWV/2dc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kPP2rW7H+RzLIW9QLdk87cJkIFQwNVxpStcw5T1kKGYTq4a1y76LRTP0JnZhPazl73CQzkQr/NT1pjSH2r9EsylV3RAf6ng6cL8wi57xd/zPQjBlduPd1gAv388B/Ng86/03phSya1KT8drDVUdjLzK1OjixlzaGLoMUbxZv0qA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hhGhkxbF; 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="hhGhkxbF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DF0AD1F00893; Wed, 9 Sep 2026 17:32:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788975189; bh=aDg0GpWzq1Pq3pQXFSFSo5nmvCOOz/Q/krQK/wQhrPE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hhGhkxbFISlcHv9SPA0uDsuBkCiT7FM/Ywv8L5DtjrH5mK8FEPRbtFXY/qfoAHX5m AuC2nxFCdMlmAaaaGlvuA5LtU7PmujyXQpktKY4X6swt7Y98dNJeXk4ibKCFOb1W8l Y/cUsyy8RqP1Wd9By4OtdJYUWJtKxj6+P99H61w29r3F8dDFYjcBLzKw4zeIg3Ub5F BBeHlObbqaheSW6s47D/BQBHcg9lLFJ1bmtiQsoJt6nLrTIPBu7KmRGWJ31J7LkHMK eCce1TAWkX/fJoaMOaoyYBXNLD2qp6feZMjf2vrI/686I9/uwObR0J9x81IWnNJP/k 5XnfYePZjDkWg== Message-ID: Date: Wed, 9 Sep 2026 19:32:57 +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)" Cc: 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 , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Guilherme Giacomo Simoes References: <20260909-vma-is-faulted-v1-1-3a701f48984a@kernel.org> <3a261c3e-4c26-41b3-b55f-5800ae1081f3@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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/9/26 18:36, Lorenzo Stoakes (ARM) wrote: > On Wed, Sep 09, 2026 at 06:24:52PM +0200, David Hildenbrand (Arm) wrote: >> 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. > > Hmm, yeah. > > I mean there's nowhere you'd do this check where you weren't checking something > that couldn't at least in theory be anon-faulted. But it's confusing vs. shared > you're right. > > vma_is_anon_faulted()? Hm, not sure. vma_had_anon_fault() Might sound better. Alternatively: vma_has_anon_vma() is the obvious thing we're checking. vma_might_have_anon_folios() would be the clearest (no anon_vma -> no anon folios). But semantically that's likely not what you want to check in the code? The whole faulted/unfaulted terminology is a bit confusing ... -- Cheers, David