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 6CF31280CD5; Tue, 21 Apr 2026 11:21:34 +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=1776770494; cv=none; b=dea+/t2FEld7scoY5yLjdnhD2XeJ5YvNlRRmDWIsO9E/CP5EJ62S3R7T6HbbHDDZa1H4XE+OksWD5VGnfQDbrC+Hm/A4PSkYBjUP8FbYt/g5w2nGZw3Q8qcwYlvQqVRmcOhVbDQgC+UVjeii7+qTnpMLgP3Dqef8CKpEhrkWj54= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776770494; c=relaxed/simple; bh=9SngLDlLSrRZtWzwN1nn6cQq5z3isskMTiIoeHQPXUY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MNFCjOPnRMkpHOcFr0aAouAguxiDX7o5ipq5gg8rRI6dn/iwV5j2q41IRfPkI3DrWwWRr0cGX36QMqmLYSgYnuC3ZDTOIVepvEt6+45nYOfhQFevGnocmu54aCsO/6173K1qS9KFPuV1xa8PfTL5BaGN2fZb1krEY51eK5HYlqY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=DZ9aeual; 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="DZ9aeual" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E2F2DC2BCB0; Tue, 21 Apr 2026 11:21:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776770494; bh=9SngLDlLSrRZtWzwN1nn6cQq5z3isskMTiIoeHQPXUY=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=DZ9aeualfrYF8+3WcZLwsJEpJIz2cA51j5ULqOULX1PxTiQg+XOAQTK212j2XfngN wYQQbLy+bmca0BeeYnHwe/1dOGQ5ZJaAWXiiLJXEH9VIGrncejeND4K3NzDiL3wA4K zvgd7vFxKb09ENWR8Oov6pCqX2MU8yvfYAB5kpOFXig19tU/qQIlMAVDM49vmEpRC5 9PGbX6lRG22bqjNnvauJQaYqCAIvK6IB9wbh7HxUziTVqKJqWTHWqrhf/tL0Dq4gO/ fRRJeUs7R+8NVUnAtfMw6mxlSI+fWq6JnU6jWG/4lOmmlX1/n4Zg4xqkk7ENA4k4ec MCzr7KpmpQsNQ== Message-ID: <76cec69e-6ee1-4630-847b-80dc1e49d643@kernel.org> Date: Tue, 21 Apr 2026 13:21:27 +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/sparse: Fix race on mem_section->usage in pfn walkers To: Muchun Song Cc: Muchun Song , Andrew Morton , Oscar Salvador , Charan Teja Kalla , Kairui Song , Qi Zheng , Shakeel Butt , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-cxl@vger.kernel.org References: <20260415022326.53218-1-songmuchun@bytedance.com> <09d1ef13-e02b-41e9-b431-4c3254e42011@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 4/15/26 11:20, Muchun Song wrote: > > >> On Apr 15, 2026, at 16:04, David Hildenbrand (Arm) wrote: >> >> On 4/15/26 04:23, Muchun Song wrote: >>> When memory is hot-removed, section_deactivate() can tear down >>> mem_section->usage while concurrent pfn walkers still inspect the >>> subsection map via pfn_section_valid() or pfn_section_first_valid(). >> >> I'll note that it's all racy either way: someone checking pfn_valid() / >> pfn_to_online_page() can race with concurrent unplug. > > Agree. When I first saw the commit message for 5ec8e8ea8b77, I was curious > because the goal of this commit was to fix an access issue with ms->usage. > Looking at the race diagram, I realized that while this only addresses the > ->usage access, subsequent accesses to struct page will still be problematic. > It's just that the former issue happened to be triggered first in this specific > commit. > >> >> We've known that for years; it's hard to fix; it never ever triggers :) > > Glad to know my analysis wasn't off! It seems I've just stumbled upon a > 'well-known secret' within the community. :) Heh, yes. > >> >> So is this really worth it, when we should in fact, work on protecting >> the users of pfn_valid() / pfn_to_online_page() with rcu or similar? > > I am not sure if it is worth fixing, especially since I just realized the > community has been aware of this issue for many years. If we do decide to > fix it, I think the most straightforward approach would be to protect it > using RCU, something like: > > # the user side of pfn_to_online_page(): > rcu_read_lock(); > page = pfn_to_online_page(); > if (!get_page_unless_zero(page)) > goto out_unlock; > rcu_read_unlock(); Right, but we'd have to protect against the sections being marked as offline as well here, though. So against a pure concurrent offline_pages(). If you're looking for a project, this is really one worth doing! :) pfn_to_online_page() is more in need for protection than pfn_valid() I think. -- Cheers, David