From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 3ADDFF8925F for ; Tue, 21 Apr 2026 11:21:40 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8E57C6B0088; Tue, 21 Apr 2026 07:21:39 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 895D06B0089; Tue, 21 Apr 2026 07:21:39 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 7AC706B008A; Tue, 21 Apr 2026 07:21:39 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 6B6426B0088 for ; Tue, 21 Apr 2026 07:21:39 -0400 (EDT) Received: from smtpin29.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 8DB068D03C for ; Tue, 21 Apr 2026 11:21:38 +0000 (UTC) X-FDA: 84682322676.29.CB57EFA Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf16.hostedemail.com (Postfix) with ESMTP id 6F9B7180008 for ; Tue, 21 Apr 2026 11:21:35 +0000 (UTC) Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=DZ9aeual; spf=pass (imf16.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1776770495; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=I5NaQp10fC21czwxYz1OlOt79WdfoTHaXfQ2WZiUuVM=; b=DGVGDLrRuMUjaEatCyt0ls1i0dGbfPPVJmzVhbnnpgzIENIKog6pIQoVizKp1VR6y58Ciz QRiWse4MiSeJtLbVRRenDjsjYQmusrJrdyOjnI/7j1sfDwxn0bDIgZfY1SGLTdiePkzh+O Xb7BUUVR6Ji7Lfj0xDquxXTk+RyAn0A= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1776770495; a=rsa-sha256; cv=none; b=tVFbBjn1z49UxFa7vgl5MV2mGyFYGLGstvkayjMSMSu8AMmqibKRwamjuV43KXIQ569ZWX 5WpVf7i1dk9f/wxeOsWDh/lKZ25xIBFOdmVSzQHUwY3btSjqEDHqiXZOnGKqtHbHsYH2jo SaUBQ5hTqJyO/mF9ilAalp9aKIic0hg= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=DZ9aeual; spf=pass (imf16.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 670A960125; Tue, 21 Apr 2026 11:21:34 +0000 (UTC) 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 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 X-Stat-Signature: qsyid3b8gg8cnhfeya97xb6c6n1t8fqw X-Rspam-User: X-Rspamd-Queue-Id: 6F9B7180008 X-Rspamd-Server: rspam05 X-HE-Tag: 1776770495-349595 X-HE-Meta: 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 AMDFXYkD +GovtwJpZ8nibCpuwZ5NYn4VGaMo1Fe4gguAsA5biYCmUqbxnalYa/XCHLwB7bhh0upxbU4/C8vsj+0cTgbVIDVM/FUYpph+FV/oGbp0eSusu8QnTzYeHsbBTjz8rwupzvpgsH9IzbQustH1Jnn/3mZ8Y/+agGl4x9ILM/vfHm03T3nqPcdPs0eNRvMKTFXpjNcal8v49Z9Mnib0uGNuCSnFqtQqoGAwwctFQG2TIDvu12XvPVjiMGNmpYQSLQoOPxWKVyMztzwjpuMqdFvtq9H9SlWyFyYNEem12/YZYRuJwTs+WEmio0s2tclDNr1jzZu9L6WmadZrIfeE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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