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 48357CDB481 for ; Wed, 24 Jun 2026 14:38:31 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 22D236B00A3; Wed, 24 Jun 2026 10:38:30 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1DD236B00A5; Wed, 24 Jun 2026 10:38:30 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0CE136B00A6; Wed, 24 Jun 2026 10:38:30 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id C77F86B00A3 for ; Wed, 24 Jun 2026 10:38:29 -0400 (EDT) Received: from smtpin16.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay02.hostedemail.com (Postfix) with ESMTP id 490F412039B for ; Wed, 24 Jun 2026 14:38:29 +0000 (UTC) X-FDA: 84915061938.16.D40A263 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf04.hostedemail.com (Postfix) with ESMTP id 866754000B for ; Wed, 24 Jun 2026 14:38:27 +0000 (UTC) Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=bVIuV7Vx; spf=pass (imf04.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-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1782311907; b=EdxD0NbnI1HUNn88vg8kQHxHc08snnoVXIIMP6+kAoEgjgNWV0YDDbQc2qeaUbDhtUHprL V7dzwAD+sPc76vEhgGPJxWZtM/pZaVMhPh23vEdG/xak0Nbw/VEg5DNnTHn2kLqrrU/NP/ U+3jdMIE5RWVLTLpcVmxG0En57EH0qM= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1782311907; 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=yhT0/LlB1rg0W3DefCcQyBJOT+qvVyNtU9gdCazTVH4=; b=eIxxBVF3jtesjdm41pGNeDZ2VCqyVfsjRziLklZrSCKPFdIEehMDQXsV9Kvy1KqQ5VK/aS nx/QmDS1tVRFBs2YmbFAadae3Riy0PMXF5SCkA5FiuGVmBFSuyHsD7MbmWX4hS/xvLTzrg pYTaG/qMj6CY+tI1pIyjM1S0IC51sXE= ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=bVIuV7Vx; spf=pass (imf04.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 (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id F32B260018; Wed, 24 Jun 2026 14:38:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3A84F1F000E9; Wed, 24 Jun 2026 14:38:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782311906; bh=yhT0/LlB1rg0W3DefCcQyBJOT+qvVyNtU9gdCazTVH4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=bVIuV7VxLcd9Iyb4ItgZDPIicoaxyovoOREIAv3mS6bxGCWSf1UR6dwmXjtaW3Hb2 gzINqdxbMqTQKYLuuwdILpR74+tToCf7tsmpY7gZeWDbtREvlshPCX8RxgqmGtvy7i 6ry/ibiDzDBGkq4XRfsfF4+P3n43NVatj9k/zbjyMGY1qnK7icBRoR1qfPz2tRlxaR tOfPOQaOcSXtuQlOdujaM1cCzyEPs//51CKSogBxrMeG6sNyMlS9qhB/B8Pe75tuc2 5h3OwImAhV1NuwoWTL3nPgeiuCjQNUrSJAlDcmCX0Osr43tZVCMa8VCLWy4xb8Wi4P woG1M3AZzHMgQ== Message-ID: <8bff931f-6e32-4506-8b3e-92a494e796e4@kernel.org> Date: Wed, 24 Jun 2026 16:38:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm: fix data-race in folio_batch_count() To: Lorenzo Stoakes , Xuewen Wang Cc: akpm@linux-foundation.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260624092606.1083449-1-wangxuewen@kylinos.cn> 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: hc1hb3e3ycs3hmgo571y75k4tcx55ybq X-Rspamd-Queue-Id: 866754000B X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1782311907-725693 X-HE-Meta: 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 JuQA20p0 s3IVGaJnK4x5ciLymjgm1t8x4o04fwJ7BeOPJXFM+YU0yB2diiQxnV9WuVmVLJnFriGHBVvRnA9zmfC0xCUjt19yxV1qGn+qfwBb8AK1k7jqWYdhzaSLI6awINuEsNNWjW0of54pO/9sD+dcA58DcwPIIJ3YLx2jIujmhvw7mUWpyPk0sOxXEDwk33tk75HdLky9taPWD3SnuNMXnfqegTNvsaFhiVeCRfunADnukG5T3r1OxIz5tLRJpfvM65dgJoAIhm5Z9J/YRCbrjLTrWApG9RlbTYLYiWatk8frG57QFrXKI7+HHaivbZA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/24/26 16:23, Lorenzo Stoakes wrote: > On Wed, Jun 24, 2026 at 05:26:06PM +0800, Xuewen Wang wrote: >> KCSAN reports: >> >> BUG: KCSAN: data-race in __lru_add_drain_all / folio_batch_add_and_move > > Where? A syzbot report? A local run? Please specify. > >> >> write to 0xffff98fe74c015f8 of 1 bytes by task 45153 on cpu 2: >> folio_batch_add+0x30/0xe0 >> >> read to 0xffff98fe74c015f8 of 1 bytes by task 45175 on cpu 0: >> folio_batch_count+0x0/0x10 >> cpu_needs_drain+0x253/0x430 >> >> The write side is a per-cpu local operation (folio_batch_add on the >> CPU that owns the per-cpu batch), while cpu_needs_drain() reads >> another CPU's per-cpu batch without locking. Reading a slightly stale >> value is harmless -- it only determines whether to schedule a drain, > > Then why are we adding a READ_ONCE() in such a core helper? > >> and a subsequent check will catch it. > > Where? Which check? Be specific. > >> >> Use READ_ONCE() to annotate the read and prevent load tearing, which >> also suppresses the KCSAN warning. > > Tearing on a single byte? Which architecture tears a single byte? > > I think you're actually more concerned about the value being optimised out on > assumption of it not being updated elsewhere right? > > But acutally you're not, because everybody else uses a stack variable or _their > own_ per-CPU value? > > Only cpu_needs_drain() is the odd one out right? > >> >> Signed-off-by: Xuewen Wang >> --- >> include/linux/folio_batch.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/include/linux/folio_batch.h b/include/linux/folio_batch.h >> index b45946adc50b..1e31e058e19d 100644 >> --- a/include/linux/folio_batch.h >> +++ b/include/linux/folio_batch.h >> @@ -53,7 +53,7 @@ static inline void folio_batch_reinit(struct folio_batch *fbatch) >> >> static inline unsigned int folio_batch_count(const struct folio_batch *fbatch) >> { >> - return fbatch->nr; >> + return READ_ONCE(fbatch->nr); > > This isn't free, you're breaking optimisations here by doing that... > > It feels like the wrong level of abstraction, but actually I think every other > case is either stack or per-CPU _on its own CPU_ (please check), in which case > we _can_ suppress the check here but I think best done with data_race(). Fully agreed. -- Cheers, David