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 37EBB3ACA79 for ; Tue, 23 Jun 2026 07:59:51 +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=1782201592; cv=none; b=hgRRnZZ1Uy1l4SBIC2GbpfrCG3ppvpqKkkuP4E113kc2uMs4QwaPKy7khRLxUadbKoBxVYOM2ktVgSUogFIaMSc/aE6gzmYyztX9WfPDeW8H/v7rDku6P9QqzRzG2hdVD1xubuKwfVM86Ds/DW68AkHFA96ssUjzCLBw6kBi58E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782201592; c=relaxed/simple; bh=djmYrBgs+oNTKNwwtfknxO1jScmzQWHgnUFQkdNnHjM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rIzC9QGYwiFZG4K8CVHmDVUf7awpaGnhnXu560ox85f+vCCMluk2BSMcAjBEnGAOsdZ0ak9vIIe4/OymtGAqVlXj+47nP5kxGbND3IVbCuYj3fUJT/WfiBr1uJHYoel7Lgys400QNllzK6fVvVCJ+Y7hNBOmE7woyR9uAaN0avw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UGrFCjlh; 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="UGrFCjlh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E4E691F000E9; Tue, 23 Jun 2026 07:59:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782201590; bh=9ReFzOAG/aop/XLXwnu7jqpdSGC0TM43hdACQPrs7w8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=UGrFCjlh1/3KCsbFeugr5KMlYdzAEX110rDTHhR7OPJdb0hkYBhpsyiLNV1y82SLa ghuhkCIK4raVycHuqabJ/g+wDVqpIQI8GtcrEbVijonVjIjR2wogh9GNM07nAxfrFy nRWZnHVXH7hEFdWCL2wsFnrvnEgKgxtr5SUyeEy+BbwjQQDufaT8Dorm7U0OPugRV1 YQUrfPvfAd0CC2soZM7OrFnvqPQIbGZHXWQ6+GQucGNdse5JHYWgULc3Ap4QkJo8hY 5j9xVS8MsDAo9c0w5mUp67sTQNwqUOa5C8e9EVkOh57Bm8k8py5NpfPyvggJTbkUMF 1HukMqUMNsnIg== Message-ID: <97f2c4ad-c302-424d-a7c8-fdfa0fc0d811@kernel.org> Date: Tue, 23 Jun 2026 09:59:45 +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: avoid KCSAN false positive in page_to_nid() To: Hui Zhu , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Hui Zhu References: <20260623074157.578113-1-hui.zhu@linux.dev> 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: <20260623074157.578113-1-hui.zhu@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/23/26 09:41, Hui Zhu wrote: > From: Hui Zhu > > KCSAN reports a data race between page_to_nid() reading page->flags > and folio_trylock()/folio_lock() doing test_and_set_bit_lock(PG_locked, > ...) on the same word from another CPU, e.g.: > > BUG: KCSAN: data-race in __lruvec_stat_mod_folio / shmem_get_folio_gfp > > The node id occupies a fixed, high bit-range of page->flags that is > set once when the page is initialized and never modified afterwards, > so it can never overlap with the low PG_locked/PG_waiters bits touched > by the folio lock path. The race is therefore harmless: page_to_nid() > always returns a consistent value regardless of how the read > interleaves with the lock bit ops. > > Wrap the flags read with data_race() to tell KCSAN this race is > intentional and benign, consistent with how page->page_type is > already annotated for similar packed-field accesses. > > Signed-off-by: Hui Zhu > --- > include/linux/mm.h | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 485df9c2dbdd..122d3b39369f 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2296,7 +2296,14 @@ static inline int memdesc_nid(memdesc_flags_t mdf) > > static inline int page_to_nid(const struct page *page) > { > - return memdesc_nid(PF_POISONED_CHECK(page)->flags); > + /* > + * The node id occupies a fixed high bit-range of page->flags > + * that is set once at page init and never changed afterwards. > + * It cannot overlap with the low PG_locked/PG_waiters bits > + * that folio_lock()/folio_unlock() concurrently update, so > + * this data race is benign. > + */ Do we really need this excessive comment? > + return memdesc_nid(data_race(PF_POISONED_CHECK(page)->flags)); In memdesc_zonenum() we use ASSERT_EXCLUSIVE_BITS. Can we do the same here inside memdesc_nid? diff --git a/include/linux/mm.h b/include/linux/mm.h index 69daeeab7fe8f..76d3bb54be844 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2290,6 +2290,7 @@ int memdesc_nid(memdesc_flags_t mdf); #else static inline int memdesc_nid(memdesc_flags_t mdf) { + ASSERT_EXCLUSIVE_BITS(mdf.f, NODES_MASK << NODES_PGSHIFT); return (mdf.f >> NODES_PGSHIFT) & NODES_MASK; } #endif -- Cheers, David