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 9CA6D3EDAD6; Mon, 29 Jun 2026 08:21:53 +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=1782721316; cv=none; b=B2D8405IXmRUOVSAor840k5ipB/E3LPkfgkJD7hbYL94BaHlzJk83NOe8dCtSPaYZkWuQ/Ypg0z+734LLfsmgZTWWUSm4mEbuA0XyiZkMlgkpzWdmTeH0ekz94YLg2/tLGSu9M534++xSf+Ey9mbY+ENRaNkmM66zbxSLHOEPRQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782721316; c=relaxed/simple; bh=CsK48eskqzrOK+DeezgWuHZH4AmHrnlPYP2SOXyzods=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VCUUCtLxobGj4YlaNU5PqyHiMqaJmojG8mMmGuvrKV57s7VBhIw6oO9AE6CQspIDm9yZpl73NUtirAtmIEmHglWyE67BgNVQZ4vX8yvpb56wIYZiUy3SP1sh2V0+c1Y2d6HKpNdH51ClzOjyIW3sGJZ6YCh01JF/YNSenuywZZg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eJM17GOj; 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="eJM17GOj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D59D11F000E9; Mon, 29 Jun 2026 08:21:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782721312; bh=PTgXkbvVLbmjixblka4EX/WB3VrVuB2RapiXZ8W+wIg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=eJM17GOjQPe7POwFKir2rKmQvXq1KG08ah06Lp5zlKQ3rzcdympN++OKKa2ZVsntv WTQC2r8gNLJclFMRpjNVhWgB3qqGuos9yaea9mmVHx/HvYLL1tYLsPlNNedCSiau3z A6eL0rLU0obT2P+QtewXrxyf3zoEYRktkL43EYOZ2YlAfHg/w9m1JkST9DS69NqUOS XMXRTj7GOruKPSgfEp4epYL12ulnANdMWVLsNJHGTLMep2UW5WAzOG6cS28huhaMUy r37erDxj9w5NjwglaiWEy56P9PIkxcjhLKOBLSrw60geyFBz9vGd0Eel4WigqDN4Wi 4FAENakVFTM0g== Message-ID: <4709b993-85aa-470f-a346-2c3ea36bc4d4@kernel.org> Date: Mon, 29 Jun 2026 10:21:42 +0200 Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/2] mm: memory-failure: fix HWPoison flag race with non-atomic page flag ops To: "Michael S. Tsirkin" , Andi Kleen Cc: linux-kernel@vger.kernel.org, Miaohe Lin , Naoya Horiguchi , Andrew Morton , Oscar Salvador , Hidehiro Kawai , Rik van Riel , Vlastimil Babka , Lorenzo Stoakes , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Hao Li , Kiryl Shutsemau , Byungchul Park , linux-mm@kvack.org, linux-cxl@vger.kernel.org References: <20260629033608-mutt-send-email-mst@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: <20260629033608-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/29/26 10:10, Michael S. Tsirkin wrote: > On Sun, Jun 28, 2026 at 07:11:58PM -0700, Andi Kleen wrote: >> On Sun, Jun 28, 2026 at 05:45:22PM -0400, Michael S. Tsirkin wrote: >>> This series fixes the race by: >>> >>> 1. Having memory_failure() call synchronize_rcu() + retry after >>> setting HWPoison, so that any in-flight non-atomic RMW that >>> read the old flags value completes before we proceed. >>> >>> 2. Wrapping all non-atomic page flag operations in >>> rcu_read_lock/rcu_read_unlock (CONFIG_MEMORY_FAILURE only), >>> so that synchronize_rcu() actually drains them. >> >> It wouldn't surprise me if your underlying performance assumptions >> -- an non contended atomic is cheaper than a rcu_read_lock/unlock -- >> are not true in various CPU/kernel configuration combinations. >> >> Modern CPUs have fast atomics when uncontended in normal circumstances. >> But it probably doesn't matter much either way because the difference >> shouldn't be very much. > > > Hmm. It's a bit silly that I didn't try. Seemed clear to me, but, > on this old xeon... > > insns/iter cycles/iter > ------------------------------------------------------- > base 12238 +/- 1.0 17889 +/- 97.9 > rcu_read_lock 12251 +/- 7.3 17991 +/-191.6 > atomic ops 12233 +/- 1.9 17733 +/-136.5 > > > The diff in the noise. > > And old, slow CPUs maybe don't have MF at all. > > So maybe just atomics instead of all this mess. That would be much better. What I was concerned about so far was that many distributions enable hwpoison handling unconditionally (independent of any specific CPU!). I recall running experiments on some not-so-dated hardware 2 years ago (when optimizing out rmap atomics) where additional atomics really hurt, even in uncontended cases. > > > > >> It seems very complicated for something that >> could be much simpler. >> >> But I guess it's fine. >> >> -Andi > > Indeed. David already said he's gonnu look at this himself, but he If we can go that simple route (I'm not sure yet), your patch would be fine. I can try finding someone to run more experiments on arm64 hardware. -- Cheers, David