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 BC95D3C9EF0; Wed, 1 Jul 2026 08:08:55 +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=1782893337; cv=none; b=eAWssBh93rrZS1R+Zcb+U1wmkdaITdF+5yVidgb7jt6k5ACS6q4uJieUM3VbvzkcDJPV/nq2UzelYWVaEi5FvOkS+6nWSEWMzRBjNi2mYPl9xk8yGIsHEH83c3qW7xMnsf27WQpc67iBkMHd1NMYY+A1KSyKf4oZRcbhHBSKghM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782893337; c=relaxed/simple; bh=db/dVU/bxlGD82xxujHYLGbXxzBJMJ5ZYyPPfW8IuAA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=R1YpvfrFCttZSXbEZglsW5Xz6KCcEw2kUVS5NRCHzNzo1NzLCQnjvNdgbJOu+etRnZfeerDRRJmHotKf014pcR0dlg4GA4j/6iW8WCVcPnbJC1PaeEl0kovQm7PBNWs7PzF73XLgvARqiWqUrU1LrvrLITm1/FnsRPGyOBQ0RRs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BlbI83cU; 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="BlbI83cU" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 489481F00A3A; Wed, 1 Jul 2026 08:08:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782893335; bh=bRfDQ94KKwfv/CvxlzEUrd912NKNt2V1LTiihODrmek=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=BlbI83cUzoFLkksudULNVKgDqYjKjAYdjLUCMofo3GXkuGAU5FbXafStIm29Up3qn P0NaQIGfQzJVswPcldyf1kNpDkU2iiMQiaGeEnWCYqMWEaHOyoa3cSN/gMSTin3o6H DJHJMsud0lh1/srvBVB73TicVBQEQnvFnzG4UJ1IMmXPr10cI4RkKn1nkK/c90y0nI gWeexF39tBly5JGpf8+NGq5F+MbAgpkKokw3X5lgHlH/VyM6IRIa4OluutXCiGJwGl EAYdRFeN515POyx26cJKlOINhqiKPJPTZ6G0sea99hrKVoMk0nF34rFrxFc1y5VJFi 5/P5MMD0AW3hg== Message-ID: <2f884bfa-3cd5-4fba-8aa4-c2e68890ab64@kernel.org> Date: Wed, 1 Jul 2026 10:08:45 +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" Cc: linux-kernel@vger.kernel.org, Miaohe Lin , Naoya Horiguchi , Andrew Morton , Oscar Salvador , Andi Kleen , 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: <0b5f8b4b-d7dc-4b79-9555-a5b36265f3a9@kernel.org> <20260629030657-mutt-send-email-mst@kernel.org> <4f5ba5d6-246c-4430-9737-e8dd8e4c5142@kernel.org> <20260629092856-mutt-send-email-mst@kernel.org> <54c8cbee-9b26-458c-93ba-5aa594f5d1e8@kernel.org> <20260629174225-mutt-send-email-mst@kernel.org> <20260630174852-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: <20260630174852-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit >> Cheers, >> >> David > > Yay. I did that + dropped the extra lock/unlock and now it's in the noise in > my testing. needs much more testing of course. Cool. I'd expect that latency-sensitive workloads (PREEMPT_RT) would not want to have hwpoison handling either way, so using the no_resched variants at these places might be doable. > > If you want me to post (including addressing your other feedback) let me > know. > Let's first discuss the options. We essentially have the following one so far: 1) Ignore the problem It's been there forever ... but I am not quite happy about that. 2) Use atomics everywhere The easiest+cleanest, but as measured, the performance hit is real. 3) Keep retrying for a couple of times The big problem is "how long". A CPU in a hypervisor might be stalled for quite a while (20s? can be longer). 4) Disable preemption around non-atomic updates + synchronize_rcu() loop I think it should work, but I don't like the possibly endless retry loop. (well, it would never be an endless loop in practice) Is there a problem with synchronize_rcu() latency, given that it can take in bad scenarios a couple of seconds? (grace periods can be large ... but also very short) 4) disable local interrupts around non-atomic updates + let all CPUs perform atomic setting/clearing of the bit through smp_call_function_many(). Disabling local interrupts is way too expensive. :( 5) disable preemption around non-atomic updates + let all CPUs perform atomic setting/clearing of the bit through schedule_on_each_cpu() Mixture of 3) + 4), but schedule_on_each_cpu() can also possibly take a long time (as long as we cannot schedule on a CPU ...). We could likely build something that remembers all pages with bits-to-set / bits-to-clear, to then kick off a per-cpu work ... and once all CPUs processed a page it was fully processed ... needs much more thought. 6) stop_machine() Big hammer. I think we'd still need to disable preemption around non-atomic updates. 7) Move hwpoison bit out of page->flags Use a sparse bitmap. Quite invasive, and any hwpoison bit checking code would get more expensive. Did I mess something up / forget something? > Or look into call_rcu_tasks/call_rcu_tasks_rude which might work without > changes in mm. Looking into call_rcu_tasks() (the first time) the semantics are interesting ("involuntary preemption is not a Tasks RCU quiescent state"). I'm curious how this interacts with random page allocations (on the IRQ path?), and which design you envision given that we only get a single callback (who prevents code to re-enter). > > Or if someone else is gonnu work on this, absolutely fine too. I can likely let someone work on that, but I think we should first figure out what can really be done. (and likely when we send it out, it should be an RFC) -- Cheers, David