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 872B33E51F6; Tue, 30 Jun 2026 06:34:55 +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=1782801298; cv=none; b=kS3Jla2IAHuFRECozepUnGFaUmLm7EWjkvCYwV+oNo/tihzmXXD9lNkFJMITNOBHPi67blZssQrVkkGHwgnbdHfVkN9XkteLEgRy+nzNBuGYF4w/UrCUKRkcvNplG2bL6Dl+JFcIn2XyPN54UI2E+/OxU7fFiB43yP2w0lJIUeA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782801298; c=relaxed/simple; bh=1Gz43i5+WG+MMG3zujDlJb7MhzjXgnTmZy0pFsi9ZbA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=o1BTp8LuTvZsYEhdw8vddqemDDd4S2l2ioaOYxFn/5peStfbbQO0EFIeDL56h4rWwuabLw6HkvYu/gWBUhpoJmJ+2Hypbp/LQ/D2YgGjonRCzDkDNJM1tiT39raKnzywSUE9Ph5mLGdR+rN/LZSGo5LgYaF8mYmPQD3UJJ0mbUY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Zb9fiMxH; 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="Zb9fiMxH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1ADC21F00A3A; Tue, 30 Jun 2026 06:34:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782801295; bh=EGuQzeJmx9gOrE/eY6AwUnnp4ycjaahX6HSrGeOyFuw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Zb9fiMxH2dvarKbg54RXe2oUt4rjfGoix58RvskFE5wWfyBEPQvVAVKlQHM5rtixK 3orrHnZxiULC3ZcUBJLyygEt1xlw6l8ADBF3V9IYDrpRtDoM8gMd6jvPK0DZmme+Xe YOlwEWui+bN/KlsjVpP74HPd/WxX4UwvnHszTQZt5INNMaGEfKcG1g9pDQMfLbmvDZ Hiy4cr1SRubbCj6NvkR6GClyJSmgybIDWamJ+CYPAymsoYgTzflQsPFB4stlyh3rAk gse5TSzFAPzwq3YNNuLTpViT51z9eq6cBOlRkGENXhxZLY15R7xN8qZYQGw2X8VrGR J/kMltGia6Utw== Message-ID: <0fb36931-9097-43db-98bb-d087b7b0fa12@kernel.org> Date: Tue, 30 Jun 2026 08:34:44 +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 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> <0a309ed3-378e-4d88-95a0-65bf47c5496d@kernel.org> <20260629193347-mutt-send-email-mst@kernel.org> <5c8ca96b-381a-4fd3-a218-6aaa87a9a3b7@kernel.org> <20260630022129-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: <20260630022129-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/30/26 08:27, Michael S. Tsirkin wrote: > On Tue, Jun 30, 2026 at 08:17:42AM +0200, David Hildenbrand (Arm) wrote: >> On 6/30/26 01:34, Michael S. Tsirkin wrote: >>> >>> Wait a sec, what about call_rcu_tasks? Use that and re-check the bit is >>> still set? >> >> So, in essence the idea I had yestarday when it was late was the following: >> >> Assume we >> >> 1) Can have a way to guarantee that a function on a CPU cannot execute within >> our critical section (while updating the flags) >> >> 2) We can request to execute a function on each CPU and wait for completion >> >> I think we could just let each CPU execute our desired action (e.g., try setting >> the bit). >> >> E.g., >> >> local_irq_save(flags); >> page->flags &= whatever; >> local_irq_restore(flags); >> >> And assume we want to set the bit, do a >> >> SetPageHWPoison(page); >> smp_call_function(set_hwpoison_smp_sync, page, 1); >> >> whereby >> >> static void set_hwpoison_smp_sync(void *info) >> { >> SetPageHWPoison(page); >> } >> >> >> The idea is (that needs double checking) that a CPU will execute the >> SetPageHWPoison() either before the local_irq_save() or after the >> local_irq_restore(). So it's own non-atomic update cannot get interrupted. >> >> Now, IIUC when it comes to "how expensive is this" I think we have (cheap to >> expensive): >> >> 1) preempt_disable() >> 2) rcu_read_lock() >> 3) local_irq_save() >> >> >> So the above wouldn't be better than an rcu-based approach we have right now. >> We'd need something that relies on disabled preemption only. >> >> Huh, but I read that "anything that disables preemption also marks an RCU-sched >> read-side critical section including preempt_disable() and preempt_enable()". >> >> So for our use case we should be able to use preempt_disable() instead of >> local_irq_save(). That should already work for your existing implementation. >> >> -- >> Cheers, >> >> David > > We have: > > #else /* #ifdef CONFIG_PREEMPT_RCU */ > > > static inline void __rcu_read_lock(void) > { > preempt_disable(); > } > > ... > > > static __always_inline void rcu_read_lock(void) > __acquires_shared(RCU) > { > __rcu_read_lock(); > __acquire_shared(RCU); > rcu_lock_acquire(&rcu_lock_map); > RCU_LOCKDEP_WARN(!rcu_is_watching(), > "rcu_read_lock() used illegally while idle"); > } > > > > So on non-debug build witout CONFIG_PREEMPT_RCU (what I tested), rcu_lock > is exactly same as preempt_disable. It's relatively cheap but not free. > > > preempt_disable is not going to be cheaper. Well, it will be cheaper in the general case (CONFIG_PREEMPT_RCU) :) But yes, not for this case. > > I can test if you want but it seems clear. > If you measured only !CONFIG_PREEMPT_RCU, then yes, it won't change a thing for that scenario. > > But IIUC task rcu might be cheaper - IIUC it does not need rcu > lock/unlock at all, it relies on readers to invoke the scheduler > instead. > No? I thought that still requires protection of sorts (preempt_disable / rcu_read_lock), because it might fire whenever the task is preempted? -- Cheers, David