From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794338230; b=KegIFYzdEU7lCP887tdZigJXdW1IwOS+woZ+kLrtEf1s+/j16blw23ZBZmOHqVgJ45rD 8dV/po0MUx9+TWsLL0Gr0EYuvzm2EtL2wL82TpX4uhO5TnZtQbLWeUtEmHKqekaNBeLLo vwi3+TKuEpYzgNOQG/T/6Hf2PsMb704sFhfVMzLMY1/jgRE3F8lRXzVPlUioiDYHIHW6B k6qvG/F04JEoyMi/8O2cL23c1RjtIn+EqZmxohiCWle37HYBoqGN2uzZQ4CXNqVt0/nXc urczcbRmucADisgpLWIUpasQb8Am3hKKX8f3+5KLUXkyh14ckIGznd1tQ4J+ufz3T5w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794338230; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=vt/CmKJodaWf9jKe7kops2ibECH9zAPqIYPvDLfmxOk=; b=dZhVZWiNc++XoVRkYR9qnYA/JqS45Jx5XYYFFnF0V4P3y4546yqCy6tat0KWykhZLC7N pX4ltyjEQpMp7a+0HOYkpiC8/jMqaJI3JFQmMOe6eQ/eehIv6yx7wklTnXAM/zjqiszQd LiY/JgNU6i3XJOptxwOeHLm0krT8kL9U2c0VDvt4CrWDv1WXxnCn4jqZnh+Tjcd+3FY/8 NlFOnxHlKxswdF6nz+IjhLqAJdoY0gO7fD5Tpti/PvCXS41d/G/hIK1DQ02/aluvcx5L2 AdRpbO5H2Ve5U3fbbc/brhwHbLmHB5pEbuggvlyvbr2CUpwqIcm2zY6gOFAZlNN7ALA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=EuMVabxB; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7C8EB606A320; Tue, 10 Nov 2026 19:17:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=EuMVabxB Received: by smtp.kernel.org (Postfix) with ESMTPSA id E2E5C1F0089F; Tue, 10 Nov 2026 19:17:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794338230; bh=vt/CmKJodaWf9jKe7kops2ibECH9zAPqIYPvDLfmxOk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=EuMVabxBkF2GedsYrWAZAlnWBD7IPltSSop/T469rSorg/PnTP8kTkbpjhyHBzGke /TY9zDrpVeEyybGn9/FRaXg1A8qtDInL4vbvYFrNvZlMdSFIwC6iFCwU4mi982D+Ic X4eTwS+FCadwSu3VdLWT26p5AOe/gwd2n3fmL/fR0aTyxw5MKWOj+6TMqcIOugSb5V Ot+0IeNagMPTL3W9GmdSUPW59aReQR9Ji1sNsBXotbcNSt2PIxunxqv9o33pWnBPB7 ysyHPWWs+NbgjnAx8lSb9efeYMXVf+PKoa0vb2NzsNJw9tsiic/CbgBdFlAXRzkfFI bzTK6l49nUS6A== Message-ID: <4bfaa882-a1ac-48a3-aa41-cb282fbf162c@kernel.org> Date: Tue, 10 Nov 2026 20:16:58 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order() To: Zi Yan Cc: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Brendan Jackman , Johannes Weiner , Ye Liu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261109-fix-split_folio_to_order-race-v3-1-1cdc509e34be@nvidia.com> <5a7bb477-5392-404c-a748-4748cec2c84f@kernel.org> <940CBBBB-D564-4D4F-9C8A-A9A272F08064@nvidia.com> <1B3B1BC7-7623-4070-ABD7-13A5EE559297@nvidia.com> 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: <1B3B1BC7-7623-4070-ABD7-13A5EE559297@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/10/26 18:03, Zi Yan wrote: > On 10 Nov 2026, at 6:31, David Hildenbrand (Arm) wrote: > >> On 11/9/26 22:05, Zi Yan wrote: >>> >>> >>> This is V3 of the patch. Instead of using prep_compound_page_atomic(), >>> PG_head is written before clear_compound_page(), after which new_head >>> is visible to the reference-less writers. >> >> I'm confused, how is it then valid to have PG_waiters set concurrently while we >> read new_folio->flags.f? Because that's what your comment states. >> >>> >>> >>> IMHO, memory_failure()’s reference-less access violates the frozen >>> page contract and should be fixed. >> >> Exactly. >> >>> But I do not think it needs to go >>> along with this patch. We discussed HWPoison flag race with >>> Michael S. Tsirkin and Miaohe before [1]: >>> >>> 1. Miaohe proposed to reset HWPoison in memory_failure() as a best effort, >>> 2. mst used RCU for all non-atomic flag accesses, >>> 3. you also proposed to move HWPoison out of ->flags. >>> >>> So memory_failure() needs a separate and proper fix series after >>> we decide the right solution. >>> >> >> Vlasta raised the idea to set hwpoison only after we actually managed to >> isolate/grab the page. Which might be the proper fix. > > I was bouncing the same idea with LLM yesterday, but it does not solve > all cases: > > 1. normal page: taking a ref just prevents it from going to be a free page, > but atomically set PG_hwpoison can still be lost when another one uses > a plain write, __folio_set/clear_*(). > 2. free page: isolating works if the free page on its free list, but it > could be on pcplist and we then need to drain all pcplists. That could > hurt performance. We can make memory failure slow. > 3. we cannot take a ref on slab page and other frozen pages, then we will > need a case by case handling for them. Yes, we need some way to isolate them, or to sync with them. > > Anyway, this probably belongs a separate discussion on hwpoison. Yes, memory-failure is the last of our priority right now when it comes to fixing things. -- Cheers, David