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 9FB733DB62D; Thu, 30 Jul 2026 09:05:56 +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=1785402357; cv=none; b=jaXJ+UavdChIZtWhGbfxJP4ResyJzTDZQUYmuoJubvQ1P6Vabmpnnc7GN4i+R+CcbCxKob4i1KXuCLPVhVGfY8umIIq0dZZtaYM6IBrUBpUA++Bdq3VoJANRWmneJ9g576Byw5JJDjNHWs6payqO4Rd3x5kAeixW7kHPoAptNMc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785402357; c=relaxed/simple; bh=Pi4kHzREh4/JC49v0qZ4m0CZbIPP2BZ0qolHzb1sGuA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MTjtmAr7kQCDrEot9zlXvN8RrboWtu+69pY4LPh2FbskwE5i1pPdX8TuX1R/ovNsCLpzduvuCGBASc7IPH/GutrTdemJIld6xOOpwj0aWY+d6iTc6sVpRw2O5T/9M7oONv47VdUhG5BDlLgmTE524ZiOBw8nCy334nCpZc9G7p8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AuqII84O; 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="AuqII84O" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9768F1F000E9; Thu, 30 Jul 2026 09:05:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785402356; bh=CJMscz0eavBVOdfValmTNV3+OuMVqWvQZLftxghzJLY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=AuqII84OYz/5m+JdsEXoOkH/HrTmljMuQUCLn3nbGpIgIW2xp/ak3YGxeoUUGWRUh 4flmrNcUiHQO3x1anAfULqNMnGYTyFxGjuqhjM+3IY/eqkJN0H6H+QCj37eHsB/pHn 9GL91iGQDsUSU5qY4BtXGcoyq8z/27ifj8AmYQ2Q8WX5UuMAk87hhiPrHaEwcTnMT8 fkXRN33Ao3Ar1MxwdLfcm92OnQytDDvDu6wcOsZarOs+NuTs6JWnXB/3IgqPrRL1Xe tfbH2ggbzMue/EwURhOArQAbITj9C+xa2Gg+IWTg/cmxcUT/2bvorgNXFd0DQcPAfr OS8A28DWUev3Q== Message-ID: <773a043f-e135-4050-9ca1-0b5190f03ec4@kernel.org> Date: Thu, 30 Jul 2026 11:05:49 +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-hotfixes 0/2] mm/huge_memory: fix huge_zero_pfn race To: "Lorenzo Stoakes (ARM)" Cc: Andrew Morton , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Pankaj Raghav , Hannes Reinecke , Hugh Dickins , Yang Shi , Kiryl Shutsemau , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hengbin Zhang , stable@vger.kernel.org References: <20260728-fix-refcounted-huge-zero-v1-0-3f261f5447b4@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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/29/26 10:15, Lorenzo Stoakes (ARM) wrote: > On Tue, Jul 28, 2026 at 09:02:57PM +0200, David Hildenbrand (Arm) wrote: >> On 7/28/26 14:05, Lorenzo Stoakes (ARM) wrote: >>> There is a subtle race in the reference-counted huge_zero_folio >>> implementation. >>> >>> The fast path atomic logic fails to account for the fact that the >>> shrinker (which drops the final huge_zero_refcount pin) can overwrite >>> huge_zero_pfn with the ~0UL sentinel value in shrink_huge_zero_folio_scan() >>> after a racing get_huge_zero_folio() installed a valid value there. >>> >>> This results in huge_zero_folio being correctly set but huge_zero_pfn being >>> set incorrectly and thus is_huge_zero_pfn() and consequently >>> is_huge_zero_pmd() will misidentify the huge zero folio as being an >>> ordinary THP folio. >>> >>> This can result in the huge zero folio being split and otherwise treated >>> incorrectly. >>> >>> The solution to this is very subtle as there is an atomic fast path, and >>> thus ordering in weakly ordered architectures has to be treated very >>> carefully. >>> >>> As a result, this series first reworks the >>> CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic so it is separated from the >>> refcounted code in order to make the subsequent fix reasonably >>> understandable. >> >> For at least somewhat easier backports, can we reverse the order? > > Since I'm going to be doing the backports can we keep it the same? :) I enjoy reviewing small targeted fixes (also for backports), so I want to understand why patch #1 is required at this point. >> The spinlock+proper ordering should be possible without #1, or am I missing >> something important? > > It's possible but this way it's a lot easier to deal with and I'd rather keep > things consistent for stable kernels. "A lot easier to deal with", can you elaborate? The shrinker code never runs with the persistent huge zero folio. So that is not a concern. So it's about get_huge_zero_folio(). But there, it's simply that concurrent races are impossible. Please enlighten me. > > I can figure things out for < persistent huge stable kernels. It'll be manual > work either way. > > (It'd be good to get some review on the actual changes also :) Sure, will get to it, but I have limited capacity. And I have more capacity for small targeted fixes :P -- Cheers, David