From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id BB027C54FDF for ; Thu, 30 Jul 2026 09:06:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id AE8296B0092; Thu, 30 Jul 2026 05:06:00 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A983F6B0096; Thu, 30 Jul 2026 05:06:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9871E6B0098; Thu, 30 Jul 2026 05:06:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 7301B6B0092 for ; Thu, 30 Jul 2026 05:06:00 -0400 (EDT) Received: from smtpin06.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 13324C08CA for ; Thu, 30 Jul 2026 09:05:59 +0000 (UTC) X-FDA: 85044860838.06.7C882DF Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf20.hostedemail.com (Postfix) with ESMTP id 41C081C000D for ; Thu, 30 Jul 2026 09:05:57 +0000 (UTC) Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=AuqII84O; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1785402357; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=CJMscz0eavBVOdfValmTNV3+OuMVqWvQZLftxghzJLY=; b=SkA90Ek3A1CnEptpa83LEn0M2CZ1SbUXfwbF1Q8kEBEGN9O8VvBNb5ywDhKPSB3uFoVFTe KsJYe0rQfmy1QtV3rcc9Xg9BMLuf2Y++rVh/C6wFfCcT+NZ9ppEqpOlxwjRnbNSLjB7hLy xVECHHoJP8ELduocVspyl44bIBfMrwo= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=AuqII84O; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1785402357; b=Inx1craktAV3plvdN2OlqOshIrMsyWE5AtqfkmYDIW1Dy9b/buhdMrLmVxraK8GpHYsCxl zUxJQrthUl7jrV5d8FgKjbh/sTSin+np9+uQ5QvaljN+rU7toSp/M2fXXnpymE6IIRulLL nWBjsNiMjQkehQz8ryF2DV7I2wTSyjc= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 618C141884; Thu, 30 Jul 2026 09:05:56 +0000 (UTC) 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 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 X-Rspam-User: X-Rspamd-Queue-Id: 41C081C000D X-Rspamd-Server: rspam01 X-Stat-Signature: eceydnjeyrnay68u37p1k4am43bthnyo X-HE-Tag: 1785402357-523689 X-HE-Meta: U2FsdGVkX181nYPRGm8mE7zRUrslS5jKmteopuqqxMbHWHOGoTD9McFIEnCTUg33fCoZWjnN+ZnTJ99WpWaJD3531qOUc8cPkXF/QlT3QWlhp9TZ7AhjHS8uQP1lwBGWB9HMchJe8LqiAlqrWpWTMoTUcgqOCNO1sYbNLv/4YMp/P4XX4WmCi3NnAybHDpS4zEsSHpxMyPR/qenXjAr+bXxBreklcV7/BRgpYuweWOWP4OGBp5niXRYQUjtxIU1QqZ6kGDiM7WwMDAzLIOqIJiUqMaAt/5mChFr72v1dhAJAQdGJ6DkH6BDIbhhEh84+TKq5Xl8T/VHSK9uzRBCyZIsb01bhVe1yYXZdPHCLJAUHo5uV6f+Un5CcrSBdGqsTmBF2OQuJqN+ho27sWunfudoa17qyO2juiX7ajJmzu2ivw1CRTr3RIg2Iec7zx4dchd35vtaGsG/wEl0zdHkPcE5dE3WT9uOLslGFovwHlNIJyISViCK6T4NcHFJP6A1QOIOQ+2VUsKgN7U/7UY95htCJUBC40ZKO96dasLqT3RBq+pJennDCNCCf61lnWjJNHlLBPiedXwmO8cm+PF2X00QYSz3/rrYiOs9+OFCVWzfSVT88GyjSxIJq2ch8sxl7WS51aSRZ8HjMiaTYjxrZl4kgg/0tet7b9OKDGL6SvRNYgUfdM6QBeleeTiKtPN+nDSPsxYhXGlKcj3BYdPae7ctXsUR1+e1htQXQqXxb1aZhOncmh7h0+XDSQiEtBfQLHmQlN62x+L7DuIM77MVby8O4eebb600TB3iKKlV1ltAo/WOCfN6vmJDO3C8ZX4XW94BtKPe1xrgDXbx0J7NdW8S61U6v19ZuqS6rajjx85/WBsatIFcCUDfyqXruiUOmsf3NWcIeeiYycrzQMmXcSHZMr9/VGC59SmU31fQT3GBfy5QTHxuF7++JRP8377bL8om4c7UIaubxuP0ITEq mM0tipMg UbjoCPBuz4jIleBzH/7yF70zbcgCgJ+w6tTsIhbWNitqnBXiSJNTiktV6J4eacQM/kLTD/mDTWScGUQH41Fr/a8T0S+TUiq/PaJIID2+cUf3B+1ho4sqFKuwAmfPYVYSw9Q7LYS1TRjU3sHyZUrzBhI5bhC0HIoqO/w7xUm7ZhtUQdPdMlsOhuxEOTxCe+XOnBLqf6Oz+hGD50DuOum5S3seG1TRzJkB9OBsjeTcfbrdcPjFRD6pzBS7jMKAXtiGprlU+kvGrSLTURzR984vjbOR9g0C1d6sWBNYO0IyKXQNS/tRJgU/oFd28AQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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