From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 6FA58606A31C for ; Thu, 12 Nov 2026 08:27:36 +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=Gk7n35q/ Received: by kanga.kvack.org (Postfix) id 880536B05BF; Thu, 12 Nov 2026 03:27:35 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 8579C6B05C0; Thu, 12 Nov 2026 03:27:35 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 76D066B05C1; Thu, 12 Nov 2026 03:27:35 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 4F7D26B05BF for ; Thu, 12 Nov 2026 03:27:35 -0500 (EST) Received: from smtpin17.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 5DCA4160480 for ; Thu, 12 Nov 2026 08:27:33 +0000 (UTC) X-FDA: 85425787986.17.A7A9EFA Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf19.hostedemail.com (Postfix) with ESMTP id 92E311A0007 for ; Thu, 12 Nov 2026 08:27:31 +0000 (UTC) Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="Gk7n35q/"; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf19.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1794472051; 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=anBOo7skxIusRxuUYrRZR6ZmK2In80o3FSHqv4V/uok=; b=FquCtioDm9lXDfdig7Ep6LCLW3kiuBxWR55TJUwnKvQdnbwOZFBdrnXFHutHb3N5YAXeG1 C0n13dRMj0DPfXoGGPRb5F6v1YQtQUXsJkOptEKSnfDXPK71GGMAVQqa+WcmR+ESofE3ic dJtP6ci5uxRbc4+hk0MPHEs2JxeF5aI= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="Gk7n35q/"; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf19.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1794472051; b=rs12MariVgfzwWBFuHMMEi8eba+W64c0D1GuYnFZjfMTpUlQvVTOHNP065qTRHWFW0hE6F X2elMKDJLwJN16yEhhWfO+YKkNl4qeopYnFyA3cortpfOVoWOjd1xHbv7LKNtsS3Pml7fl mti5/TwKGq83FtiB/6XyJgu8GYtvKyA= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 0F232401F5; Thu, 12 Nov 2026 08:27:30 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 429EE1F00A05; Thu, 12 Nov 2026 08:27:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794472049; bh=anBOo7skxIusRxuUYrRZR6ZmK2In80o3FSHqv4V/uok=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Gk7n35q/j5Z+w6T3RsNBVb9DiyZBYKFdQ4QjiOe2NWhUI4yStdbyFMa7Z9OuGxsrL eBkm0wVrusupAswCs6V+aTvkeTdlQmYgf4aSPorSOgSEZgUswRmrqsGWKaRzWBsuqp cHF05739MSi2RXCvo5e6agVYH4F6IOIPF9/LuYkQT7w+zdURILFMccGS6dwSmI7SBz lEiBLtTAvGH9cFiaDmoFWLT/JED9ZQz5q7Bccj6zPtu+3LAs6XfTnNiRgGQTe10m0T I+7DSK1425xCvtWZbD301dL+SrgL+LuxZGTNnYtkAZRX18L9XExgUIsfSplpzR/bSd rMbZzoBvXC71g== Message-ID: Date: Thu, 12 Nov 2026 09:27:21 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order() To: Zi Yan , 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261110-fix-split_folio_to_order-race-v4-1-cc5042f33408@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: <20261110-fix-split_folio_to_order-race-v4-1-cc5042f33408@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 92E311A0007 X-Stat-Signature: t3ej6wadp4gyatpj9imqq1qmezbjri79 X-Rspam-User: X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1794472051-248364 X-HE-Meta: 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 QsHvBrHm ve5Z9mZMLLv2DEH8gO3UA7/XKm6J0xlPF5rPf1CLsUZUb9DoqWQBus/2UjsHfqxplrkq6tAS1BjuephKfZmm49H8+hDbywLPeMe6UduaXM75YfsqT+UWT5K5VWEdfbaPIzsLKesYcSw2LgqNoeqiYhRb32Dm9wFiz9m1KtSHSMhIovup/uhsrv7yaiPPippQJ/mtvl3fxqSER5XKIGMSupMr4lBcSeGOprAcA70wANN8GuRKmpkGG+lasClTRJduwIrgaJMFFF0CF8sQqfzqjO9TFzWj6H6FAnJ8cs7BnHT/e2+MsOmUOWpY0PDW6fp3PSa/MPvW1JE861k2cSA9g75cOaPn5APfmLSYP Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/11/26 04:19, Zi Yan wrote: > __split_folio_to_order() calls prep_compound_page() on each new head page > to create a compound page after clear_compound_head(). prep_compound_page() > uses non atomic __SetPageHead(). It is mostly OK, since the folio is locked > and frozen and no one else is supposed to modify the folio's flags. > > However, code calling migration_entry_wait() can reach the folio by PFN, > then access its flags. It is because softleaf_entry_wait_on_locked() used > by migration_entry_wait() sets PG_waiters concurrently without taking a > reference and the frozen refcount during split cannot block it. Meanwhile, > __SetPageHead() does a plain read and write, so PG_waiters atomically set > by softleaf_entry_wait_on_locked() will be lost if the set happens after > __SetPageHead()'s read and before its write. > > One example code interleaving: > > CPU0: __split_folio_to_order(new_order>0) CPU1: do_swap_page() > > 1. clear_compound_head(new_head) > new_head->compound_info = 0 > (page_folio(new_head) == new_head now) > > 2. migration_entry_wait() > softleaf_to_folio(entry) > page_folio() -> new_head > (not the old head any more) > > 3. prep_compound_page(new_head, new_order) > __SetPageHead(new_head) > tmp = new_head->flags.f <-- load; PG_waiters == 0 in tmp > > 4. folio_set_waiters(new_head) > set_bit(PG_waiters, &flags.f) > enqueue on waitqueue, sleep > > 5. new_head->flags.f = tmp | (1UL << PG_head) > <-- store; overwrites step 4, PG_waiters == 0 again > > Later, folio_unlock(new_folio) sees no PG_waiters, skips folio_wake_bit(), > and CPU1 can remain blocked indefinitely. > > Fix it by setting PG_head along with other flags in new_folio->flags before > smp_wmb() and clear_compound_head(). The remaining compound page metadata > is set after clear_compound_head(). In that way, after > clear_compound_head() makes new_folio visible, nothing overwrites the > PG_waiters from softleaf_entry_wait_on_locked() by modifying > new_folio->flags non-atomically. > > While at it, use data_race() to suppress the KCSAN issue reported by Ye > Liu's patch (the second Closes) on reading folio->flags in > __split_folio_to_order(). > > Suggested-by: Kiryl Shutsemau > Reported-by: David Hildenbrand > Closes: https://lore.kernel.org/all/DMHE8BEEER1H.2BIY52NXATPIA@nvidia.com/ > Reported-by: Ye Liu > Closes: https://lore.kernel.org/all/20261029-data_race_huge_mem-v1-1-112e331291c1@linux.dev/ > Fixes: c010d47f107f6 ("mm: thp: split huge page to any lower order pages") > Cc: stable@vger.kernel.org > Signed-off-by: Zi Yan > --- Reviewed-by: David Hildenbrand (Arm) -- Cheers, David