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 E77FDC53209 for ; Mon, 27 Jul 2026 13:40:47 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 614EF6B00E8; Mon, 27 Jul 2026 09:40:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 5AE5B6B00E9; Mon, 27 Jul 2026 09:40:46 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 49DF66B00EA; Mon, 27 Jul 2026 09:40:46 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 086346B00E8 for ; Mon, 27 Jul 2026 09:40:45 -0400 (EDT) Received: from smtpin24.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 75998A188E for ; Mon, 27 Jul 2026 13:40:45 +0000 (UTC) X-FDA: 85034666850.24.D2E86E4 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf10.hostedemail.com (Postfix) with ESMTP id 93CB9C0006 for ; Mon, 27 Jul 2026 13:40:43 +0000 (UTC) Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="NciMao/7"; spf=pass (imf10.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=1785159643; b=Tm33Ld8AQikodiueyCvTDOyj5iz0hYMT8k2fBs+hJMDjqSrrhB5reVyVfcDrGCwu9TTIXt +LR1yXBV8GQJZtX7ib9dtQHDwjLQDLROxJcYP7rIFX96kRhjtOgPa7W0ly5jsIDHhS0oHe ajjlekCTc167vpZ1HdaWaBeewBBd9YA= ARC-Authentication-Results: i=1; imf10.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="NciMao/7"; spf=pass (imf10.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=1785159643; 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=cm+6RO6NKrmQhCTAXomhESN+8SE1hTWFq0tnUoVm+go=; b=lJaT0oRrUszllMgs1MKRlGvsCOX0tLcwE9Y8549Sy3vqFkiV7Z2SiaM3i+04ApYHsOi/Ah wBBjETpfrIJwPkhFvccgXe0axRmkrYM/xB6MUb9HZTY6QXgVEUsCUrx4hVe26c9pdmshS6 bS7zYX66F70Md8YkgssJsH3CqkAHkjM= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 648244026A; Mon, 27 Jul 2026 13:40:42 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 805BF1F000E9; Mon, 27 Jul 2026 13:40:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785159642; bh=cm+6RO6NKrmQhCTAXomhESN+8SE1hTWFq0tnUoVm+go=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=NciMao/7Bbc7u/PPvksd4RtB7y9NUjZzNmswpiTdU9O5cxf/tThmsVnswGl5G3fP+ cEF0+uzWKff/hO8HHckYSaz7Jy04adTELzgaheA02Ryc3nAvRnIOeWi9G9/j70OEYX rBKG6QZsqZ8TFSyamCNdVHWeQerRcGPlFQKc9xFYV18Qj55K0fDht3+3BLMGsbcdhD ljNeTOEtAJdm5twueyvyuoNGGHpWJoA67imPhW5CmaqtNeD3yFXjtQ/k+lDcFXQ0Hk wYXOKwgDxTZZHm8Wn8otqZzjye+vTUjgyJuFShlNZin7M+jD74dCqwUFIjjEl0W1i8 WZqT7BYEGbrdA== Message-ID: <4146d4f5-c8fb-42cf-9dc8-a9d4c26055da@kernel.org> Date: Mon, 27 Jul 2026 15:40:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2] mm/thp: serialize huge-zero folio state transitions To: Hengbin Zhang , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif References: <177a1767-c782-4210-b377-44c520c74619@kernel.org> <20260727131814.3983064-1-uqbarz@gmail.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: <20260727131814.3983064-1-uqbarz@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: b9ycqtoifjq5zab8sc9guor7zyqy633e X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 93CB9C0006 X-Rspam-User: X-HE-Tag: 1785159643-320719 X-HE-Meta: U2FsdGVkX19f8Z977V+pjY8UmuY73E8a3tzfOwuWSV4nEdMProUcAEnowVakq6vbkNqAIEKHLCIrAScVI+SLg24vrxXzJggW7h/8UUwF/GEQW/px6XXzDAoZnwX/gvMVeAJ/qMLi0nvOuMeZgX/0vlkRnFSpO2VlvPtRD/a+qb4LlLkxHGOx8vRa5/tZjpkPYvDnvXCT2Jx0FEuNFjo7rsBC3SiPhdunDzDDs2hHD1ZsRTGhPDMDb9rKkBnnusrg6vahK0D2ZkoGzHjzVC29Se9NoAr3AhD1cNssJUeCAqZoBYi7evP4uTtrF1jCF8D/OMpxP8FhxJQ+0KyAVWZAq26qOnuqqIs5XcqZsyBQKPA4XekgSoopZCGfCUY7zImu8bpdOzzHy8q5bW6a/VhK89OkCX9nqQj1eW3XWEA+jv/Tzq+Kv+1HvCFwSe8uCZTjnMYCUh+XotjAOmS/DpT0+fGWU+8MyEPWrgSVQlg2QAG6qZIgTyLUjPw80QICSjHGXU9rS2uZ5WwVnCIMuP1cV8evE9VvE/54ffOSyFHZdgrAWuhUfXwBmNtIU7fbloEHWaldojOFgJ/jOUEVK1fHE/D7yRSMTymgl40wH4wx3+d6TXAJivttxZxpjUaDxStltNERw4nM7dJ++dNTCu2JLhcA6AgxWTi51okZJ+JPF/feDICZ3gOZQgRCZPlyWb7PN+NMZ23rAn/8LG0u8CGxEpIUaGUonkgSMaGRkMdNNNvWU7x0bsg+KFZoruN3+Dc/187TKgO1G96cRuHCM5rodLPu/za8wlEOrxTZFPQmDXC2q0KLkB+H9bgYWF4Dxeo7pWybbd5jE0fz6uAL4vt1LTn7YZS+SEyUapWyyHX4RNMXpQOQ8CpiiujX+OZCa6qfeag3mncppbkKP0EJB8tc4F9jpEoBHnyhtsMOLfkUyhxwg3MmtTh2GKSvbbCaWl16ePRU/CLs0T3R3rkT2Wg CnfR2Vbl RyWPXHotx98LJiNenoGHnNu3tYA6mJIvgY/XCEgcI0J/vxBqAajk+CVFjuXCciTdEdT+HZCgyc+L9cxtk7Rblqp1nFmlxs51XXcBf7lbiSZy/WFL8S85gwhAjKWDWCtbaNQtolfthCVFPJONg9V6R+jEFr8OvSRqH+f42s2CbGzh4Qd5UNvNdyfTpugUCr9hwIyelYrKs2H/chNFF7fx/Ghj+hTQynfTPi6x13w8BstaX4nyaFAxZFZfqKmnlN7WJB1q/N2O5pCaDMvaLeA336zD4YVYfqvOMgnUVJyUuEcSXgTDEBfs50SPh/w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/27/26 15:18, Hengbin Zhang wrote: > The nonpersistent huge-zero shrinker clears huge_zero_folio and then > invalidates huge_zero_pfn. A concurrent fault can publish a replacement > folio and PFN between those updates, after which the old shrinker > invalidates the replacement generation's PFN identity. > > A later partial mprotect() can then misidentify the live special PMD and > enter the ordinary anonymous THP split path. > > A writer-side spinlock keeps huge_zero_folio and huge_zero_pfn updates > from different generations together. Lockless getters still use the > refcount fast path. atomic_set_release() publishes an initialized new > generation before a successful atomic_inc_not_zero() can admit a getter > on weakly ordered architectures. > > The race was reproduced using test-only instrumentation that widens the > shrinker/allocation window; that instrumentation is not included here. > > Suggested-by: David Hildenbrand > Link: https://lore.kernel.org/r/20260724100509.2300200-1-uqbarz@gmail.com > Signed-off-by: Hengbin Zhang > --- > mm/huge_memory.c | 36 +++++++++++++++++++++++------------- > 1 file changed, 23 insertions(+), 13 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index b5d1e9d4463d..89771a9b282d 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -78,6 +78,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, > static bool split_underused_thp = true; > > static atomic_t huge_zero_refcount; > +static DEFINE_SPINLOCK(huge_zero_lock); > struct folio *huge_zero_folio __read_mostly; > unsigned long huge_zero_pfn __read_mostly = ~0UL; > unsigned long huge_anon_orders_always __read_mostly; > @@ -237,17 +238,19 @@ static bool get_huge_zero_folio(void) > } > /* Ensure zero folio won't have large_rmappable flag set. */ > folio_clear_large_rmappable(zero_folio); > - preempt_disable(); > - if (cmpxchg(&huge_zero_folio, NULL, zero_folio)) { > - preempt_enable(); > + spin_lock(&huge_zero_lock); > + if (READ_ONCE(huge_zero_folio)) { > + spin_unlock(&huge_zero_lock); > folio_put(zero_folio); > goto retry; > } > WRITE_ONCE(huge_zero_pfn, folio_pfn(zero_folio)); > + WRITE_ONCE(huge_zero_folio, zero_folio); > + > + /* Publish the identity before admitting lockless getters. */ > + atomic_set_release(&huge_zero_refcount, 2); > + spin_unlock(&huge_zero_lock); Can't we just keep the atomic_set(&huge_zero_refcount, 2); after the spin_unlock() ? > > - /* We take additional reference here. It will be put back by shrinker */ > - atomic_set(&huge_zero_refcount, 2); > - preempt_enable(); > count_vm_event(THP_ZERO_PAGE_ALLOC); > return true; > } > @@ -297,15 +300,22 @@ static unsigned long shrink_huge_zero_folio_count(struct shrinker *shrink, > static unsigned long shrink_huge_zero_folio_scan(struct shrinker *shrink, > struct shrink_control *sc) > { > - if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) { > - struct folio *zero_folio = xchg(&huge_zero_folio, NULL); > - BUG_ON(zero_folio == NULL); > - WRITE_ONCE(huge_zero_pfn, ~0UL); > - folio_put(zero_folio); > - return HPAGE_PMD_NR; > + struct folio *zero_folio; > + > + spin_lock(&huge_zero_lock); > + if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) != 1) { > + spin_unlock(&huge_zero_lock); > + return 0; > } Do we really have to move that refcount modifications under the lock? I'd assume it's sufficient to only move setting huge_zero_pfn+huge_zero_folio under the spinlock. -- Cheers, David