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 6B16CC54FDF for ; Thu, 30 Jul 2026 09:22:21 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6BE426B00A3; Thu, 30 Jul 2026 05:22:20 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 66F466B00A6; Thu, 30 Jul 2026 05:22:20 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 537316B00A7; Thu, 30 Jul 2026 05:22:20 -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 2BCAA6B00A3 for ; Thu, 30 Jul 2026 05:22:20 -0400 (EDT) Received: from smtpin13.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 8FDC01408E5 for ; Thu, 30 Jul 2026 09:22:19 +0000 (UTC) X-FDA: 85044901998.13.B9E8A6F Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf21.hostedemail.com (Postfix) with ESMTP id B75171C0003 for ; Thu, 30 Jul 2026 09:22:17 +0000 (UTC) Authentication-Results: imf21.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=GX8vXUoM; spf=pass (imf21.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=1785403337; 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=tG6cmXvm95wjXcqB8dvCOqPznntDpzBFT2+tmIJffq4=; b=iNleAe+dCUupPMI2EFt4SE+7fA7Nu7CQD0GWKi0es/qIOeruopRhJ1oDWFeMUMEIzzTNeB bwIxPc6gLjgVvSfFEmEKzDgsqw1aRhlNw+aNnOGKlrNQMNxtv9LPfZMex6SUNf9S0Vz/5H wMLbafn4VgLJSsJkn7j1k/9DD5Cdnao= ARC-Authentication-Results: i=1; imf21.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=GX8vXUoM; spf=pass (imf21.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=1785403337; b=PqfbffQmwD7b2T2rcRqN2yR4UZnGA82l6QV7Suvydy3trNIe6r/n7Wm21LCdpJCDrZ1ZeQ j8u85IWbMjcyej4r4799SXJ/OVFayMjNGGaOeJsA2UtddYQoocCtqN3serX/LKEM+BwANp AKaIvHgr270xBJNYgIcDfRBbnzKQ6Ms= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id F3820400C3; Thu, 30 Jul 2026 09:22:16 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1C4731F000E9; Thu, 30 Jul 2026 09:22:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785403336; bh=tG6cmXvm95wjXcqB8dvCOqPznntDpzBFT2+tmIJffq4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=GX8vXUoMPS74Qq8B+Tq0b69jgz1uhRRrIyHKabEiUKKHoZ3ViacaCkpqYYKZxIznj qKZujjl4n7lvWZvgjgYibkx1/Q6L1sJivWDRu9PqF3gpRocIh3EfbVSQiv42bW3l8Z H6cQ6UXiyy3c+DwXRrc21ng5ngJkQm7p2lN89KowbtQNAUA3kxIGG5At/zQx1P4Y5p hMSTe1AjsBHgikiEWt5QFdZjCvbmTZNg3IwjkZia+m9H40VM8RNB10KIzgPRPTcAMh WIoAmUZQen4gm8AKrSCNKIMRyxdx374WRlzA7z6xPR9p120jYLbXl20S/OZPoVTImi LC6oeUUSdNBiA== Message-ID: <25d560df-6d7e-462a-a987-60a8695793ba@kernel.org> Date: Thu, 30 Jul 2026 11:22:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-hotfixes 2/2] mm/huge_memory: fix huge_zero_pfn race To: "Lorenzo Stoakes (ARM)" , 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 Cc: 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> <20260728-fix-refcounted-huge-zero-v1-2-3f261f5447b4@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: <20260728-fix-refcounted-huge-zero-v1-2-3f261f5447b4@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: B75171C0003 X-Rspamd-Server: rspam01 X-Stat-Signature: hokebsxuk7mceymw7h75h3riuaz3o38p X-HE-Tag: 1785403337-664618 X-HE-Meta: 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 qIH7GycA VK2EXTLjxCzCovWZ9P9fZdM9hfkb4w+Tg62lj2F/9NLVLPnnV8UqVQc8GC0LoyBC3bqdfzgk9ZqZrYhAABP7VP2z9KSbF7zE2izKM+Ud6YM14eq/kegHMxm+tgRJYAsEv8G+zFC/lW8dSG8at1BYPK/hY6NsU0l2slR5gg3CYDFi9ujTatqqZ4mnaD/ggFUtsNkuSa7Yv5ghinAq4WKwuUJ+Ay6n/EvJjvPic4s96fZoHcqM4AEvLMF8S8RQhBZD8tD9tTG8CrsYX/ewXdTLtE58vieAt0ScTys0cbwswpIyVX6aCslBT8lxdlQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: [...] > So invariants are required - huge_zero_refcount MUST: > > * Only be set in the huge_zero_lock critical section to ensure > serialisation of huge_zero_pfn, huge_zero_folio and huge_zero_refcount > writes. > > * Be set non-zero only AFTER huge_zero_[pfn, folio] are set to valid values > so installation of the huge zero folio on read page fault ensures > concurrent is_huge_zero_*() calls correctly identify the huge zero folio. > > * Be set zero only BEFORE huge_zero_[pfn, folio] are set to NULL and ~0UL > respectively, and atomically. > > Establish these by: > > * Only updating huge_zero_refcount in the huge_zero_lock critical section > in get_huge_zero_folio() and shrink_huge_zero_folio_scan(). That is imprecise. huge_zero_refcount is updated (incremented) outside of huge_zero_lock in get_huge_zero_folio(). > > * Using atomic_set_release(&huge_zero_refcount) in get_huge_zero_folio() > after huge_zero_[pfn, folio] are set. This is paired with > atomic_inc_not_zero() to ensure atomic_inc_not_zero() only observes a > non-zero value if huge_zero_[pfn, folio] are set. That makes sense, yes. > > * Using atomic_cmpxchg() in shrink_huge_zero_folio_scan() to ensure that it > is set zero only when equal to 1 and set atomically. What we already do, yes. > > * atomic_cmpxchg() being fully ordered ensures this is done prior to > huge_zero_[folio, pfn] being set to NULL and ~0UL respectively. Which is always the case for atomic_cmpxchg() I think, yes. [...] > > @@ -270,7 +272,8 @@ void mm_put_huge_zero_folio(struct mm_struct *mm) > static bool get_huge_zero_folio(void) > { > struct folio *zero_folio; > -retry: > + > + /* Paired with atomic_set_release(). */ > if (likely(atomic_inc_not_zero(&huge_zero_refcount))) > return true; > > @@ -278,17 +281,21 @@ static bool get_huge_zero_folio(void) > if (unlikely(!zero_folio)) > return false; > > - preempt_disable(); > - if (cmpxchg(&huge_zero_folio, NULL, zero_folio)) { > - preempt_enable(); > + /* Paired with critical section in shrink_huge_zero_folio_scan(). */ > + spin_lock(&huge_zero_lock); > + if (huge_zero_folio) { > + /* Somebody else already installed it. */ > + atomic_inc(&huge_zero_refcount); You replace the retry loop by an atomic_inc() here. That works now by moving the atomic_cmpxchg() in shrink_huge_zero_folio_scan() under the lock as well, so it cannot go away concurrently. Worth spelling that out in the patch description (unless I missed it). > + spin_unlock(&huge_zero_lock); > folio_put(zero_folio); > - goto retry; > + return true; > } > + WRITE_ONCE(huge_zero_folio, zero_folio); > WRITE_ONCE(huge_zero_pfn, folio_pfn(zero_folio)); > + /* Paired with atomic_inc_not_zero(). +1 for shrinker pin. */ > + atomic_set_release(&huge_zero_refcount, 2); Ah, that must now go into the lock as well, otherwise the concurrent atomic_inc() would be problematic as well. > + spin_unlock(&huge_zero_lock); > > - /* 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; > } > @@ -312,15 +319,22 @@ 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); > + struct folio *zero_folio; > + > + /* Paired with critical section in get_huge_zero_folio(). */ > + scoped_guard(spinlock, &huge_zero_lock) { > + /* Paired with atomic_inc_not_zero() in get_huge_zero_folio(). */ > + if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) != 1) > + return 0; > + > + zero_folio = huge_zero_folio; > + VM_WARN_ON_ONCE(!huge_zero_folio); > + WRITE_ONCE(huge_zero_folio, NULL); > WRITE_ONCE(huge_zero_pfn, HUGE_ZERO_UNSET_PFN); > - folio_put(zero_folio); > - return HPAGE_PMD_NR; > } One important point: The folio can't get freed and reallocated before we updated huge_zero_folio/huge_zero_pfn I think. The folio_put() should involve a full memory barrier (atomic RMW) and not get reordered. So we cannot misclassify the folio in new context as still being the zero folio. LGTM, thanks! -- Cheers, David