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 4B74D4028F7; Thu, 30 Jul 2026 09:22:17 +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=1785403338; cv=none; b=Vt5HFWUM2Dre3Qqm2/CbeWqhHNYwrbag6gyX8M/MZYJS1v/vI5ryWs6C5H/6mvA9UnfDafRdE0o+D+qWBXtgmIxIaadaeJ51AapvXHDYXDE5cOgKB0eFfzB5TOrca7YFc3Mlqs1u/a8P7OE/Quy6G+CjAQ+Uosqas3EsE8nYY/Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785403338; c=relaxed/simple; bh=U6W/ER119pnEKBoJgtpOFBN3zpfsqoLZvrELCbUSj9s=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oP5dChph0PQYlQo2GDaqdo6QkfkAP51cffu/jLMAnxEQ5UHqXi/24BCWXJhQvMH5ldDd25icsJLhzGtfxD4N2sEEc7J+B4nmnnQol5jsmiXCWgjb8AM5BiTjnq/PhK3NgYrvrW1rYmkgT5FSxqJm9jHNyVUFBtEdaSF4TLt/5Kc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GX8vXUoM; 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="GX8vXUoM" 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 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 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 [...] > 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