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 012E33B38BF; Mon, 27 Jul 2026 16:08:59 +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=1785168541; cv=none; b=rhsSTXCsfoIEWD3Zn4NIi9XYY3Hscl6MaX0CgNueYQ9X5iDw56bQPylbJCutyP+MQag0M0K3uI8wYJx6E7C3g3PfjysqD7CQ/5h92P7zSKFdjOyj3sYSQwzGKGc72yFEUnItfBTrkvNgUHYCMwOOINIIODUQ/wwkNoRUj4b0ofY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785168541; c=relaxed/simple; bh=QiCRQ8uy/+rds76IKIMt5bEdNPoYdSWYI7n/H3t7hWQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MFlANvbkcVZUzB6znr17rdmve4keGLEWkTgCKP4rkTZ8FWtGdFCg1doVw86SyYFVkcRfSXJr12HeyJIW07LMniA8CX1KPh5rfcMQ1FO1tmCKeikvdw1OJ91Tyrx6p9NJsweQ+EPIkwXgNtLCzTtrQlw35WZC6qv29TtpBl+Q0Bk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PW8tOqTC; 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="PW8tOqTC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5839B1F000E9; Mon, 27 Jul 2026 16:08:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785168539; bh=Z0LvAO0ah3SsYMLwJ96GBLTr20qyrgX9+ER60RqNQcE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PW8tOqTCRonmn5YwYODzuh7/6xiFucn+teoQBtbAYeXY5H5wOcR0RyQWihpjbgu2X VWDI4A9oLfUyWlLWgGtLSQ39uZj4pfA8u10n7xpxukiQ2te96QACxV18Mbzse7Q0CI HAttNqykj4lylZ6tBeoaTs8nr/q7jS1FtBMUO0OiAAXw2GxniwrpLFhZdI++XKXFtC 25Xm1u3f+yu6KJvMQTw6S9/N4oTnO0QeWBCEA83At62zNclFHnMhzfg0nSnf5A8Mrp SVFWfeRg57CU3vYwBht9DmU2SIvS4zsuFpCMSPTpYN0dTPl+ZEhIKVseuWvEGKNeUO enSJbfZh3bObg== Message-ID: Date: Mon, 27 Jul 2026 18:08:53 +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: [RFC PATCH v3] 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: <4146d4f5-c8fb-42cf-9dc8-a9d4c26055da@kernel.org> <20260727154001.4102341-1-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: <20260727154001.4102341-1-uqbarz@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/27/26 17:40, 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. > > Suggested-by: David Hildenbrand > Link: https://lore.kernel.org/r/20260724100509.2300200-1-uqbarz@gmail.com We want a Fixes: tag + Cc: stable. Can you dig up the proper Fixes commit? Thanks > Signed-off-by: Hengbin Zhang > --- > mm/huge_memory.c | 22 +++++++++++++++------- > 1 file changed, 15 insertions(+), 7 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index b5d1e9d4463d..79bec4495540 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 +238,18 @@ 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)) { Same comment as below: who could possibly modify this value concurrently? I don't think we need the READ_ONCE. > + 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); > + 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(); > + /* Publish the identity before admitting lockless getters. */ > + atomic_set_release(&huge_zero_refcount, 2); Why do we need the _release semantics here when we just did a spin_unlock()? > count_vm_event(THP_ZERO_PAGE_ALLOC); > return true; > } > @@ -298,9 +300,15 @@ 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); > + struct folio *zero_folio; > + > + spin_lock(&huge_zero_lock); > + zero_folio = READ_ONCE(huge_zero_folio); Why do we need a READ_ONCE? Nobody should be legally be allowed to change this variable concurrently, no? > BUG_ON(zero_folio == NULL); > WRITE_ONCE(huge_zero_pfn, ~0UL); > + WRITE_ONCE(huge_zero_folio, NULL); > + spin_unlock(&huge_zero_lock); > + > folio_put(zero_folio); > return HPAGE_PMD_NR; > } -- Cheers, David