From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 B19D636E49F for ; Mon, 9 Mar 2026 08:59:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773046782; cv=none; b=YnsHhSf5Ez8HXNl4EK4qCVl8fB3H+fWLXfA7ja2F3dCTHGGCd3DzT5LJFoqXmluEPfm7v+Wn1v1VcabgjvOcFF2kgOeyloQy8MGfVqQWTn4t6HLtypCe9Y5QY01vrs0IDRaP8XX0zGpGJfq8H5L0wKpc9sl5CwI96j5qI8uUhgk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773046782; c=relaxed/simple; bh=bX4xlQ29q52Zuy39lIHfXpnK++YzZXX3b9xKFu0LXp8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=aHCHs9gkhr9EJB7bdzh6HVLsAiwyIjT+fI9ufhv1jSljMc6uapnxjHBAyVzuFvqsyPxZW5UyH8c939ngZKTGqiMnelkJp+DddjFYVCrVgjx5xf1q++QaGfeY1acTm/rMvFqBO59FOWnX8xBIm9+rMyFIOL/LBZgGiTcEL+43N1c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=vPt6R1/H; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="vPt6R1/H" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 193E7C4CEF7; Mon, 9 Mar 2026 08:59:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773046782; bh=bX4xlQ29q52Zuy39lIHfXpnK++YzZXX3b9xKFu0LXp8=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=vPt6R1/HoyVIkg+NIib8EF3iauR8+9F1GyELDtfyyFpbWCgJ+erG1B0hbFgOoZYb6 gkykNCgbHGfAN9b8NRl8xj6p1PZyynSsilCgMX6uEmzMvDD4tPhZkUQmsjsmjuPhjJ jsCUsKmHbMiD6u8YCO9l0PQBBDVWly/PoCq388lbINMulXpfPUgkUfTltj7J+fZ/ik 2WF+MhDkFjE+WKMhlNJ51FEQSTf2kRRiWb2apmfxfOpTOM6mUaBhBSRJA89G482K8y XTm37GOSOj7so6Jq6y8iwyEx5eKf0d1aweFCHX1I/Z6mrGLqiMReo8AyHGUCBL9TX6 +dR9R+6k5PYNA== Message-ID: <440a5424-45b8-439e-b4f9-9e80e26029fa@kernel.org> Date: Mon, 9 Mar 2026 09:59:33 +0100 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 1/1] mm: implement page refcount locking via dedicated bit To: Gladyshev Ilya Cc: Andrew Morton , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Harry Yoo , Matthew Wilcox , Yu Zhao , Baolin Wang , Alistair Popple , Gorbunov Ivan , Muchun Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kiryl Shutsemau , Linus Torvalds References: <6bf6eba6e2e6a74e2045a3bd08d58fd91bece7be.1772120327.git.gladyshev.ilya1@h-partners.com> <902d821b-e903-4bf5-89db-070851c95a1a@h-partners.com> <54cc6e6d-bda7-4c5b-a998-df854ead1985@kernel.org> <75aa8f34-4d91-4758-847d-248a5be3db62@h-partners.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: <75aa8f34-4d91-4758-847d-248a5be3db62@h-partners.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/6/26 15:29, Gladyshev Ilya wrote: >>> >>> You are right that refcount = 0 is tricky. However, for a bad outcome >>> you will need: >>> 1. Some external reference to this page, through which you try to >>> increment the refcount; >>> 2. set_page_count(0) somewhere between freeing and "it is safe to alloc" >>> state. >> >> That is way, way, way too dodgy. >> >> What you should likely do is >> >> (a) Make set_page_count(0) set it to PAGEREF_LOCKED_BIT >> (b) Make any places that might skip set_page_count(0) to use it >> (c) Document this all extremely thoroughly. > Okay, I agree. My reasoning was more about "it's not _that_ bad if we > miss something outside the allocator" than "everything is fine as it > is", but maybe it is really that bad... > >> Alternatively, disallow set_page_count(0) ccompletely and add something >> like "initialize_page_as_frozen()" or sth. like that. > > (Below are my thoughts, not active proposal) > > Actually, we can keep "zero is locked" property if we change scheme a > little: > > If we invert locked bit so that 0 means locked, then we don't care about > memset() / GFP_ZERO / set_page(0). But then there is extra bit on each > active refcount, and (un)masking will be required on each refcount read/ > set/etc operation. > > We can place locked bit in the highest bit like now, or move it to the > lowest. This doesn't make any difference except for how masking is > performed (and or shift). I can't really tell which approach is faster > (and more optimizable after inline). > > This approach doesn't look appealing to me, mostly because of redundant > masking. So for now I will try to add proper initialization / > documentation in all risky places. I agree. refcount == 0 will only be allowed temporarily while transitioning to the frozen state. In fact, you should then rename PAGEREF_LOCKED_BIT to PAGEREF_FROZEN_BIT and page_ref_add_unless_zero() to page_ref_add_unless_frozen. That then matches the terminology we use for inactive refcounts (page_ref_freeze(), alloc_frozen_pages()). And it will be a lot easier to comprehend compared to using a different "locking" terminology when traditionally the "page lock" is something completely different. -- Cheers, David