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 7E43634CDD; Mon, 8 Jun 2026 12:46:46 +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=1780922807; cv=none; b=JB1jC3PX1UZi/mPhK5Y5ciF7XZnZsug3L4WZv5enm6JCaNGjYwgEphJ2cyxKZ5scLF2+7Z/yp/ssEGV6tIo3nMTKcA1HmBPyeCBukdX7sLvvSEC/eadxIqY5LhB8G5NiF4w3r4ol/d1yOo4DjTCEnnjE94biG2NwArvypPgaTIY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780922807; c=relaxed/simple; bh=kb9dXQf5klDYChJwAr1QzF2ERIvKcH3T1s5AbgbKPoY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gihp81RTVZKuzn5R2TgWb+xOHpfjZt2fk9gEiS9FdWvGy8dJLO+yn70DSE808BOIWO/qT05ZGOEWbWRydRQgfzkbyQdm+r9lV/I9nyKAqfdfu/2OyZzXaRTVLyU9xxEHfGTBfexm0ZfJV4Dvg3h9orx7L6GHSHpEhPqmbxaD6ok= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JeHD5VtC; 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="JeHD5VtC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 515F71F00893; Mon, 8 Jun 2026 12:46:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780922806; bh=pC89nERQStWmeKYuCWAC1b9Ikp/Z8/vSsZHxe88ImJc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=JeHD5VtCQu8/oU1OSlGmBc8VeSDAgg/2WMHzTprzW1NS9u9I0wejqQLJoQafl/0yO yh3bCNNCZI15QGxHdgD49EiD7yLZmMC4vJOxjIZSNUNSSJ3gca4SYr9a8F7S0Q5jmd cn4FTS+1rVkKn0KF4FpRPOG4lVkbvIR5adUgS69u0NGrWJ/lv9XMToTHoBypGxfET1 8UDJLAGxcgxHaD74QXh0EdlHB7k4YFdfDHPc4oVr9uhzZaZQk6lQqXEaOcWQ+qaG4Z Fy7OX3KTLnuqQbHJg/XkFg8BMxyQEN+a2GotqSoTo53OYfKKwqVpxJpF0o/lvu6+nB TpC2blC1uSfZQ== Message-ID: <8ec82477-b497-466c-902b-82e108ae2b7b@kernel.org> Date: Mon, 8 Jun 2026 14:46:34 +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 v10 24/37] mm: add put_page_zeroed and folio_put_zeroed To: Lorenzo Stoakes , "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Jason Wang , Xuan Zhuo , =?UTF-8?Q?Eugenio_P=C3=A9rez?= , Muchun Song , Oscar Salvador , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Hugh Dickins , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Axel Rasmussen , Yuanchu Xie , Wei Xu , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , virtualization@lists.linux.dev, linux-mm@kvack.org, Andrea Arcangeli References: 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/8/26 14:25, Lorenzo Stoakes wrote: > On Mon, Jun 08, 2026 at 04:38:54AM -0400, Michael S. Tsirkin wrote: >> Add put_page_zeroed() / folio_put_zeroed() for callers that hold >> a reference to a page known to be zeroed. >> >> If this drops the last reference, the zeroed hint is >> propagated to the buddy allocator. If someone else still holds a >> reference, the hint is simply lost - this is best-effort. >> >> This is useful for balloon drivers during deflation: the host >> has already zeroed the pages, and the balloon is typically the >> sole owner. But if the page happens to be shared, silently >> dropping the hint is safe and avoids the need for callers to >> check the refcount. >> >> Note: put_page_zeroed uses folio_put_testzero() which only >> detects sole ownership at the instant of the atomic decrement. >> A concurrent reference holder (e.g. migration) means the hint >> is silently lost. This is by design: the zeroed hint is a >> performance optimization, not a correctness requirement. >> Losing it just means the next allocation re-zeroes the page. > > Do not put comments about specific expected races like this in the commit > message but not in the code. Subtleties need to be called out. > > The commit message also doesn't at all explain why PG_zeroed doesn't > suffice here. > >> >> Signed-off-by: Michael S. Tsirkin >> Assisted-by: Claude:claude-opus-4-6 > > I really don't understand why you have a 'zeroed' folio flag but need to > also have new API calls to detect that? > > They're also HORRIBLY named. Zeroed as in what? Zero page? Huge zero page? > Memory zeroed by kernel? Pages that userland happen to have zeroed? Or host > VM zeroed? > > Each are cases we address individually and relate to folios. > > You absolutely fail to clarify _which one_ you mean, and provide absolutely > no documentation and add an exported mm API with no description. > > This is just I think not something we want to add? Especially on something > so fundamental? I raised previously that providing a folio helper is odd, and that I suggested that we defer this change. Maybe we'd want to add such an interface for frozen pages later (to be used by the balloon), but I don't think we want that for folios. [1] https://lore.kernel.org/all/5f76af6e-9818-42ea-a305-c0fc1d920dca@kernel.org/ -- Cheers, David