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 F1541190473; Tue, 14 Apr 2026 15:37:57 +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=1776181078; cv=none; b=CJ2gKxgtC7VhxIVnAmSXTlcld852v5gN5OFrjiwRsUtTl7rvaiVZ/YdJNDMCHxc4N6vZIIp7JuUCkXsboxKL/ciJkQY6macSyKuxufG5MKb7XT+F2+NtCaP39WTtZTkde2UKU6cgkASjW3rgIOxGXl3kj3On2cnytHqSI2rpIQ0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776181078; c=relaxed/simple; bh=vLt1zIsK1rIcZO3TGQDorDFsNmmezc0A83+t9iQrN+k=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ur21lBNQlyaO1cBghC7G11EP/5PIRy/Eee9bTsv87vG/0LvpZbHcjh709wL/YiNw/CeA4mND1NVSg/IybU+QkX/p2Io+BEuAI6YXAJT2BeS4Q3NS6Vqz+Mmjtyl4gHeXg00kFfh3oqDir93LgZswYi9q0eIhvRmwsYMvqjiUbB8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NjdxK0h2; 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="NjdxK0h2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9E91AC19425; Tue, 14 Apr 2026 15:37:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776181077; bh=vLt1zIsK1rIcZO3TGQDorDFsNmmezc0A83+t9iQrN+k=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=NjdxK0h2GlaLeB2t7hArNWCRSqIDWcr2FCJVvMvkYdIxeOUoHWUm6aU7MhJs3gNKJ FypuwAHi2hMrXIc+l1an1CDDfivVpNt3PY4pT08Q3K6e54kreCwdbViRtNdtlQ0Efl HhJ7XbrsOkzRKkr11b1F0kC/SM4p+8VNc49XaTj12yQFL8rdupj0JBa2P+C8Jvt6Qv rvkA6/+pPJvk1QsfcB2eOgjvWNzL1PMJqkn4K95cb8JxYMxdfD/mdkgqzoKAGljVae pkA6KH1CRimrpkUlpvNirlN9XHIyNfbtgr4zdhbyFhwE17ncHUFz5dMqwLNcvOaZg7 +0KeKpVCjDscA== Message-ID: <55019037-4f1c-4d9c-83ee-3a844d8f3d5e@kernel.org> Date: Tue, 14 Apr 2026 17:37:50 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC, PATCH 00/12] userfaultfd: working set tracking for VM guest memory To: "Kiryl Shutsemau (Meta)" , Andrew Morton Cc: Peter Xu , Lorenzo Stoakes , Mike Rapoport , Suren Baghdasaryan , Vlastimil Babka , "Liam R . Howlett" , Zi Yan , Jonathan Corbet , Shuah Khan , Sean Christopherson , Paolo Bonzini , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org References: <20260414142354.1465950-1-kas@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: <20260414142354.1465950-1-kas@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 4/14/26 16:23, Kiryl Shutsemau (Meta) wrote: > This series adds userfaultfd support for tracking the working set of > VM guest memory, enabling VMMs to identify cold pages and evict them > to tiered or remote storage. > > == Problem == > > VMMs managing guest memory need to: > 1. Track which pages are actively used (working set detection) > 2. Safely evict cold pages to slower storage > 3. Fetch pages back on demand when accessed again > > For shmem-backed guest memory, working set tracking partially works > today: MADV_DONTNEED zaps PTEs while pages stay in page cache, and > re-access auto-resolves from cache. But safe eviction still requires > synchronous fault interception to prevent data loss races. > > For anonymous guest memory (needed for KSM cross-VM deduplication), > there is no mechanism at all — clearing a PTE loses the page. > > == Solution == > > The series introduces a unified userfaultfd interface that works > across both anonymous and shmem-backed memory: > > UFFD_FEATURE_MINOR_ANON: extends MODE_MINOR registration to anonymous > private memory. Uses the PROT_NONE hinting mechanism (same as NUMA > balancing) to make pages inaccessible without freeing them. I would rather tackle this from the other direction: it's another form of protection (like WP), not really a "minor" mode. Could we add a UFFDIO_REGISTER_MODE_RWP (or however we would call it) and support it for anon+shmem, avoiding the zapping for shmem completely? -- Cheers, David