From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id B018DF9D0D3 for ; Tue, 14 Apr 2026 15:38:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 27A706B0096; Tue, 14 Apr 2026 11:38:01 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 251136B0098; Tue, 14 Apr 2026 11:38:01 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1675B6B0099; Tue, 14 Apr 2026 11:38:01 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 03DF46B0096 for ; Tue, 14 Apr 2026 11:38:01 -0400 (EDT) Received: from smtpin04.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id D2432E0279 for ; Tue, 14 Apr 2026 15:38:00 +0000 (UTC) X-FDA: 84657567120.04.E2CC8A5 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf21.hostedemail.com (Postfix) with ESMTP id EFF331C000A for ; Tue, 14 Apr 2026 15:37:58 +0000 (UTC) Authentication-Results: imf21.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=NjdxK0h2; spf=pass (imf21.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1776181079; a=rsa-sha256; cv=none; b=Kfbwpb+4oscuCg20Wh2t9JTKfQN1gcoQYKUPsqVkowXIX459Ykv5V6SEEfGovREZgMnG6+ kbcM8gdgLLdXyAaId6/2N/bA1hXPXo0sfAnYPtKlwTv1jUYouYcruJGUoB29a4cxyujoGj scA2XxaeZtDJr4qFP6QytUi2T6dL/dY= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1776181079; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=ANrqflZwQDfczuzqrIcfJ0PCScTyhm32PWssrqXSJBQ=; b=0kc/N4UsCnSOWtqPVlnCnYbi3zZdnD73H9oauTnyX1h2zxnGEUDHMizlcOQiXeMKZWultD je2GLJfQetOPqtWriHw/Sfj6GUxq8hU7G0pTwl7hBZsRHS/ehtI3zM4yVpiuwRymvZFEZM Bl3mhVUp1TGselmieUSV+Vw1pZEZHfE= ARC-Authentication-Results: i=1; imf21.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=NjdxK0h2; spf=pass (imf21.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id C2DBD41841; Tue, 14 Apr 2026 15:37:57 +0000 (UTC) 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 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 X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: EFF331C000A X-Stat-Signature: uiyixa17az3ey3jza41ogi3yrqb1fu69 X-HE-Tag: 1776181078-539892 X-HE-Meta: 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 SV/BAdnN CBSHtcd8sKq+Iqg2rX7DbQaO9UGi/ab73ktPjLSmAMNjsKEKf+c6VTvT77z1SBPLsE+MPd0W7kE9gIlcHGj8Z4piMszvsa+WTpkLgKH+GV03Z7spRlMpVMQqIlry8Xw6C7xR+kaGlCkdf0q8/ZYf8d5sDsdG/p+s4e5fQHISHPkvPOXz7hc3c8xUi6Ze5dzCuiaEvifcW7eECmXdxa7fAGlQeEs9E2ywYodB5UTRNgK56Ffx/VnQlT3mhwMlItNepx19LDVV9gg302uK7XwaktpC0n+nMm6rP5JUK Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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