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 6891EF4368D for ; Fri, 17 Apr 2026 11:43:47 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CFD046B00E1; Fri, 17 Apr 2026 07:43:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id CD5256B00E3; Fri, 17 Apr 2026 07:43:46 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BC35F6B00E4; Fri, 17 Apr 2026 07:43:46 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id AB8266B00E1 for ; Fri, 17 Apr 2026 07:43:46 -0400 (EDT) Received: from smtpin22.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 720DCC02B9 for ; Fri, 17 Apr 2026 11:43:46 +0000 (UTC) X-FDA: 84667863252.22.238E835 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf24.hostedemail.com (Postfix) with ESMTP id 8D411180005 for ; Fri, 17 Apr 2026 11:43:44 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b="rh1kWv/p"; spf=pass (imf24.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-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1776426224; 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=5XZA8Z7Wwen7bvvhGV+9K7epD3nJl6JbbaIA3EMgD1Q=; b=tGODEP2uAvoBW48/0sFs/A5qhp4bhZlGrHaeEVfOiqVHo6nE5iLNo3kmh7a6DRMOIvYwhJ P2geR0NMyG6MakNtcuYFMnDEGp8lfaf1QcIzISJzOi4oQ4/CsYEYw9ypiGTSgWrM72Krp+ 3UKlresh/vG/LuWDLHqlYFniarkYFM8= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1776426224; a=rsa-sha256; cv=none; b=xTVFLc/wDN5W1XFbxOE/22XqJ5l8v0QsTgHMOYMO2YHr4JdU5kfDyCAagMcAPTrhTPy7/p HwNUq73Ww105eH1nVLPC8E151fcaokvvwRdEOhkZkzN1btwdItxphYd19BGNN8+OSEHx+v cWA0Umct9Dt/OF5Ld3IZSGH4TSCTQ98= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b="rh1kWv/p"; spf=pass (imf24.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 88F454370F; Fri, 17 Apr 2026 11:43:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A65EEC19425; Fri, 17 Apr 2026 11:43:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776426223; bh=JGgbvpvWm+Ik8ZBTXf+YL1qrGBJYpDxeijGIxjtDHkQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=rh1kWv/pWbdLIgEVHPwauDYHLUIFVZUP6FH3zD4hG9r2CQf6vxykMagcdtJFPSIIh tvy0ByCX5em5C880qRgnFn/S8aaUQANfgMCf8to2uMjMCmTq2s3rrHdNmb9hInzEHJ WMIIFTxxRM3gWA7aH/HuEO+uIXvDNsBGHX3agIUwDr2FHjMtyg8Z4+nod8NYhvHVyY HvA1V02mYmjnHe0YwVr8667tXr1ErTSWKzesTg2Nd0qQHyqrFx43Y8UNCVAvkN9ctn CF3h766uicbcARhlCRZdj/jdFjB+2gYrJdUX0NPbizpPhrgpBKqOuWk/uv54W/F0xX OHjknsEsvoAnQ== Message-ID: <4c635703-3d8d-4cfa-bb98-7f6f5fcbe547@kernel.org> Date: Fri, 17 Apr 2026 13:43:36 +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 Cc: Andrew Morton , 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> <55019037-4f1c-4d9c-83ee-3a844d8f3d5e@kernel.org> <1a499781-1115-44bc-adbf-2ac3769354ca@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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 8D411180005 X-Rspamd-Server: rspam07 X-Stat-Signature: crdj5u1m7qoktmmxcebn3jedekrho8mj X-Rspam-User: X-HE-Tag: 1776426224-117323 X-HE-Meta: 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 kYJcjwBV x8KUmBuF+gxBmSlLEmleDLJqELhI2Rfe7Smzxu+rs4G3yQRtm6WGkX7y8CxFhSpLC+QtJA1Qd5yIudL3vhFWT+UVwx5TurmGOBkP2n1mZL1Ftq7v8ENOwCSHp/ayAbsUibGKzk8lE6RZz7XH69hcQweWlH+5NWtOuMDpb0MWiXnO1oMwuEwq+FT3FaYIYyIuaCAdHD+LA5GikQMLo5jpZnBoxg5NknpyCea/3keuKvMCL0SfI4EDEm6PbQVJviECPbFORbTEKDCTkBWHqNvh8K/ygLQXG/Nou5LrH Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/16/26 22:25, Kiryl Shutsemau wrote: > On Thu, Apr 16, 2026 at 08:32:19PM +0200, David Hildenbrand (Arm) wrote: >> On 4/16/26 15:49, Kiryl Shutsemau wrote: >>> >>> Here is an updated version: >>> >>> https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/log/?h=uffd/rfc-v2 >>> >>> will post after -rc1 is tagged. >>> >>> I like it more. It got substantially cleaner. >> >> I don't have time to look into the details just yet, but my thinking was >> that >> >> a) It would avoid the zap+refault > > Yep. > >> b) We could reuse the uffd-wp PTE bit + marker to indicate/remember the >> protection, making it co-exist with NUMA hinting naturally. >> >> b) obviously means that we cannot use uffd-wp and uffd-rwp at the same >> time in the same uffd area. I guess that should be acceptable for the >> use cases we you should have in mind? > > I took a different path: I still use PROT_NONE PTEs, so it cannot > co-exist with NUMA balancing [fully], but WP + RWP should be fine. I > need to add a test for this. > > I didn't give up on NUMA balancing completely. task_numa_fault() is > called on RWP fault. So it should help scheduler decisions somewhat. > > I think an RWP user might want to use WP too. > > Do you see this trade-off as reasonable? One reason why the PTE bit was added for the WP case was to distinguish it from other write faults. I assume without a dedicated PTE bit your design will always suffer from false positive notifications. Leaving NUMA-balancing aside, a simple mprotect(PROT_NONE)+mprotect(PROT_READ) would already be problematic to distinguish both cases. Zap+refault for shmem would likely have similar problems (we'd need a marker). I don't think a design that allows for false positives is what we really want, especially as it would diverge from what we already have for WP. Yes, using the PTE bit (that we already have) implies that we could, for now, not allow the combination of WP + RWP. -- Cheers, David