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 8A596263F34; Sat, 25 Apr 2026 05:57:00 +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=1777096620; cv=none; b=aeuH5xJTDTpbJX6R/0SawTZJ5IEebS6X7lMbrJa9hT0q7ri7iLwSNC2R1M/ftlH+52QgRyB6hrLpz2HRpXW/Dg6N29DtUij4NTeD29zbz2RkaQM8cXH9OF/8TLUZJVINZNRiJN07lPcFWnQ57veNxP3tL5w7ToJWbRdiaHDNtzA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777096620; c=relaxed/simple; bh=Tm1RVF/otoy7t8GU5QzG858Hip+jjhcSOcigpbnnIKY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=K+DDL6EA4xiCL6zPciHIwQE64OzK+4DNGK+uLjXznQx1VGSEnd42Rbd5uQ6vXUoBQQmbqgLSBgiTmbLkg6bFI+8ooCgvXM6D4saL9BjAzebtU35LFDsv1qHX9YJ1SvZuo0w7xPtaAriCGGetLhPmmYT14PNhtlhN40fUiubLoDw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=csaj2/sP; 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="csaj2/sP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BD955C2BCB0; Sat, 25 Apr 2026 05:56:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777096620; bh=Tm1RVF/otoy7t8GU5QzG858Hip+jjhcSOcigpbnnIKY=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=csaj2/sPuUjak144yih/NYw1vSry/TXCHzWpoDZcz5rAcuuVYu5iInicKb4zlCr44 w9BepnEXbI6L9DWYUaTMPSi6oeEGk7YcprFxmNZus0KmJ4M5azn8XHPlou0JYacw+h bDvFfwrtUnik8NSo1Gm7EzIXQazOuvmYF2mWkscyHv85tIwo2FAqHrxyOovalwtkGE FHiuWIzk5oN8en9lg33iJ/GvC8hEbbBfsyEi5oGw6YUXwmHbVDdhDNCIbdUtvfSzXj 8W3+FjlTTLkY0fd5L+eBYd6NWQQo70jpddYtlprwAlT/rvNwT11pUuqG12oFU3uNA3 lIqP29VveexVA== Message-ID: <602b431e-244e-4171-bc82-6ba7ee71fe65@kernel.org> Date: Sat, 25 Apr 2026 07:56:52 +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: Peter Xu Cc: Kiryl Shutsemau , Andrew Morton , 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: <34f75083-29a3-4860-8a6e-94551d37ac6a@kernel.org> <17b0dc02-eee3-46d6-9afb-5f81a3a20216@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 On 4/23/26 22:10, Peter Xu wrote: > On Thu, Apr 23, 2026 at 09:25:30PM +0200, David Hildenbrand (Arm) wrote: >>> >>> The other thing is, as I mentioned in the other email, I still don't know >>> how the current RW protection would work for anonymous. I don't yet think >>> the user swapper can read the anon page with RW-protected pgtables. So far >>> my understanding is maybe you only care about shmem so it's fine, but it'll >>> always be great to confirm with you. >> >> I wonder if uffdio_move could be used for a swapper implementation instead? > > If RW is justified to be useful first, maybe. > > I had a gut feeling Kirill's use case doesn't use anon at all, then if > nobody needs it we can still decide to not support anon. > >> >> If we ever have to read from a protnone page, maybe we could teach ptrace access >> to do it, or have something that can read from prot_none areas -- like >> uffdio_copy, which can write to prot-none areas. > > Somethinig like swap_access() in my proposal can also partly achieve that. Looks more like the hammer for the nail here: we could fault the page in just fine, while keeping it mapped prot_none and keeping the uffd-rwp pte bit set. I was rather thinking of some uffd-specific thing that can read from a uffd-rwp protected pte without trigger uffd. > > https://lore.kernel.org/all/aYuad2k75iD9bnBE@x1.local/ > > There, it was only about reading from swap so far, though. But that one > might be easier to be extended to read PROT_NONE and directly put data into > buffer user specified (ps: in my local tree impl I named it maccess() to > pair with mincore(), but it doesn't really matter; it doesn't even need to > be a syscall..). > > To me, the interfacing is not a major issue. The major question I have is > why RW protection can help in swap system impl when we already have uffd-wp. > > So I want to make sure the use case can't be implemented by uffd-wp already. > Because that's really what we might do for QEMU. There has to be some added value indeed. -- Cheers, David