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 C845A35B63C; Wed, 22 Apr 2026 18:39:57 +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=1776883197; cv=none; b=RKtHoCvm1ZkxZN18xuKqooIsLqVe5A5GpFWVnEAM8y7xBhayE3o1Iej2yJZlroOdVbuVp5u19uNd0B2S0NLoGgHH67RaHMgK724zJEEqU9lz4BacyO0TMz6DAe4oXffRIgcrSMIxnkMiCiqVCfJ0YZs2lWMsV6fygYly/hp1T8c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776883197; c=relaxed/simple; bh=gPBkLTS8xvviMRNP8SEcmV5ut7acgVc3jDTKipEUsns=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Hv8NvnPpoulgpkFgHey7C+8Y6Ws5Gsclfy0oApV4X3Z95jpP7BIIlFbCMih114bt22Sx/nYv3MqfX/Sn1QHheFyYAvBDZP6I3/H7aPz4bfft3yREhFIZHHPLuDP3DzlvVF4/PhgOpW8kJs1njaSuZ/P5DvOND8af9u2CzRD4t0Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RG1AIUof; 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="RG1AIUof" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6A4F7C19425; Wed, 22 Apr 2026 18:39:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776883197; bh=gPBkLTS8xvviMRNP8SEcmV5ut7acgVc3jDTKipEUsns=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=RG1AIUofKfe1bYx9Q4cRXAEWj+ON9FElIcMnnfMFF5BknmDNEGf66PIK9UkMmih4x q4NIPGJeGozprMQtH0pH+1BGB0EycqP3DdE4hsePDKc9wLbrk0rCcr0QGuyZi/inG0 Q14x9hD9OKGhJslLLO4vekucewh5KWpQQh/pFsm2mIyNW6/ib4cYFwNT0njGDjRqcz ZiZXIEM9kE+YJMcvfUsW4MF20wGd+/oxvvcfR4drh233AJLyRUYZmR009wFJVXp2am 9b6/ARH061RBy+hrJ/gWkrclE4F/ERVWbBtDGpVIzd6ISnUXHEzAHNEQWAm89JrQne 5K/NY4RJaPr1Q== Message-ID: Date: Wed, 22 Apr 2026 20:39:50 +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: [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> <4c635703-3d8d-4cfa-bb98-7f6f5fcbe547@kernel.org> <34f75083-29a3-4860-8a6e-94551d37ac6a@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 4/21/26 16:33, Kiryl Shutsemau wrote: > On Tue, Apr 21, 2026 at 03:03:56PM +0200, David Hildenbrand (Arm) wrote: >> On 4/19/26 16:33, Kiryl Shutsemau wrote: >>> >>> See https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git uffd/rfc-v3 >>> >> >> Quick feedback from skimming over it: >> >> >> 1) ARCH_SUPPORTS_PROT_NONE needs some thought, because I am pretty sure all >> architectures support something like mprotect(PROT_NONE), and the config >> option might be misleading. >> >> So you very likely want to express different semantics here. You want to >> know whether pte_protnone()/pmd_protnone() works. > > We do support mprotect(PROT_NONE) everywhere, but we don't always have a > way to distinguish such entries from others without VMA in hands. Like, > there are other PTEs that don't have present bit set. In my and NUMA > balancing context we cannot rely on VMA, because we want to install > PAGE_NONE entires into accessible VMA. Exactly. So it's not ARCH_SUPPORTS_PROT_NONE. > > So we need two things; pte/pmd_protnone() checks and PAGE_NONE itself. > The first to test PTE for PAGE_NONE, the second for pte/pmd_modify() to > make the entry protnone. > > Currently, generic code only use this functionality for NUMA balancing > and gated by NUMA balancing config option. So I moved it under separate > config option. > > Do you want it to be named differently? Would ARCH_SUPPORTS_PXX_PROTNONE or sth. like that better describe that pte_protnone()/pmd_protnone() do what we want? > >> 2) The other stuff is really just an extension of existing WP handling. >> I suspect we want to have some reasonable cleanups to not end up in >> common code with >> >> @@ -1841,7 +1841,7 @@ static void copy_huge_non_present_pmd( >> add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR); >> mm_inc_nr_ptes(dst_mm); >> pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable); >> - if (!userfaultfd_wp(dst_vma)) >> + if (!userfaultfd_wp(dst_vma) && !userfaultfd_rwp(dst_vma)) >> pmd = pmd_swp_clear_uffd_wp(pmd); >> set_pmd_at(dst_mm, addr, dst_pmd, pmd); >> >> All the uffd handling should be better isolated (i.e., a single vma check?), >> and likely the uffd bit should be abstracted away from being called "wp" to >> something more generic. >> >> Maybe it's simply a "uffd" flag which's semantics depend >> on the vma flags. >> >> Maybe something like: >> >> @@ -1841,7 +1841,7 @@ static void copy_huge_non_present_pmd( >> add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR); >> mm_inc_nr_ptes(dst_mm); >> pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable); >> if (!userfaultfd_uses_pte_bit(dst_vma)) >> pmd = pmd_swp_clear_uffd(pmd); >> set_pmd_at(dst_mm, addr, dst_pmd, pmd); >> >> Not sure, needs another thought. But I think there are some decent >> cleanups to be had. > > That's fair. Maybe userfaultfd_protected() name is better for the VMA > check? Yes, something like that could also work. > > And about UFFD_WP bit name. Maybe we can just drop _WP: _PAGE_UFFD_WP -> > _PAGE_UFFD, pte_uffd_wp() -> pte_uffd()? Yes, I hinted at the above with pmd_swp_clear_uffd(). > > But it is a lot of changes. Can I do the bit rename as a follow up > patchset? Let's get this clean. There is no need to rush that in ;) I suspect it's a fairly mechanical change. > >> 3) Some other stuff needs a second thought, like >> >> diff --git a/mm/gup.c b/mm/gup.c >> index 8e7dc2c6ee738..08fc18f1290d4 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -695,7 +695,8 @@ static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page, >> /* ... and a write-fault isn't required for other reasons. */ >> if (pmd_needs_soft_dirty_wp(vma, pmd)) >> return false; >> - return !userfaultfd_huge_pmd_wp(vma, pmd); >> + return !userfaultfd_huge_pmd_wp(vma, pmd) && >> + !userfaultfd_huge_pmd_rwp(vma, pmd); >> } >> >> How can a pte be writable and prot_none at the same time? Maybe just confused AI >> output that you should carefully double check before sending that out officially. > > Note that this path is for !pmd_write() case to begin with. It serves > FOLL_FORCE case. I believe this check is correct: we don't want to allow > to write to such pages even with FOLL_FORCE. > > But looking around, I missed gup_can_follow_protnone() modification. It > has to return false for RWP. Right, read-permission checks come before the write-permission checks. > >> 4) How do we want to handle PM_UFFD_WP? >> >> We are pretty much out of flags soon. Overloading PM_UFFD_WP means that we will not >> be able to easily support using a separate bit. >> >> But our internal design will not easily allow that either, and I am not really >> sure we want to go down that path any time soon. >> >> Maybe we could document this for now as "In WP VMAs, indicated WP PTEs. >> Otherwise, in RWP VMAs, indicates RWP.". Whenever we would allow both at the >> same time, we could change the semantics. User space would fail to create one >> with both protection types for now either way. > > Yeah. I think about doing documentation-only update for PM_UFFD_WP for > now. Ok, good! -- Cheers, David