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 43593C5B572 for ; Tue, 11 Aug 2026 17:57:21 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 0A1F56B00BD; Tue, 11 Aug 2026 13:57:20 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 052026B00BF; Tue, 11 Aug 2026 13:57:19 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E5C7C6B00C0; Tue, 11 Aug 2026 13:57:19 -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 ABCA26B00BD for ; Tue, 11 Aug 2026 13:57:19 -0400 (EDT) Received: from smtpin11.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 37DCF401A7 for ; Tue, 11 Aug 2026 17:57:19 +0000 (UTC) X-FDA: 85089745398.11.E115357 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf08.hostedemail.com (Postfix) with ESMTP id 6FE79160004 for ; Tue, 11 Aug 2026 17:57:17 +0000 (UTC) Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=X4emHB7M; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf08.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1786471037; 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=g/k363n5Lvqavno9fCoFoqYAurRBI/pjeBdWiyEPQAI=; b=dX3VcdgDoMeoYNa+ZAsKRtjqzjsdPUIvVZtOvEOotL94hOvm8MbPvxmCZnZLNPxYh3STJC o9qhxcDH4kBjRPwwTTBaOwsPwnRU6zLQFDrz4ZpQmvSW8fpQztDciPs7NIG0A1p+UQLpcs 0UqiriyDGOxhLdEt5pHhSG7YcQOF7uY= ARC-Authentication-Results: i=1; imf08.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=X4emHB7M; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf08.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1786471037; b=BBplurSIrNc51ezDC+jVUelp6q1wQZijUX9RQvZiwbaK18+dArPYlpEnO7QxWSp40Pu3po ZVurwTSSENbMmdPgF4mzXimsCG+rop9lKtxBs6Zej/DMhUuKibJQ0gTVwnAGcCiGD+lD7E Nvx4Jbb1gm4oHhTVFPUtI0iAkNV/WMw= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id EF03D600AD; Tue, 11 Aug 2026 17:57:16 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 086321F000E9; Tue, 11 Aug 2026 17:56:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786471036; bh=g/k363n5Lvqavno9fCoFoqYAurRBI/pjeBdWiyEPQAI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=X4emHB7MFkge8y440tlJsYBHJo5+N8Li59e/0ORXOxym8rC+y/HdLt15DOSVbQWZ1 4HO9ZhlCXXfzjkUtIBxtAGmKsTbVUwax3Osp2lGhzk5U6PlORe0UqDJZF2aS14FEkG AncR9K+gj0URdsSSUZaOsznpEDLwe/cJqEhunPqw+hFAeWd3SOCySFNhesYu2UURLH 3x+CVp0x/ZkCUXlvWC5Vm+AnAcaZt7I+AF0LdrdCIhLZoEehItGo3B1D8Zo7s9HOQL KUGYasF/2xyLJJbsiIa9x3ecW1a3ytFa0oB+z3tdXlL8ksBKDj0DPO6yA1//U+UCRc A8CF2e7vWz+TQ== Message-ID: <4c638fc5-5a74-43d0-b5bf-d7ef893cb994@kernel.org> Date: Tue, 11 Aug 2026 19:56:57 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 11/41] KVM: guest_memfd: Ensure pages are not in use before conversion To: Sean Christopherson , Ackerley Tng Cc: aik@amd.com, andrew.jones@linux.dev, binbin.wu@linux.intel.com, brauner@kernel.org, chao.p.peng@linux.intel.com, jmattson@google.com, jthoughton@google.com, michael.roth@amd.com, oupton@kernel.org, pankaj.gupta@amd.com, qperret@google.com, rick.p.edgecombe@intel.com, rientjes@google.com, shivankg@amd.com, steven.price@arm.com, tabba@google.com, willy@infradead.org, wyihan@google.com, yan.y.zhao@intel.com, forkloop@google.com, pratyush@kernel.org, suzuki.poulose@arm.com, aneesh.kumar@kernel.org, liam@infradead.org, Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Shuah Khan , Shuah Khan , Vishal Annapurve , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Baoquan He , Jason Gunthorpe , John Hubbard , Peter Xu , tarunsahu@google.com, Vlastimil Babka , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev References: <20260807-gmem-inplace-conversion-v10-0-2fc18ee6d3ba@google.com> <20260807-gmem-inplace-conversion-v10-11-2fc18ee6d3ba@google.com> 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-Stat-Signature: onhazugj6q1q95gn1gfdjd33xa4ictzg X-Rspamd-Queue-Id: 6FE79160004 X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1786471037-808410 X-HE-Meta: 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 V8XhzCUt ogVrZCaIXXc2LvYYaIzvvsbkEp+xwanJ57gITv6HWky+Z/C4I0CArc2LNdfhp994uKsNFyvKJnC0sj1hsqZ0/TflnHiznBDqi/RhBKtMMjjzBxNnfX6NymnJSpW3G9mY46UM0loXEog7+DK30g3mYCTk1whPAvi4rQJqB+9NO616Rd0q5+a1ZTgeRJfqnLb2iBR1tUD+QOII3WPyQNPPlC/5pHvQcqN95APMS/nU1eNTPBzxkO1OGy0CFFmnhDiajRfjCDpSXHD5142SLGuQ3rQQ+qGa/vG0DgC9p0RecP1YSGCpDSwpHPaEKcA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > helps the reader understand what's being checked without having to look at the > details, and also helps communicate the ordering dependency without needing a > comment. > > There are definitely times where the usage of a function bleeds into its name, > but usually that's because the name and the usage are on and the same. E.g. > get_user() describes both the usage and the "what". And it's easy/possible to > go too far in the opposite direction, e.g. by giving a play-by-play of what a > function is doing, but that's why we have bikshedding sessions :-) Note that the problem I have with kvm_gmem_is_safe_for_conversion() that it is all about *conversion to private*, not *conversion to shared*. In that sense, the function name is just confusing. > >> Perhaps a little ahead of its time, > > Ya. > >> but later with restructuring for huge pages, we also need no additional >> refcounts other than gmem's own so that restructuring is safe, hence this >> function name was meant to extend there as well. > > Given that I've read that at least five times and still don't understand the > nuance, I think it's safe (ha!) to say we'll need to revisit and review those > changes no matter what. :-) > > >> In this case "unexpected" (especially since the next patch adds checks >> for maybe dma pinned and unmapping), begs the question "unexpected in >> what way"? > > Ya, that's why I like "outstanding", it succinctly captures that one or more > references have been "loaned" but not yet "repaid". > >>> >>> I'd rather add a comment than have this filemap_get_folios_refcount. > > +1, the local variable just made me scratch my head. > >>> >>> /* >>> * We expect one reference per folio-page in the pagecache and one >>> * reference from filemap_get_folios(). > > Nit, please no pronouns in KVM code. Whatever floats KVM's boat :) > >>> */ >>> if (folio_ref_count(folio) != folio_nr_pages(folio) + 1) >>> >> >> This comment explains what's "unexpected". I can do this and switch it >> to kvm_gmem_mem_has_unexpected_refs() unless people have other >> suggestions. >> >> I wish there was a folio_pagecache_refs(folio) that >> folio_expected_ref_count() can share with this, and also >> folio_swapcache_refs(), to solidify the definition of refcounts taken by >> the pagecache. > > ... > >>> >>> I'd add a comment here for the "why are we unmapping". >>> >> >> Does this sound right: >> >> Unmap here to ensure that userspace page tables have no mappings, which >> also ensures refcounts from those mappings are dropped. > > How about: > > /* > * Forcefully unmap the pages from all userspace page tables, > * and then verify there are no outstanding references, e.g. > * acquired via GUP or similar. Tell userspace to try again if > * there are oustanding references and hope that whatever has > * pinned the page will put its reference "soon". > */ > unmap_mapping_pages(mapping, start, nr_pages, false); > > if (!kvm_gmem_is_safe_for_conversion(inode, start, nr_pages, > err_index)) { > mas_destroy(&mas); > r = -EAGAIN; > goto out; > } Sounds good besides the function still not being clear about *which* kind of conversion. -- Cheers, David