From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 0976030F7FF; Mon, 10 Aug 2026 09:20:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786353611; cv=none; b=A/p3Qlcy5SxnPng1VLp9jgactHcsh1HD1w073O7IEegozIdKLZ56sh8OjuC8FEIEmgY29/GiZ8M0AMqm7Qblnd9lCQPVWtcqHUYqVsZFveCAI+UDZnhT1KeuIaIoyWB5qUTMvtK30hmRZDzeZOQ5n9L6Ah20EwTuk2autaiLf4g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786353611; c=relaxed/simple; bh=7vge6SQNAp4DZJJsAeFyFclIvAG4Wz4oqgpUkveaZR4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rHr5SaFXVYRSwqScj8qAHHkilZKhZ5dYHgq3TqfTK/4iNWjWTjqqLmBmkmaM6JibRFnbuCcY5pDKhS5crZKcSWkrLYqNcXDG10cbmXN1b39/yo74Wl00z08GZDcQMuuEWQwyBS3FNivCP8b75Le6/AYsA9pi7YsYyDS2kFcGvDo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Srx5NcHv; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Srx5NcHv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C7EA81F000E9; Mon, 10 Aug 2026 09:19:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786353609; bh=NOaEX7G7WbL3WRc3qOqS40hHvnBJZDj9XRocVTD2BrE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Srx5NcHvuYWHX7NKDmEKead6xMO2DdHxbktyD9Wsiv3rNza+l3o4CtjVjAv1VubgE 3w3ZeQ7jZ+1VmfNPGmp/BFpMnSDsI9RIQtrg0a2v8irfeFG6aMZadFAobQ1+sDZ6ly 4UW4Zj7DfHuZjUIWGda7tQp5AhJ981xhEgzC4cz5rW1S4qzCs7DdoWdLOR7X0Cbtt9 IU3a8mFD5qgLgfiOk59J7AgohHhlXa52xlis0TM8yJMkYifzxw49Fad1jB5MwWHHPm Xi1xUcLa9D3aOkNAvOT+o2xZ4xRddHQ+h1ExEA7gUCBQQ4G1fXw8ms9QYrrHIJYr3m BjXmxfFWKGSEw== Message-ID: Date: Mon, 10 Aug 2026 11:19:48 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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: ackerleytng@google.com, 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 , Sean Christopherson , 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 Cc: 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: <20260807-gmem-inplace-conversion-v10-11-2fc18ee6d3ba@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote: > From: Ackerley Tng > > When converting memory to private in guest_memfd, it is necessary to ensure > that the pages are not currently being accessed by any other part of the > kernel or userspace to avoid any current user writing to guest private > memory. > > guest_memfd checks for unexpected refcounts to determine whether a page is > still in use. The only expected refcounts after unmapping the range > requested for conversion are those that are held by guest_memfd itself. > > Update the kvm_memory_attributes2 structure to include an error_offset > field. This allows KVM to report the exact offset where a conversion > failed to userspace. If the safety check fails, return -EAGAIN and copy > the error_offset back to userspace so that it can potentially retry the > operation or handle the failure gracefully. > > Update documentation to document the error_offset field and the possible > -EAGAIN error. > > Suggested-by: David Hildenbrand > Co-developed-by: Vishal Annapurve > Signed-off-by: Vishal Annapurve > Reviewed-by: Fuad Tabba > Tested-by: Shivank Garg > Signed-off-by: Ackerley Tng > --- [...] > #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3) > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 3783e63476569..13c3989136f67 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -524,8 +524,42 @@ static int kvm_gmem_mas_preallocate(struct ma_state *mas, u64 attributes, > return mas_preallocate(mas, xa_mk_value(attributes), GFP_KERNEL); > } > > +static bool kvm_gmem_is_safe_for_conversion(struct inode *inode, pgoff_t start, > + size_t nr_pages, pgoff_t *err_index) I would focus on the "to_private" aspect or abstract it to "kvm_gmem_mem_has_unexpected_refs" or sth like that. > +{ > + struct address_space *mapping = inode->i_mapping; > + const int filemap_get_folios_refcount = 1; > + pgoff_t last = start + nr_pages - 1; > + struct folio_batch fbatch; > + bool safe = true; > + pgoff_t next; > + int i; > + > + folio_batch_init(&fbatch); > + > + next = start; > + while (safe && filemap_get_folios(mapping, &next, last, &fbatch)) { > + for (i = 0; i < folio_batch_count(&fbatch); ++i) { > + struct folio *folio = fbatch.folios[i]; > + > + if (folio_ref_count(folio) != > + folio_nr_pages(folio) + filemap_get_folios_refcount) { I'd rather add a comment than have this filemap_get_folios_refcount. /* * We expect one reference per folio-page in the pagecache and one * reference from filemap_get_folios(). */ if (folio_ref_count(folio) != folio_nr_pages(folio) + 1) > + safe = false; > + *err_index = max(start, folio->index); > + break; > + } > + } > + > + folio_batch_release(&fbatch); > + cond_resched(); > + } > + > + return safe; > +} > + > static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, > - size_t nr_pages, uint64_t attrs) > + size_t nr_pages, uint64_t attrs, > + pgoff_t *err_index) > { > bool to_private = attrs & KVM_MEMORY_ATTRIBUTE_PRIVATE; > struct address_space *mapping = inode->i_mapping; > @@ -542,8 +576,21 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, > > mas_init(&mas, mt, start); > r = kvm_gmem_mas_preallocate(&mas, attrs, start, nr_pages); > - if (r) > + if (r) { > + *err_index = start; > goto out; > + } > + > + if (to_private) { I'd add a comment here for the "why are we unmapping". -- Cheers, David