From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 58B207E59A for ; Tue, 19 Mar 2024 09:54:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710842068; cv=none; b=SQPIlbN+WfF5pZFUj0HEkznQoC0R05chXXWk/SGUqQpnuOK6d0wjwJpR7pdoQ6AYScYbgsNnY7LZnDycDamKQdWS4BlvsWNZ2KXrGuuqDhfg3Jl36EYlHdZLIeZbZnkxLZipkHGghWkfH3TAriJrIhjIFqc4jQrgPhzCCDbMNpk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710842068; c=relaxed/simple; bh=K/4PfF+tYHqStM63SgGuf/2JKa15Uw5JC/Vkgi2ushE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=f/TwT0M0AJBv8FiLoA27rads079TERMtm/Y40TRIWYdjdcaJOGFHPkKHqyxyH3gqmaFwwhm0Mxs4fSHZJluIm1iWqaLerrm0iXn82u2pAvs7STjZ4KDN1XGHmX6PHGTXyo47SaMyNBHmZWbXlk+NF5yZCrdqaO55+xVsOakAnQ4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=LgngeGq0; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="LgngeGq0" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1710842066; h=from:from: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:autocrypt:autocrypt; bh=h75rJNbjTnf0OcHQw3x/FkbJWebdes8x11NFS4+PgT8=; b=LgngeGq0HdBO6fcMPRfFpU/v4Z58V6rBE01zavOU2JRLyqa6r14dgvF7HWMzUzzsGmKBHN tUfp9xrReDkTkUF3HJDi0FiM6gBSJ6dxkFq5uxYf0yUA2ZbX6rEd1SCjIVJX+wFV6lbtLO bFY4ZpnEihR0bg55ZBjbEy+DjUubXm4= Received: from mail-wm1-f72.google.com (mail-wm1-f72.google.com [209.85.128.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-689-2T7sJ99ZNHuFh3vS3PFwpQ-1; Tue, 19 Mar 2024 05:54:24 -0400 X-MC-Unique: 2T7sJ99ZNHuFh3vS3PFwpQ-1 Received: by mail-wm1-f72.google.com with SMTP id 5b1f17b1804b1-4140a47f514so14681165e9.0 for ; Tue, 19 Mar 2024 02:54:24 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1710842063; x=1711446863; h=content-transfer-encoding:in-reply-to:organization:autocrypt:from :references:cc:to:content-language:subject:user-agent:mime-version :date:message-id:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=h75rJNbjTnf0OcHQw3x/FkbJWebdes8x11NFS4+PgT8=; b=PLpAub1s7QeoIN98iVBCPKzAnlGqnPuTi2pWU0OHyvHHVsgftoXdVPfrGB0ZY6BCsj 7IVCcF3qfqJq0PknP/Zg3FNPSA0fr/na3DT6UlTu7gWYRhX6DXfSGtLZmVm6UE0pMkja bR2I2gXc/2U69ABHc5xAIcKGUi3nfFlS/UvnFpv7tvM1njPO9pNpmkz15a5b0btCuEUo f03vE1rverLv7ckcSNH6bjo3gBa7lX8fG+w+n0X3+t8QJ93W4YlIBnS4R3KVqFLrPUNq PCdcF5c7O7uwAXV5npiNAty7gduusxfA3BJ+egYvDKJK6bUwjmiBlF4mwrsFJtDNgk0F r03g== X-Forwarded-Encrypted: i=1; AJvYcCXNZTTgmL981l1ZYTVYSz121c/5QLeKjnqZmEiPqONqV9mpb38JLYv/EEUw/3bXJMCcY6MsKnvjkyza6HQbmZfjKYT4 X-Gm-Message-State: AOJu0Yz/Qm5fBJC3IlWNemqQANyU8NMmeXgVFgw2+xgU+uWALGyw55N5 10u7ELBp5fFjFht01ffpNrVXZr6HMjM69J4e/l4iWbsBA+tElH4i0ay0Fh408NnC+Kiqv/suK1I nyNMNSOcm79biNzLfvKTsQQJliptUGJFGhhLa3r6AZQJl9Flseg== X-Received: by 2002:a05:600c:1c84:b0:412:e70a:ab8a with SMTP id k4-20020a05600c1c8400b00412e70aab8amr10839500wms.25.1710842063508; Tue, 19 Mar 2024 02:54:23 -0700 (PDT) X-Google-Smtp-Source: AGHT+IE5Ggr0h0CsR5zaVuPj0wHJgcPzLTcPmrLF5jw5gTfCMJdgtAjQppU2RxG5vLt59MOXWtZstQ== X-Received: by 2002:a05:600c:1c84:b0:412:e70a:ab8a with SMTP id k4-20020a05600c1c8400b00412e70aab8amr10839469wms.25.1710842063066; Tue, 19 Mar 2024 02:54:23 -0700 (PDT) Received: from ?IPV6:2003:cb:c741:2200:2adc:9a8d:ae91:2e9f? (p200300cbc74122002adc9a8dae912e9f.dip0.t-ipconnect.de. [2003:cb:c741:2200:2adc:9a8d:ae91:2e9f]) by smtp.gmail.com with ESMTPSA id e13-20020a05600c218d00b00412cfdc41f7sm439884wme.0.2024.03.19.02.54.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 Mar 2024 02:54:22 -0700 (PDT) Message-ID: <73f5f05b-2826-49ea-a3fd-3f1a5b14a223@redhat.com> Date: Tue, 19 Mar 2024 10:54:19 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: folio_mmapped Content-Language: en-US To: Quentin Perret Cc: Sean Christopherson , Matthew Wilcox , Fuad Tabba , kvm@vger.kernel.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, keirf@google.com, linux-mm@kvack.org References: <755911e5-8d4a-4e24-89c7-a087a26ec5f6@redhat.com> <99a94a42-2781-4d48-8b8c-004e95db6bb5@redhat.com> <20240304132732963-0800.eberman@hu-eberman-lv.qualcomm.com> <4b0fd46a-cc4f-4cb7-9f6f-ce19a2d3064e@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 19.03.24 10:47, Quentin Perret wrote: > On Monday 04 Mar 2024 at 22:58:49 (+0100), David Hildenbrand wrote: >> On 04.03.24 22:43, Elliot Berman wrote: >>> On Mon, Mar 04, 2024 at 09:17:05PM +0100, David Hildenbrand wrote: >>>> On 04.03.24 20:04, Sean Christopherson wrote: >>>>> On Mon, Mar 04, 2024, Quentin Perret wrote: >>>>>>> As discussed in the sub-thread, that might still be required. >>>>>>> >>>>>>> One could think about completely forbidding GUP on these mmap'ed >>>>>>> guest-memfds. But likely, there might be use cases in the future where you >>>>>>> want to use GUP on shared memory inside a guest_memfd. >>>>>>> >>>>>>> (the iouring example I gave might currently not work because >>>>>>> FOLL_PIN|FOLL_LONGTERM|FOLL_WRITE only works on shmem+hugetlb, and >>>>>>> guest_memfd will likely not be detected as shmem; 8ac268436e6d contains some >>>>>>> details) >>>>>> >>>>>> Perhaps it would be wise to start with GUP being forbidden if the >>>>>> current users do not need it (not sure if that is the case in Android, >>>>>> I'll check) ? We can always relax this constraint later when/if the >>>>>> use-cases arise, which is obviously much harder to do the other way >>>>>> around. >>>>> >>>>> +1000. At least on the KVM side, I would like to be as conservative as possible >>>>> when it comes to letting anything other than the guest access guest_memfd. >>>> >>>> So we'll have to do it similar to any occurrences of "secretmem" in gup.c. >>>> We'll have to see how to marry KVM guest_memfd with core-mm code similar to >>>> e.g., folio_is_secretmem(). >>>> >>>> IIRC, we might not be able to de-reference the actual mapping because it >>>> could get free concurrently ... >>>> >>>> That will then prohibit any kind of GUP access to these pages, including >>>> reading/writing for ptrace/debugging purposes, for core dumping purposes >>>> etc. But at least, you know that nobody was able to optain page references >>>> using GUP that might be used for reading/writing later. >>> >>> Do you have any concerns to add to enum mapping_flags, AS_NOGUP, and >>> replacing folio_is_secretmem() with a test of this bit instead of >>> comparing the a_ops? I think it scales better. >> >> The only concern I have are races, but let's look into the details: >> >> In GUP-fast, we can essentially race with unmap of folios, munmap() of VMAs >> etc. >> >> We had a similar discussion recently about possible races. It's documented >> in folio_fast_pin_allowed() regarding disabled IRQs and RCU grace periods. >> >> "inodes and thus their mappings are freed under RCU, which means the mapping >> cannot be freed beneath us and thus we can safely dereference it." >> >> So if we follow the same rules as folio_fast_pin_allowed(), we can >> de-reference folio->mapping, for example comparing mapping->a_ops. >> >> [folio_is_secretmem should better follow the same approach] > > Resurecting this discussion, I had discussions internally and as it > turns out Android makes extensive use of vhost/vsock when communicating > with guest VMs, which requires GUP. So, my bad, not supporting GUP for > the pKVM variant of guest_memfd is a bit of a non-starter, we'll need to > support it from the start. But again this should be a matter of 'simply' > having a dedicated KVM exit reason so hopefully it's not too bad. Likely you should look into ways to teach these interfaces that require GUP to consume fd+offset instead. Yes, there is no such thing as free lunch :P -- Cheers, David / dhildenb