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(p200300cbc70b05006a0617fc810bb054.dip0.t-ipconnect.de. [2003:cb:c70b:500:6a06:17fc:810b:b054]) by smtp.gmail.com with ESMTPSA id z9-20020adff749000000b00323287186aasm3887984wrp.32.2023.10.19.01.26.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Oct 2023 01:26:43 -0700 (PDT) Message-ID: <0e436056-95c3-4ed6-a17e-a46a780e5ab9@redhat.com> Date: Thu, 19 Oct 2023 10:26:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/4] softmmu/physmem: Warn with ram_block_discard_range() on MAP_PRIVATE file mapping To: Xiaoyao Li , qemu-devel@nongnu.org Cc: "Michael S. Tsirkin" , Juan Quintela , Peter Xu , Leonardo Bras , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= , Peng Tao , Mario Casquero References: <20230706075612.67404-1-david@redhat.com> <20230706075612.67404-2-david@redhat.com> <23d209c1-f860-4915-935e-816d9077b65c@intel.com> Content-Language: en-US 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: 8bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 18.10.23 18:27, Xiaoyao Li wrote: > On 10/18/2023 5:26 PM, David Hildenbrand wrote: >> On 18.10.23 11:02, Xiaoyao Li wrote: >>> On 10/18/2023 3:42 PM, David Hildenbrand wrote: >>>> On 18.10.23 05:02, Xiaoyao Li wrote: >>>>> David, >>>>> >>>>> On 7/6/2023 3:56 PM, David Hildenbrand wrote: >>>>>> ram_block_discard_range() cannot possibly do the right thing in >>>>>> MAP_PRIVATE file mappings in the general case. >>>>>> >>>>>> To achieve the documented semantics, we also have to punch a hole into >>>>>> the file, possibly messing with other MAP_PRIVATE/MAP_SHARED mappings >>>>>> of such a file. >>>>>> >>>>>> For example, using VM templating -- see commit b17fbbe55cba >>>>>> ("migration: >>>>>> allow private destination ram with x-ignore-shared") -- in >>>>>> combination with >>>>>> any mechanism that relies on discarding of RAM is problematic. This >>>>>> includes: >>>>>> * Postcopy live migration >>>>>> * virtio-balloon inflation/deflation or free-page-reporting >>>>>> * virtio-mem >>>>>> >>>>>> So at least warn that there is something possibly dangerous is >>>>>> going on >>>>>> when using ram_block_discard_range() in these cases. >>>>>> >>>>>> Acked-by: Peter Xu >>>>>> Tested-by: Mario Casquero >>>>>> Signed-off-by: David Hildenbrand >>>>>> --- >>>>>>     softmmu/physmem.c | 18 ++++++++++++++++++ >>>>>>     1 file changed, 18 insertions(+) >>>>>> >>>>>> diff --git a/softmmu/physmem.c b/softmmu/physmem.c >>>>>> index bda475a719..4ee157bda4 100644 >>>>>> --- a/softmmu/physmem.c >>>>>> +++ b/softmmu/physmem.c >>>>>> @@ -3456,6 +3456,24 @@ int ram_block_discard_range(RAMBlock *rb, >>>>>> uint64_t start, size_t length) >>>>>>                  * so a userfault will trigger. >>>>>>                  */ >>>>>>     #ifdef CONFIG_FALLOCATE_PUNCH_HOLE >>>>>> +            /* >>>>>> +             * We'll discard data from the actual file, even though >>>>>> we only >>>>>> +             * have a MAP_PRIVATE mapping, possibly messing with >>>>>> other >>>>>> +             * MAP_PRIVATE/MAP_SHARED mappings. There is no easy >>>>>> way to >>>>>> +             * change that behavior whithout violating the promised >>>>>> +             * semantics of ram_block_discard_range(). >>>>>> +             * >>>>>> +             * Only warn, because it work as long as nobody else >>>>>> uses that >>>>>> +             * file. >>>>>> +             */ >>>>>> +            if (!qemu_ram_is_shared(rb)) { >>>>>> +                warn_report_once("ram_block_discard_range: >>>>>> Discarding RAM" >>>>>> +                                 " in private file mappings is >>>>>> possibly" >>>>>> +                                 " dangerous, because it will modify >>>>>> the" >>>>>> +                                 " underlying file and will affect >>>>>> other" >>>>>> +                                 " users of the file"); >>>>>> +            } >>>>>> + >>>>> >>>>> TDX has two types of memory backend for each RAM, shared memory and >>>>> private memory. Private memory is serviced by guest memfd and shared >>>>> memory can also be backed with a fd. >>>>> >>>>> At any time, only one type needs to be valid, which means the opposite >>>>> can be discarded. We do implement the memory discard when TDX converts >>>>> the memory[1]. It will trigger this warning 100% because by default the >>>>> guest memfd is not mapped as shared (MAP_SHARED). >>>> >>>> If MAP_PRIVATE is not involved and you are taking the pages directly out >>>> of the memfd, you should mark that thing as shared. >>> >>> Is it the general rule of Linux? Of just the rule of QEMU memory discard? >>> >> >> MAP_SHARED vs. MAP_PRIVATE is a common UNIX principle, and that's what >> this flag and the check is about. >> >> From mmap(2) >> >> MAP_SHARED: Share this mapping.  Updates to the mapping are visible to >> other processes mapping the same region, and (in the case of file-backed >> mappings) are carried through to the underlying file. >> >> MAP_PRIVATE: Create a private copy-on-write mapping.  Updates to the >> mapping are not visible to other processes mapping the same file, and >> are not carried through to the underlying file.  It is unspecified >> whether changes made  to the file after the mmap() call are visible in >> the mapped region. >> >> For your purpose (no mmap() at all), we behave like MAP_SHARED -- as if >> nothing special is done. No Copy-on-write, no anonymous memory. >> >>>> Anonymous memory is never involved. >>> >>> Could you please elaborate more on this? What do you want to express >>> here regrading anonymous memory? (Sorry that I'm newbie for mmap stuff) >> >> Anonymous memory is memory that is private to a specific process, and >> (see MAP_PRIVATE) modifications remain private to the process and are >> not reflected to the file. >> >> If you have a MAP_PRIVATE file mapping and write to a virtual memory >> location, you'll get a process-private copy of the underlying pagecache >> page. that's what we call anonymous memory, because it does not belong >> to a specific file. fallocate(punch) would not free up that anonymous >> memory. > > For guest memfd, it does implement kvm_gmem_fallocate as .fallocate() > callback, which calls truncate_inode_pages_range() [*]. > > I'm not sure if it frees up the memory. I need to learn it. > > [*] > https://github.com/kvm-x86/linux/blob/911b515af3ec5f53992b9cc162cf7d3893c2fbe2/virt/kvm/guest_memfd.c#L147C73-L147C73 > >>> >>>> >>>> "Private memory" is only private from the guest POV, not from a mmap() >>>> point of view. >>>> >>>> Two different concepts of "private". >>>> >>>>> >>>>> Simply remove the warning will fail the purpose of this patch. The >>>>> other >>>>> option is to skip the warning for TDX case, which looks vary hacky. Do >>>>> you have any idea? >>>> >>>> For TDX, all memory backends / RAMBlocks should be marked as "shared", >>>> and you should fail if that is not provided by the user. >>> >>> As I asked above, I want to understand the logic clearly. Is mapped as >>> shared is a must to support the memory discard? i.e., if we want to >>> support memory discard after memory type change, then the memory must be >>> mapped with MAP_SHARED? >> >> MAP_PIRVATE means that it's not sufficient to only fallocate(punch) the >> fd to free up all memory for a virtual address, because there might be >> anonymous memory in a private mapping that has to be freed up using >> MADV_DONTNEED. > > I can understand this. But it seems unrelated to my question: Is mapped > as shared is a must to support the memory discard? Sorry, I don't quite get what you are asking that I haven't answered yet. Let's talk about the issue you are seeing below. > > e.g., if use below parameters to specify the RAM for a VM > > -object memory-backend-memfd,id=mem0,size=2G \ > -machine memory-backend=mem0 > > since not specifying "share" property, the ram_block doesn't have > RAM_SHARED set. If want to discard some range of this memfd, it triggers > the warning. Is this warning expected? That should not be the case. See "memfd_backend_instance_init" where we set share=true. In memfd_backend_memory_alloc(), we set RAM_SHARED. We only default to share=off for memory-backend-file (well, and memory-backend-ram). So are you sure you get this error message in the configuration you are describing here? > > I know it is not a possible case for current QEMU, but it's true for > future TDX VM. TDX VM can have memory-backend-memfd as the backend for > shared memory and kvm gmem memfd for private memory. At any time, for > any memory range, only one type is valid, thus the range in opposite > memfd can be fallocated. Right. > > Here I get your message as "the ramblock needs to have RAM_SHARED flag > to allow the fallocate of the memfd". This is what I don't understand. The problem I am seeing is that either (a) Someone explicitly sets share=off for some reason for memory-backend-memfd, triggering the warning. (b) We somehow lose RAM_SHARED in above configuration, which would be bad and trigger the warning. Can you make sure that (a) is not the case? -- Cheers, David / dhildenb