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(p200300cbc70ad3005c7b51cee3ef6e01.dip0.t-ipconnect.de. [2003:cb:c70a:d300:5c7b:51ce:e3ef:6e01]) by smtp.gmail.com with ESMTPSA id o18-20020a05600c4fd200b0040472ad9a3dsm1177473wmq.14.2023.10.18.02.26.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Oct 2023 02:26:15 -0700 (PDT) Message-ID: Date: Wed, 18 Oct 2023 11:26:14 +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 Content-Language: en-US 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> 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: <23d209c1-f860-4915-935e-816d9077b65c@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; 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_H3=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 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. > >> >> "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. That's why this functions handles both cases differently, and warns if something possibly nasty is being done. -- Cheers, David / dhildenb