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(p200300cbc70b05006a0617fc810bb054.dip0.t-ipconnect.de. [2003:cb:c70b:500:6a06:17fc:810b:b054]) by smtp.gmail.com with ESMTPSA id h19-20020a05600c315300b004064741f855sm3941125wmo.47.2023.10.19.02.43.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Oct 2023 02:43:28 -0700 (PDT) Message-ID: <5a40b152-115b-4dfd-9821-876d340148e2@redhat.com> Date: Thu, 19 Oct 2023 11:43:27 +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> <0e436056-95c3-4ed6-a17e-a46a780e5ab9@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: 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 19.10.23 11:26, Xiaoyao Li wrote: > On 10/19/2023 4:26 PM, David Hildenbrand wrote: >> 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? > > Sorry, I made an mistake. I was using "-object > memory-backend-ram,id=mem0,size=2G" instead of "memory-backend-memfd". > > yes, when using "memory-backend-memfd" as the backend for TDX shared > memory, it doesn't trigger the warning because > memfd_backend_instance_init() set "share" to true. > > When using "memory-backend-ram" as the backend for TDX shared memory, > the warning is triggered converting memory from private (kvm gmem) to > shared (backend-ram). In this case, there is a valid fd (kvm gmem fd), > so it goes to the path of need_fallocate. However, > qemu_ram_is_shared(rb) returns false because "memory-backend-ram" > doesn't have "share" default on. Thus, the warning is triggered. > > It seems I need figure out a more proper solution to refactor the > ram_block_discard_range(), to make it applicable for kvm gmem discard case. You should probably completely ignore any ramblock flags when fallocate(punch) the kvm_gmem_fd. kvm_gmem_fd is a rather special "secondary storage that's never mapped", independent of the ordinary RAMBlock memory. -- Cheers, David / dhildenb