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[2003:cb:c722:3700:6501:8925:6f9:fcdc]) by smtp.gmail.com with ESMTPSA id r20-20020a05600c35d400b0040c05c768afsm7033321wmq.9.2023.12.04.01.21.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Dec 2023 01:21:24 -0800 (PST) Message-ID: <0b6dacd2-a7c0-469f-830a-9162dfae75bf@redhat.com> Date: Mon, 4 Dec 2023 10:21:23 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] hostmem: Round up memory size for qemu_madvise() in host_memory_backend_memory_complete() Content-Language: en-US To: =?UTF-8?B?TWljaGFsIFByw612b3puw61r?= , qemu-devel@nongnu.org Cc: imammedo@redhat.com References: <9b8a2863-1264-4058-b367-0b61a75921ac@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=170.10.133.124; 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=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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 01.12.23 10:07, Michal Prívozník wrote: > On 11/27/23 14:55, David Hildenbrand wrote: >> On 27.11.23 14:37, David Hildenbrand wrote: >>> On 27.11.23 13:32, Michal Privoznik wrote: >>>> Simple reproducer: >>>> qemu.git $ ./build/qemu-system-x86_64 \ >>>> -m size=8389632k,slots=16,maxmem=25600000k \ >>>> -object >>>> '{"qom-type":"memory-backend-file","id":"ram-node0","mem-path":"/hugepages2M/","prealloc":true,"size":8590983168,"host-nodes":[0],"policy":"bind"}' \ >>>> -numa node,nodeid=0,cpus=0,memdev=ram-node0 >>>> >>>> With current master I get: >>>> >>>> qemu-system-x86_64: cannot bind memory to host NUMA nodes: Invalid >>>> argument >>>> >>>> The problem is that memory size (8193MiB) is not an integer >>>> multiple of underlying pagesize (2MiB) which triggers a check >>>> inside of madvise(), since we can't really set a madvise() policy >>>> just to a fraction of a page. >>> >>> I thought we would just always fail create something that doesn't really >>> make any sense. >>> >>> Why would we want to support that case? >>> >>> Let me dig, I thought we would have had some check there at some point >>> that would make that fail (especially: RAM block not aligned to the >>> pagesize). >> >> >> At least memory-backend-memfd properly fails for that case: >> >> $ ./build/qemu-system-x86_64 -object >> memory-backend-memfd,hugetlb=on,size=3m,id=tmp >> qemu-system-x86_64: failed to resize memfd to 3145728: Invalid argument >> >> memory-backend-file ends up creating a new file: >> >>  $ ./build/qemu-system-x86_64 -object >> memory-backend-file,share=on,mem-path=/dev/hugepages/tmp,size=3m,id=tmp >> >> $ stat /dev/hugepages/tmp >>   File: /dev/hugepages/tmp >>   Size: 4194304         Blocks: 0          IO Block: 2097152 regular file >> >> ... and ends up sizing it properly aligned to the huge page size. >> >> >> Seems to be due to: >> >>     if (memory < block->page_size) { >>         error_setg(errp, "memory size 0x" RAM_ADDR_FMT " must be equal to " >>                    "or larger than page size 0x%zx", >>                    memory, block->page_size); >>         return NULL; >>     } >> >>     memory = ROUND_UP(memory, block->page_size); >> >>     /* >>      * ftruncate is not supported by hugetlbfs in older >>      * hosts, so don't bother bailing out on errors. >>      * If anything goes wrong with it under other filesystems, >>      * mmap will fail. >>      * >>      * Do not truncate the non-empty backend file to avoid corrupting >>      * the existing data in the file. Disabling shrinking is not >>      * enough. For example, the current vNVDIMM implementation stores >>      * the guest NVDIMM labels at the end of the backend file. If the >>      * backend file is later extended, QEMU will not be able to find >>      * those labels. Therefore, extending the non-empty backend file >>      * is disabled as well. >>      */ >>     if (truncate && ftruncate(fd, offset + memory)) { >>         perror("ftruncate"); >>     } >> >> So we create a bigger file and map the bigger file and also have a >> RAMBlock that is bigger. So we'll also consume more memory. >> >> ... but the memory region is smaller and we tell the VM that it has >> less memory. Lot of work with no obvious benefit, and only some >> memory waste :) >> >> >> We better should have just rejected such memory backends right from >> the start. But now it's likely too late. >> >> I suspect other things like >>  * qemu_madvise(ptr, sz, QEMU_MADV_MERGEABLE); >>  * qemu_madvise(ptr, sz, QEMU_MADV_DONTDUMP); >> >> fail, but we don't care for hugetlb at least regarding merging >> and don't even log an error. >> >> But QEMU_MADV_DONTDUMP might also be broken, because that >> qemu_madvise() call will just fail. >> >> Your fix would be correct. But I do wonder if we want to just let that >> case fail and warn users that they are doing something that doesn't >> make too much sense. >> > > Yeah, what's suspicious is: if the size is smaller than page size we > error out, but if it's larger (but still not aligned) we accept that. > I'm failing to see reasoning there. Looks like the ROUND_UP() was added > in v0.13.0-rc0~1201^2~4 (though it's done with some bit magic) and the > check itself was added in v2.8.0-rc0~30^2~23. So it's a bit late, yes. Yeah. > > OTOH - if users want to waste resources, should we stop them? For It's all inconsistent, including memfd handling or what you noted above. For example, Having a 1025 MiB guest on gigantic pages, consuming 2 GiB really is just absolutely stupid. Likely the user wants to know about such mistakes instead of making QEMU silence all side effects of that. :) > instance, when user requests more vCPUs than physical CPUs a warning is > printed: > > $ ./build/qemu-system-x86_64 -accel kvm -smp cpus=128 > qemu-system-x86_64: -accel kvm: warning: Number of SMP cpus requested > (128) exceeds the recommended cpus supported by KVM (8) But that case is still reasonable for testing guest behavior with many vCPUs, or migrating from a machine with more vCPUs. Here, the guest will actually see all vCPUs. In comparison, the memory waste here will never ever be consumable by the VM. > > but that's about it. So maybe the error can be demoted to just a warning? The question is what we want to do, for example, with the qemu_madvise(QEMU_MADV_DONTDUMP). It will similarly simply fail. I'm curious, are there real customers running into that? We could fix it all that but always warn when something like that is being done. -- Cheers, David / dhildenb