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[2003:d8:2f45:5500:8267:647f:4209:dedd]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-442d685c2d7sm20802795e9.30.2025.05.09.01.18.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 May 2025 01:18:07 -0700 (PDT) Message-ID: <96ab7fa9-bd7a-444d-aef8-8c9c30439044@redhat.com> Date: Fri, 9 May 2025 10:18:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 07/13] ram-block-attribute: Introduce RamBlockAttribute to manage RAMBLock with guest_memfd To: Chenyi Qiang , Baolu Lu , Alexey Kardashevskiy , Peter Xu , Gupta Pankaj , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= , Michael Roth Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, Williams Dan J , Peng Chao P , Gao Chao , Xu Yilun , Li Xiaoyao References: <20250407074939.18657-1-chenyi.qiang@intel.com> <20250407074939.18657-8-chenyi.qiang@intel.com> <013b36a9-9310-4073-b54c-9c511f23decf@linux.intel.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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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: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.416, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=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 >>> >>> Signed-off-by: Chenyi Qiang >> >> <...> >> >>> + >>> +int ram_block_attribute_realize(RamBlockAttribute *attr, MemoryRegion >>> *mr) >>> +{ >>> +    uint64_t shared_bitmap_size; >>> +    const int block_size  = qemu_real_host_page_size(); >>> +    int ret; >>> + >>> +    shared_bitmap_size = ROUND_UP(mr->size, block_size) / block_size; >>> + >>> +    attr->mr = mr; >>> +    ret = memory_region_set_generic_state_manager(mr, >>> GENERIC_STATE_MANAGER(attr)); >>> +    if (ret) { >>> +        return ret; >>> +    } >>> +    attr->shared_bitmap_size = shared_bitmap_size; >>> +    attr->shared_bitmap = bitmap_new(shared_bitmap_size); >> >> Above introduces a bitmap to track the private/shared state of each 4KB >> page. While functional, for large RAM blocks managed by guest_memfd, >> this could lead to significant memory consumption. >> >> Have you considered an alternative like a Maple Tree or a generic >> interval tree? Both are often more memory-efficient for tracking ranges >> of contiguous states. > > Maybe not necessary. The memory overhead is 1 bit per page > (1/(4096*8)=0.003%). I think it is not too much. It's certainly not optimal. IIRC, QEMU already maintains 3 dirty bitmaps in ram_list.dirty_memory (DIRTY_MEMORY_NUM = 3) for guest ram. With KVM, we also allocate yet another dirty bitmap without KVM_MEM_LOG_DIRTY_PAGES. Assuming a 4 TiB VM, a single bitmap should be 128 MiB. -- Cheers, David / dhildenb