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mx.microsoft.com 1; spf=pass smtp.mailfrom=intel.com; dmarc=pass action=none header.from=intel.com; dkim=pass header.d=intel.com; arc=none Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=intel.com; Received: from DM3PR11MB8735.namprd11.prod.outlook.com (2603:10b6:0:4b::20) by IA1PR11MB6098.namprd11.prod.outlook.com (2603:10b6:208:3d6::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9228.16; Mon, 20 Oct 2025 11:48:10 +0000 Received: from DM3PR11MB8735.namprd11.prod.outlook.com ([fe80::3225:d39b:ca64:ab95]) by DM3PR11MB8735.namprd11.prod.outlook.com ([fe80::3225:d39b:ca64:ab95%4]) with mapi id 15.20.9228.015; Mon, 20 Oct 2025 11:48:10 +0000 Message-ID: Date: Mon, 20 Oct 2025 19:48:02 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] ram-block-attributes: Avoid the overkill of shared memory with hugetlbfs backend To: David Hildenbrand , Peter Xu , "Alexey Kardashevskiy" , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= CC: , Gao Chao , Li Xiaoyao References: <20251017081445.175342-1-chenyi.qiang@intel.com> <08e0ea1a-34dd-4700-917c-157996586c73@intel.com> <944cfa69-c7ed-4e18-aab9-db23f38da1a7@redhat.com> From: Chenyi Qiang Content-Language: en-US In-Reply-To: <944cfa69-c7ed-4e18-aab9-db23f38da1a7@redhat.com> Content-Type: text/plain; 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envelope-from=chenyi.qiang@intel.com; helo=mgamail.intel.com X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 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_MED=-2.3, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_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 On 10/20/2025 7:31 PM, David Hildenbrand wrote: > On 20.10.25 12:32, Chenyi Qiang wrote: >> >> >> On 10/17/2025 11:13 PM, David Hildenbrand wrote: >>> On 17.10.25 10:14, Chenyi Qiang wrote: >>>> Currently, private memory and shared memory have different backend in >>>> CoCo VMs. It is possible for users to specify the shared memory with >>>> hugetlbfs backend while private memory with guest_memfd backend only >>>> supports 4K page size. In this case, ram_block->page_size is different >>>> from the host page size which will trigger the assertion when getting >>>> block size. Relax the restriction to allow shared memory to use >>>> hugetlbfs backend. >>>> >>>> Fixes: 5d6483edaa92 ("ram-block-attributes: Introduce RamBlockAttributes to manage RAMBlock with guest_memfd") >>>> Signed-off-by: Chenyi Qiang >>>> --- >>>>    system/ram-block-attributes.c | 7 ++++--- >>>>    1 file changed, 4 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/system/ram-block-attributes.c b/system/ram-block-attributes.c >>>> index 68e8a027032..0f39ccf9090 100644 >>>> --- a/system/ram-block-attributes.c >>>> +++ b/system/ram-block-attributes.c >>>> @@ -28,10 +28,11 @@ ram_block_attributes_get_block_size(const RamBlockAttributes *attr) >>>>         * Because page conversion could be manipulated in the size of at least 4K >>>>         * or 4K aligned, Use the host page size as the granularity to track the >>>>         * memory attribute. >>>> +     * When hugetlbfs is used as backend of shared memory, ram_block->page_size >>>> +     * is different from host page size. So it is not appropriate to use >>>> +     * ram_block->page_size here. >>> >>> But are we sure everything else is working as expected and that this is not a check that prevents other code from doing the wrong thing? >> >> I think so. The block size must be 4K due to the page conversion could be in the size of 4K and we use "bitmap" to track the status. > > Indeed. > >> I originally missed the case of hugetlb so added an assert() here. But it is allowed to use hugetlb as shared memory backend >> before shared device assignment patches were introduced. >> >>> >>> I recall that punching holes was problematic as the VM shares/unshared 4k chunks. >> >> I can see the kvm_convert_memory() will skip ram_block_discard_range() if using hugetlb backend. >> It will cause the double-memory consumption (*). Any other problem? > > Right. > > > What we should be doing is unifying the retrieval of the block size in ram_block_attributes_create() as well. That's where we allocate it. > > So either > > a) Use qemu_real_host_page_size() everywhere. > > b) Use ram_block_attributes_get_block_size() everywhere. > > Could be done in a separate patch. Agree. Will clean up it and modify the commit message mentioned by you and Peter in other mail threads. >