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Shutemov" , luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com, david@redhat.com, aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com, Muchun Song References: <20220706082016.2603916-1-chao.p.peng@linux.intel.com> <20220713075738.GC2831541@chaop.bj.intel.com> <13d25d2e-ff79-5762-ddb8-87df56f5cbcf@amd.com> <20220713235922.GB2881285@chaop.bj.intel.com> From: "Gupta, Pankaj" In-Reply-To: <20220713235922.GB2881285@chaop.bj.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: AM5PR0601CA0067.eurprd06.prod.outlook.com (2603:10a6:206::32) To CY4PR1201MB0181.namprd12.prod.outlook.com (2603:10b6:910:1f::11) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: ead61a78-b9f2-4e38-26ac-08da6552e31c X-MS-TrafficTypeDiagnostic: DM6PR12MB4139:EE_ X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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The patches are based on latest kvm/queue branch >>>>> commit: >>>>> >>>>> b9b71f43683a (kvm/queue) KVM: x86/mmu: Buffer nested MMU >>>>> split_desc_cache only by default capacity >>>>> >>>>> Introduction >>>>> ------------ >>>>> In general this patch series introduce fd-based memslot which provides >>>>> guest memory through memory file descriptor fd[offset,size] instead of >>>>> hva/size. The fd can be created from a supported memory filesystem >>>>> like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM >>>> >>>> Thinking a bit, As host side fd on tmpfs or shmem will store memory on host >>>> page cache instead of mapping pages into userspace address space. Can we hit >>>> double (un-coordinated) page cache problem with this when guest page cache >>>> is also used? >>> >>> This is my understanding: in host it will be indeed in page cache (in >>> current shmem implementation) but that's just the way it allocates and >>> provides the physical memory for the guest. In guest, guest OS will not >>> see this fd (absolutely), it only sees guest memory, on top of which it >>> can build its own page cache system for its own file-mapped content but >>> that is unrelated to host page cache. >> >> yes. If guest fills its page cache with file backed memory, this at host >> side(on shmem fd backend) will also fill the host page cache fast. This can >> have an impact on performance of guest VM's if host goes to memory pressure >> situation sooner. Or else we end up utilizing way less System RAM. > > (Currently), the file backed guest private memory is long-term pinned > and not reclaimable, it's in page cache anyway once we allocated it for > guest. This does not depend on how guest use it (e.g. use it for guest > page cache or not). Even if host shmem backed memory always be always un-reclaimable, we end up utilizing double RAM (both in guest & host page cache) for guest disk accesses? I am considering this a serious design decision before we commit to this approach. Happy to be enlightened on this and know the thoughts from others as well. Thanks, Pankaj