linux-kernel - Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

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Message-Id: <b0c726d4-2ad3-47e7-90cf-d67b36e7d59e@www.fastmail.com>
Date:   Wed, 13 Jul 2022 21:29:08 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Gupta, Pankaj" <pankaj.gupta@....com>,
        "Chao Peng" <chao.p.peng@...ux.intel.com>
Cc:     "kvm list" <kvm@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        "Linux API" <linux-api@...r.kernel.org>, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, linux-kselftest@...r.kernel.org,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Jonathan Corbet" <corbet@....net>,
        "Sean Christopherson" <seanjc@...gle.com>,
        "Vitaly Kuznetsov" <vkuznets@...hat.com>,
        "Wanpeng Li" <wanpengli@...cent.com>,
        "Jim Mattson" <jmattson@...gle.com>,
        "Joerg Roedel" <joro@...tes.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Hugh Dickins" <hughd@...gle.com>,
        "Jeff Layton" <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Shuah Khan" <shuah@...nel.org>, "Mike Rapoport" <rppt@...nel.org>,
        "Steven Price" <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Vishal Annapurve" <vannapurve@...gle.com>,
        "Yu Zhang" <yu.c.zhang@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Andi Kleen" <ak@...ux.intel.com>,
        "David Hildenbrand" <david@...hat.com>, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        "Quentin Perret" <qperret@...gle.com>,
        "Michael Roth" <michael.roth@....com>,
        "Michal Hocko" <mhocko@...e.com>,
        "Muchun Song" <songmuchun@...edance.com>
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest
 private memory



On Wed, Jul 13, 2022, at 3:35 AM, Gupta, Pankaj wrote:
>>>> This is the v7 of this series which tries to implement the fd-based KVM
>>>> guest private memory. 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.

Is this in any meaningful way different from a regular VM?

--Andy