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

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Message-ID: <9f868fd3-ae47-d44e-fd55-566793405121@amd.com>
Date:   Thu, 14 Jul 2022 06:39:37 +0200
From:   "Gupta, Pankaj" <pankaj.gupta@....com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        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>,
        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>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, 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


>>>>> 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.
> 
> (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