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

Gupta, Pankaj
2022-07-14T04:56:05Z

>>>>> 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