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

[Gupta, Pankaj]

> > > > > 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