Re: [RFC][Patch v8 6/7] KVM: Enables the kernel to isolate and report free pages

[Alexander Duyck]

On Fri, Feb 8, 2019 at 1:38 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Fri, Feb 08, 2019 at 03:41:55PM -0500, Nitesh Narayan Lal wrote:
> > >> I am also planning to try Michael's suggestion of using MAX_ORDER - 1.
> > >> However I am still thinking about a workload which I can use to test its
> > >> effectiveness.
> > > You might want to look at doing something like min(MAX_ORDER - 1,
> > > HUGETLB_PAGE_ORDER). I know for x86 a 2MB page is the upper limit for
> > > THP which is the most likely to be used page size with the guest.
> > Sure, thanks for the suggestion.
>
> Given current hinting in balloon is MAX_ORDER I'd say
> share code. If you feel a need to adjust down the road,
> adjust both of them with actual testing showing gains.

Actually I'm left kind of wondering why we are even going through
virtio-balloon for this? It seems like this would make much more sense
as core functionality of KVM itself for the specific architectures
rather than some side thing. In addition this could end up being
redundant when you start getting into either the s390 or PowerPC
architectures as they already have means of providing unused page
hints.

I have a set of patches I proposed that add similar functionality via
a KVM hypercall for x86 instead of doing it as a part of a Virtio
device[1].  I'm suspecting the overhead of doing things this way is
much less then having to make multiple madvise system calls from QEMU
back into the kernel.

One other concern that has been pointed out with my patchset that
would likely need to be addressed here as well is what do we do about
other hypervisors that decide to implement page hinting. We probably
should look at making this KVM/QEMU specific code run through the
paravirtual infrastructure instead of trying into the x86 arch code
directly.

[1] https://lkml.org/lkml/2019/2/4/903