Re: [RFC PATCH 0/4] kvm: Report unused guest pages to host

[Alexander Duyck]

On Thu, 2019-02-07 at 09:48 -0500, Nitesh Narayan Lal wrote:
> On 2/4/19 1:15 PM, Alexander Duyck wrote:
> > This patch set provides a mechanism by which guests can notify the host of
> > pages that are not currently in use. Using this data a KVM host can more
> > easily balance memory workloads between guests and improve overall system
> > performance by avoiding unnecessary writing of unused pages to swap.
> > 
> > In order to support this I have added a new hypercall to provided unused
> > page hints and made use of mechanisms currently used by PowerPC and s390
> > architectures to provide those hints. To reduce the overhead of this call
> > I am only using it per huge page instead of of doing a notification per 4K
> > page. By doing this we can avoid the expense of fragmenting higher order
> > pages, and reduce overall cost for the hypercall as it will only be
> > performed once per huge page.
> > 
> > Because we are limiting this to huge pages it was necessary to add a
> > secondary location where we make the call as the buddy allocator can merge
> > smaller pages into a higher order huge page.
> > 
> > This approach is not usable in all cases. Specifically, when KVM direct
> > device assignment is used, the memory for a guest is permanently assigned
> > to physical pages in order to support DMA from the assigned device. In
> > this case we cannot give the pages back, so the hypercall is disabled by
> > the host.
> > 
> > Another situation that can lead to issues is if the page were accessed
> > immediately after free. For example, if page poisoning is enabled the
> > guest will populate the page *after* freeing it. In this case it does not
> > make sense to provide a hint about the page being freed so we do not
> > perform the hypercalls from the guest if this functionality is enabled.
> 
> Hi Alexander,
> 
> Did you get a chance to look at my v8 posting of Guest Free Page Hinting
> [1]?
> Considering both the solutions are trying to solve the same problem. It
> will be great if we can collaborate and come up with a unified solution.
> 
> [1] https://lkml.org/lkml/2019/2/4/993

I haven't had a chance to review these yet.

I'll try to take a look later today and provide review notes based on
what I find.

Thanks.

- Alex