Re: [Qemu-devel] [RFC PATCH RDMA support v4: 03/10] more verbose documentation of the RDMA transport

[Michael R. Hines]

OK, so I did a quick test and the cgroup does appear to be working 
correctly for zero pages.

Nevertheless, this still doesn't solve the chunk registration problem 
for RDMA.

Even with a cgroup on the sender *or* receiver side, there is no API 
that I know
that would correctly indicate to the migration process which pages are 
safe to register
or not with the hardware. Without such an API, even a "smarter" chunked 
memory
registration scheme would not work with cgroups because we would be 
attempting
to pin zero pages (for no reason) that cgroups has already kicked out, 
which would
defeat the purpose of using cgroups.

So, if I submit a separate patch to fix this, would you guys review it? 
(Using /dev/pagemap).

Unless there is a better idea? Does KVM expose the necessary mappings?

- Michael

On 03/19/2013 01:14 PM, Paolo Bonzini wrote:
> Il 19/03/2013 18:09, Michael R. Hines ha scritto:
> > Allowing QEMU to swap due to a cgroup limit during migration is a viable
> > overcommit option?
> >
> > I'm trying to keep an open mind, but that would kill the migration
> > time.....
> Would it swap?  Doesn't the kernel back all zero pages with a single
> copy-on-write page?  If that still accounts towards cgroup limits, it
> would be a bug.
>
> Old kernels do not have a shared zero hugepage, and that includes some
> distro kernels.  Perhaps that's the problem.
>
> Paolo