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

[Michael R. Hines]

On 03/19/2013 11:16 AM, Michael S. Tsirkin wrote:
> On Tue, Mar 19, 2013 at 11:08:24AM -0400, Michael R. Hines wrote:
> > This is actual a much bigger problem that I thought, not just for RDMA:
> >
> > Currently the *sender* side is does not support overcommit
> > during a regular TCP migration.......I assume because the
> > migration_bitmap does not know which memory is mapped or
> > unmapped by the host kernel.
> >
> > Is this a known issue?
> >
> > - Michael
> I don't really understand what you are saying here.
> Do you see some bug with migration where we might use
> more memory than allowed by cgroups?

Yes: cgroups does not coordinate with the list of pages
that have "not yet been mapped" or touched by the
virtual machine, right?

I may be missing something here from what I read in
the code, but even if I set a cgroups limit on memory,
QEMU will still attempt to access that memory if the
migration_bitmap tells it to, as far as I can tell.

Is this an accurate observation?

A simple solution would be to just have QEMU consult with /dev/pagemap, no?

- Michael