Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots

[Peter Krempa]

On Tue, Dec 01, 2020 at 11:42:18 +0300, Andrey Gruzdev wrote:
> On 01.12.2020 10:08, Peter Krempa wrote:
> > On Thu, Nov 26, 2020 at 18:17:28 +0300, Andrey Gruzdev via wrote:
> > > This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas 
> > > he's

[...]

> > Note that in cases when qemu can't guarantee that the
> > background_snapshot feature will work it should not advertise it. We
> > need a way to check whether it's possible to use it, so we can replace
> > the existing --live flag with it rather than adding a new one and
> > shifting the problem of checking whether the feature works to the user.
> > 
> 
> Hi,
> 
> May be you are using hugetlbfs as memory backend?

Not exactly hugepages, but I had:

  <memoryBacking>
    <access mode='shared'/>
  </memoryBacking>

which resulted into the following commandline to instantiate memory:

-object 
memory-backend-file,id=pc.ram,mem-path=/var/lib/libvirt/qemu/ram/6-upstream-bj/pc.ram,share=yes,size=33554432000,host-nodes=0,policy=bind
 \

When I've removed it I got:

-object memory-backend-ram,id=pc.ram,size=33554432000,host-nodes=0,policy=bind \

And the migration didn't fail in my quick test. I'll have a more
detailed look later, thanks for the pointer.

> I totally agree that we need somehow check that kernel and VM memory backend
> support the feature before one can enable the capability.
> Need to think about that..

Definitely. Also note that memory backed by memory-backend-file will be
more and more common, for cases such as virtiofs DAX sharing and
similar.