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

[Andrey Gruzdev]

On 01.12.2020 21:40, Dr. David Alan Gilbert wrote:
> * Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> > On 01.12.2020 13:53, Peter Krempa wrote:
> > > 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.
> >
> > Yep, seems that current userfaultfd supports hugetlbfs and shared memory for
> > missing pages but not for wr-protected..
>
> For hugepages, you'd need kernel support - but also you'd want to make
> sure you write the whole hugepage at once.
>
> For shared, there's a harder problem to ask; what happens if RAM is
> written by the other process - for postcopy, we get the other process
> to send us a userfaultfd that they have registered with their VM.
>
> Dave

Yep. May be problematic. But if used for vhost-user external backend - 
seems it should work.

> > > > 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.
> >
> > I see.. That needs support from kernel side, so far 'background-snapshots'
> > are incompatible with memory-backend-file sharing.
> >
> > --
> > Andrey Gruzdev, Principal Engineer
> > Virtuozzo GmbH  +7-903-247-6397
> >                  virtuzzo.com


--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                virtuzzo.com