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

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Cc: Peter Krempa <pkrempa@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	qemu-devel@nongnu.org, Peter Xu <peterx@redhat.com>,
	Markus Armbruster <armbru@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>, Den Lunev <den@openvz.org>
Subject: Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots
Date: Tue, 1 Dec 2020 18:40:55 +0000	[thread overview]
Message-ID: <20201201184055.GM4338@work-vm> (raw)
In-Reply-To: <d3b0c767-14d8-b7e7-345d-0314acb3d07e@virtuozzo.com>

* 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

> > > 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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK