* Andrey Gruzdev (andrey.gruz...@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 / dgilb...@redhat.com / Manchester, UK