Re: [Qemu-devel] [Migration Bug? ] Occasionally, the content of VM's memory is inconsistent between Source and Destination of migration

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Mar 26, 2015 at 11:29:43AM +0100, Juan Quintela wrote:
> Wen Congyang <wency@cn.fujitsu.com> wrote:
> > On 03/25/2015 05:50 PM, Juan Quintela wrote:
> >> zhanghailiang <zhang.zhanghailiang@huawei.com> wrote:
> >>> Hi all,
> >>>
> >>> We found that, sometimes, the content of VM's memory is
> >>> inconsistent between Source side and Destination side
> >>> when we check it just after finishing migration but before VM continue to Run.
> >>>
> >>> We use a patch like bellow to find this issue, you can find it from affix,
> >>> and Steps to reprduce:
> >>>
> >>> (1) Compile QEMU:
> >>>  ./configure --target-list=x86_64-softmmu  --extra-ldflags="-lssl" && make
> >>>
> >>> (2) Command and output:
> >>> SRC: # x86_64-softmmu/qemu-system-x86_64 -enable-kvm -cpu
> >>> qemu64,-kvmclock -netdev tap,id=hn0-device
> >>> virtio-net-pci,id=net-pci0,netdev=hn0 -boot c -drive
> >>> file=/mnt/sdb/pure_IMG/sles/sles11_sp3.img,if=none,id=drive-virtio-disk0,cache=unsafe
> >>> -device
> >>> virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0
> >>> -vnc :7 -m 2048 -smp 2 -device piix3-usb-uhci -device usb-tablet
> >>> -monitor stdio
> >> 
> >> Could you try to reproduce:
> >> - without vhost
> >> - without virtio-net
> >> - cache=unsafe is going to give you trouble, but trouble should only
> >>   happen after migration of pages have finished.
> >
> > If I use ide disk, it doesn't happen.
> > Even if I use virtio-net with vhost=on, it still doesn't happen. I guess
> > it is because I migrate the guest when it is booting. The virtio net
> > device is not used in this case.
> 
> Kevin, Stefan, Michael, any great idea?

You must use -drive cache=none if you want to use live migration.  It
should not directly affect memory during migration though.

> >>> We have done further test and found that some pages has been
> >>> dirtied but its corresponding migration_bitmap is not set.
> >>> We can't figure out which modules of QEMU has missed setting bitmap
> >>> when dirty page of VM,
> >>> it is very difficult for us to trace all the actions of dirtying VM's pages.
> >> 
> >> This seems to point to a bug in one of the devices.

I think you'll need to track down which pages are different.  If you are
lucky, their contents will reveal what the page is used for.

Stefan

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