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

[Wen Congyang]

On 03/25/2015 05:50 PM, Juan Quintela wrote:
> zhanghailiang <address@hidden> 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.

I can use e1000 to reproduce this problem.

> 
> What kind of load were you having when reproducing this issue?
> Just to confirm, you have been able to reproduce this without COLO
> patches, right?

I can reproduce it without COLO patches. The newest commit is:
commit 054903a832b865eb5432d79b5c9d1e1ff31b58d7
Author: Peter Maydell <address@hidden>
Date:   Tue Mar 24 16:34:16 2015 +0000

    Update version for v2.3.0-rc1 release
    
    Signed-off-by: Peter Maydell <address@hidden>

> 
>> (qemu) migrate tcp:192.168.3.8:3004
>> before saving ram complete
>> ff703f6889ab8701e4e040872d079a28
>> md_host : after saving ram complete
>> ff703f6889ab8701e4e040872d079a28
>>
>> DST: # x86_64-softmmu/qemu-system-x86_64 -enable-kvm -cpu qemu64,-kvmclock 
>> -netdev tap,id=hn0,vhost=on -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 -incoming tcp:0:3004
>> (qemu) QEMU_VM_SECTION_END, after loading ram
>> 230e1e68ece9cd4e769630e1bcb5ddfb
>> md_host : after loading all vmstate
>> 230e1e68ece9cd4e769630e1bcb5ddfb
>> md_host : after cpu_synchronize_all_post_init
>> 230e1e68ece9cd4e769630e1bcb5ddfb
>>
>> This happens occasionally, and it is more easy to reproduce when issue 
>> migration command during VM's startup time.
> 
> OK, a couple of things.  Memory don't have to be exactly identical.
> Virtio devices in particular do funny things on "post-load".  There
> aren't warantees for that as far as I know, we should end with an
> equivalent device state in memory.
> 
>> 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.
> 
>> Actually, the first time we found this problem was in the COLO FT 
>> development, and it triggered some strange issues in
>> VM which all pointed to the issue of inconsistent of VM's memory. (We have 
>> try to save all memory of VM to slave side every time
>> when do checkpoint in COLO FT, and everything will be OK.)
>>
>> Is it OK for some pages that not transferred to destination when do 
>> migration ? Or is it a bug?
> 
> Pages transferred should be the same, after device state transmission is
> when things could change.
> 
>> This issue has blocked our COLO development... :(
>>
>> Any help will be greatly appreciated!
> 
> Later, Juan.
>