Re: [Qemu-devel] [PATCH v2] balloon: Fix failure of updating guest memory status

[Ladi Prosek]

On Tue, Aug 2, 2016 at 2:11 AM, Michael S. Tsirkin <address@hidden> wrote:
> On Mon, Aug 01, 2016 at 11:59:31PM +0000, Li, Liang Z wrote:
>> > On Wed, Jul 06, 2016 at 12:49:06PM +0000, Li, Liang Z wrote:
>> > > > > > > After live migration, 'guest-stats' can't get the expected
>> > > > > > > memory status in the guest. This issue is caused by commit
>> > 4eae2a657d.
>> > > > > > > The value of 's->stats_vq_elem' will be NULL after live
>> > > > > > > migration, and the check in the function
>> > > > > > > 'balloon_stats_poll_cb()' will prevent the 'virtio_notify()'
>> > > > > > > from executing. So guest will not update the memory status.
>> > > > > > >
>> > > > > > > Commit 4eae2a657d is doing the right thing, but 
>> > > > > > > 's->stats_vq_elem'
>> > > > > > > should be treated as part of balloon device state and migrated
>> > > > > > > to destination if it's not NULL to make everything works well.
>> > > > > > >
>> > > > > > > Signed-off-by: Liang Li <address@hidden>
>> > > > > > > Suggested-by: Paolo Bonzini <address@hidden>
>> > > > > > > Cc: Michael S. Tsirkin <address@hidden>
>> > > > > > > Cc: Ladi Prosek <address@hidden>
>> > > > > > > Cc: Paolo Bonzini <address@hidden>
>> > > > > >
>> > > > > > I agree there's an issue but we don't change versions anymore.
>> > > > > > Breaking migrations for everyone is also not nice.
>> > > > > >
>> > > > > > How about queueing virtio_balloon_receive_stats so it will get
>> > > > > > invoked when vm starts?
>> > > > > >
>> > > > >
>> > > > > Could you give more explanation about how it works?  I can't catch 
>> > > > > you.
>> > > > >
>> > > > > Thanks!
>> > > > > Liang
>> > > >
>> > > > virtqueue_discard before migration
>> > > >
>> > > > virtio_balloon_receive_stats after migration
>> > > >
>> > >
>> > > Sorry, I still can't catch you. Maybe it's easier for you to submit a
>> > > patch than writing a lot a words to make me understand your idea.
>> >
>> > I'm rather busy now.  I might look into it towards end of the month.
>> >
>> > > I just don't understand why not to use the version to make things
>> > > easier, is that not the original intent of version id?
>> >
>> > This was the original idea but we stopped using version ids since they have
>> > many shortcomings.
>> >
>> > > If we want to extend the device and more states are needed, the idea
>> > > you suggest can be used as a common solution?
>> > >
>> > > Thanks!
>> > > Liang
>> >
>> > The idea is to try to avoid adding more state. that's not always possible 
>> > but in
>> > this case element was seen but not consumed yet, so it should be possible
>> > for destination to simply get it from the VQ again.
>> >
>> > > > --
>> > > > MST
>>
>> Hi Michel,
>>
>> Do you have time for this issue recently?
>>
>> Thanks!
>> Liang

Hi Liang,

I should be able to look into it this week if you help me with testing.

Thanks,
Ladi

> Sorry, doesn't look like I will.
> Idea is to make sure balloon_stats_poll_cb runs
> on source. This will set stats_vq_elem to NULL.
>
>
> --
> MST