Re: [Qemu-devel] [PATCH] migration: not send zero page header in ram bulk stage

[Hailiang Zhang]

On 2016/1/19 11:11, Hailiang Zhang wrote:
> On 2016/1/19 9:26, Li, Liang Z wrote:
> > > On 2016/1/15 18:24, Li, Liang Z wrote:
> > > > > It seems that this patch is incorrect, if the no-zero pages are
> > > > > zeroed again during !ram_bulk_stage, we didn't send the new zeroed
> > > > > page, there will be an error.
> > > >
> > > > If not in ram_bulk_stage, still send the header, could you explain why it's
> > > wrong?
> > > > Liang
> > >
> > > I have made a mistake, and yes, this patch can speed up the live migration
> > > time, especially when there are many zero pages, it will be more obvious.
> > > I like this idea. Did you test it with postcopy ? Does it break postcopy ?
> >
> > Not yet, I saw Dave's comment's, it will beak post copy, it's not hard to fix 
> > this.
> > A more important thing is Paolo's comments, I don't know in which case this 
> > patch will break LM. Do you have any idea about this?
> > Hope that QEMU don't write data to the block 'pc.ram'.
>
> Paolo is right, for VM in destination, QEMU may write VM's memory before VM 
> starts.
> So your assumption that "VM's RAM pages are initialized to zero" is incorrect.
> This patch will break LM.

Actually, someone has done like that before and cause a migration bug,
See commit f1c72795af573b24a7da5eb52375c9aba8a37972, and
the fixing patch is
commit 9ef051e5536b6368a1076046ec6c4ec4ac12b5c6
Revert "migration: do not sent zero pages in bulk stage"

> > Liang
> >
> > > Thanks,
> > > zhanghailiang
> >
> > .