Re: [Qemu-devel] RFC migration of zero pages

[Gleb Natapov]

On Thu, Jan 31, 2013 at 04:36:25PM +0200, Avi Kivity wrote:
> On Thu, Jan 31, 2013 at 4:30 PM, Gleb Natapov <address@hidden> wrote:
> >> >
> >> > Avi/Michael do you remember why mincore can't be used to check if a guest
> >> page is unmapped?
> >>
> >> A page may be not in core, but also nonzero (for example swap).
> > Yes, mincore() should not be used as zero page check, but it can be used
> > as an indicator that page can be dealt with in dead stage of migration
> > since it is likely zero and will not take much time to send.
> 
> Or it's swapped, in which case we know nothing about it.
> 
> > It is
Of course, that is why I said "likely' :)

> > possible to call madvise(MADV_WILLNEED) on them meanwhile to pre-load
> > swap without faulting on each page individually.
> 
> During migration you're faulting like mad anyway.
That's guest faulting on dirty bit logging, as far as I understand Orit
says that in addition to that she sees a lot of faults generated by
migration reading unmapped guest memory. She wants to eliminate at least
those.

--
                        Gleb.