Re: Error restoring DomU when using GPLPV

[Mukesh Rathor <mukesh.rathor@oracle.com>]

Keir Fraser wrote:
> On 15/09/2009 22:25, "Keir Fraser" <keir.fraser@eu.citrix.com> wrote:
> 
>> No, it doesn't. I agree that after the first migration tot_pages will have
>> increased to 0x83ed. But I do not agree that it will continue to increase by
>> three pages on each future migration. Look at it this way -- three GPFNs
>> (guest-physical pages) have changed from xenheap pages to domheap pages
>> across that first migration. On future migrations they will be migrated just
>> like any other ordinary domheap page, since that's what they now are. And
>> tot_pages will therefore not change. Right?
> 
> Actually of course you do the right thing with the shinfo page, so actually
> one page per migration does get switched back to being a Xenheap page (the
> shinfo page) and tot_pages actually increases by 3 on the first migration,
> then decreases by 1 when shinfo gets remapped by the PV drivers. Then
> increases by 1 on every future migration (which is the shinfo Xenheap page
> getting changed into a domheap page), and then decreases by 1 when shinfo
> gets remapped by the PV drivers.
> 
> But even setting things out exactly right as above, the end result is the
> same: I *still* cannot explain Annie's result.

The bug in her driver is that its only remapping shinfo page, and NOT
the 2 shared grant frames. tot_pages hence increases by 2 every
migration. I can see all in kdb.  tot_pages goes up by 3, then down by 1
as shared info frame is remapped, and remains there. Next migration, it
goes up by 3, down by 1 again.  So each migration leaks 2 frames. The initial
difference is 21 frames between tot and max, hence after 10 migrations
it fails. (BTW, no max_mem specified in config file, I'm told it means no POD).

On linux side, driver remaps shinfo page + both grant frames. So, it goes up
by 3 for a moment, then comes remap and down by 3, back to where it was. If
tot_pages == max_pages, then mig will fail. Which brings me to a question,
to test out balloon changes, what would be the best way to get tot_pages
equal to max_pages. xm mem-set doesn't quite get me there. Occassionally
I see the two same after starting guest, but I've not figured out what
causes that to happen.

thakns
Mukesh



>  -- Keir
> 
>> This is why I still cannot understand or explain Annie's experimental
>> result.
>