Re: [Xen-devel] Rebooting domu fails in nfs share exported from another domu on the same dom0

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Jul 18, 2014 at 05:43:23PM -0400, annie li wrote:
[...]
> >>
> >>When doing grantcopy for netback, xen requires the source is dom0. However,
> >>it is vm2 in this case.
> >Would it be possible in the hypervisor to have an extra check where you
> >look to see if the source (dom0 in this case) has this page mapped
> >from another guest? As in, this is a bit of A->B->C transition
> >and we want to do A->C (B is dom0). If you figure out that the PFN
> >belongs to A and you are doing a copy of A's page to C's page from B (dom0)
> >page (and B PFN is actually mapped to be A's page), then why
> >not just copy from A to C directly?
> >
> >Hmm. Linux kernel could actually also have this information (it does
> >already I think) and we could search for that if the grant copy fails
> >and if so alter the source and retry the grant copy. Instead of dom0
> >being the source it is the C guest (vm2)? Thought I don't know if
> >the hypercall allows us to make a grant_copy on behalf of two
> >other guests.
> 
> Oh... I remember netback did have some grant_copy code on behalf of two
> guests on same server, and it was removed later on. So I think grantcopy
> A->C works for this case with the precondition that we can recognize this
> page is mapped from another guest.
> 

I have not followed this thread closely.

The tracking facility was removed because it was dead at that time. See
43e9d19 ("xen-netback: remove page tracking facility").

However I think the latest netback with mapping scheme does have
something similar. I can see there's a "foreign_queue" check in
xenvif_gop_frag_copy. Is that not enough?

Wei.

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