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

[annie li <annie.li@xxxxxxxxxx>]

On 2014/7/18 17:07, Konrad Rzeszutek Wilk wrote:
> On Fri, Jul 18, 2014 at 04:31:13PM -0400, annie li wrote:
> > On 2014/7/18 16:22, Konrad Rzeszutek Wilk wrote:
> > > On Fri, Jul 18, 2014 at 04:17:02PM -0400, annie li wrote:
> > > > On 2014/7/18 15:43, Konrad Rzeszutek Wilk wrote:
> > > > > On Fri, Jul 18, 2014 at 03:31:43PM -0400, annie li wrote:
> > > > > > On 2014/7/18 14:53, Konrad Rzeszutek Wilk wrote:
> > > > > > > On Thu, Jul 17, 2014 at 12:56:12PM -0400, annie li wrote:
> > > > > > > > On 2014/7/17 11:49, Roger Pau Monné wrote:
> > > > > > > > > On 16/07/14 22:36, annie li wrote:
> > > > > > > > > > Hi
> > > > > > > > > >
> > > > > > > > > > I hit a problem in such scenario: vm1 is running and export nfs service,
> > > > > > > > > > dom0 mount this nfs, and vm2 is booted in this nfs location. vm1 and vm2
> > > > > > > > > > are running on the same dom0.
> > > > > > > > > >
> > > > > > > > > > When this bug happens, the data flow is:  vm2 blkfront-> vm2 blkback->
> > > > > > > I am a bit confused here. 'vm2 blkfront -> vm2 blkback'? Did you
> > > > > > > mean 'dom0 blkback'?
> > > > > > Yes, Dom0 blkback.
> > > > > >
> > > > > > > > > > loop -> nfs file -> nfs client -> bridge priv1 -> vm1 vif -> vm1 netback
> > > > > > > > > > -> vm1 netfront.
> > > > > > > So both netback and netfront run in the same guest? I think you
> > > > > > > want these two swapped around (netfront -> netback).
> > > > > > No, the netfront in above routine means the one in guest vm1, and this is
> > > > > > network RX path in vm1.
> > > > > OK, so 'dom0 netback' then. As the netback thread is running in the
> > > > > initial domain?
> > > > Netback thread is running in Dom0 as normal, and the two vms are all
> > > > normal vm, not driver domain. Probably my initial description causes
> > > > some confusion.
> > > >
> > > > > So a revised view is:
> > > > >
> > > > >    vm2 blkfront -> dom0 blkback -> loop -> nfs file -> nfs client
> > > > >         -> bridge priv1 -> vm1 vif -> dom0 netback -> vm1 netfront
> > > > >
> > > > > So with the grant map (blkfront -> blkback) the source is dom0
> > > > > and the destination is vm1.
> > > > You mean destination is vm2 here, right?
> > > Yes :-)
> > > > But I am little confused. Blkfront of vm2 sends out request to
> > > > blkback, and the data page is allocated from vm2. So the grant map
> > > > source is vm2, destination is dom0. Anything I missed here?
> > > >
> > > > >   For the grant copy, the source is
> > > > > dom0 and the destinatation is vm2 right? (or did I get my src
> > > > > and dst confused?).
> > > > You mean vm1 here, right?
> > > Heh.
> > > > If so, yes, source is dom0, and destination is vm1.
> > > > In grantcopy code, it checks whether the source is from dom0, if
> > > > not, then an error is thrown out. In this situation, the source is
> > > >from vm2 which is grant mapped.
> > > In both cases dom0 is part of the equation. In one
> > > case it is the destination and in another it is the source - both cases
> > > for the same page right?
> > Yes.
> > blkfront->blkback, dom0 is destination
> > netback->netfront, dom0 is the source
> >
> > 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.
Thanks
Annie
> > Thanks
> > Annie
> >
> > > > Thanks
> > > > Annie
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