Re: [Xen-devel] Problems using xl migrate

[M A Young <m.a.young@xxxxxxxxxxxx>]

On Mon, 24 Nov 2014, Ian Campbell wrote:

> On Mon, 2014-11-24 at 12:06 +0000, M A Young wrote:
> > On Mon, 24 Nov 2014, George Dunlap wrote:
> >
> > > On Mon, Nov 24, 2014 at 12:07 AM, M A Young <m.a.young@xxxxxxxxxxxx> wrote:
> > > > On Sat, 22 Nov 2014, M A Young wrote:
> > > >
> > > > > While investigating a bug reported on Red Hat Bugzilla
> > > > > https://bugzilla.redhat.com/show_bug.cgi?id=1166461
> > > > > I discovered the following
> > > > >
> > > > > xl migrate --debug domid localhost does indeed fail for Xen 4.4 pv (the
> > > > > bug report is for Xen 4.3 hvm ) when xl migrate domid localhost works. There
> > > > > are actually two issues here
> > > > >
> > > > > * the segfault in libxl-save-helper --restore-domain (as reported in the
> > > > > bug above) occurs if the guest memory is 1024M (on my 4G box) and is
> > > > > presumably because the allocated memory eventually runs out
> > > >
> > > > I have found a bit more out about this. The segfault at at line 1378 of
> > > > tools/libxc/xc_domain_restore.c which is
> > > >                 DPRINTF("************** pfn=%lx type=%lx gotcs=%08lx "
> > > >                         "actualcs=%08lx\n", pfn, pagebuf->pfn_types[pfn],
> > > >                         csum_page(region_base + (i + curbatch)*PAGE_SIZE),
> > > >                         csum_page(buf));
> > > > and is because pfn in pagebuf->pfn_types[pfn] is beyond the end of the
> > > > array. This occurs in the verification phase.
> > > >
> > > > > * the segfault doesn't occur if the guest memory is 128M, but the
> > > > > migration still fails. The first attached file contains the log from a run
> > > > > with xl -v migrate --debug domid localhost (with mfn and duplicated lines
> > > > > stripped out to make the size manageable).
> > > >
> > > > The difference actually seems to be down to how active the VM is rather than
> > > > the memory size (my small memory test system was doing very little, my
> > > > larger system was a full OS install). In the non-segfault case the problem
> > > > was the printf and printf_info commands in the create_domain() routine in
> > > > tools/libxl/xl_cmdimpl.c . As xl migrate uses stdout to pass status messages
> > > > back from the restoring dom0, these commands cause an unexpected message. If
> > > > you move them onto stderr then the migration completes in the non-segfault
> > > > case.
> > > Good job tracking those down -- are there patches in the works?
> > I have a partial patch for the printf printf_info problem, which works for
> > me but doesn't cover printing the info in sxp format.
> Am I right that is all related to the use of --debug and or -vm? and
> that a plain "xl migrate" works ok?
>
> It's still a bug of course, but changes the severity (somehow, not sure
> to what extent IMHO it does etc).
A plain xl migrate does work on 4.4. I didn't get xl migrate working on 
4.5-rc1 but was waiting for rc3 before trying to debug that.
        Michael Young

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