Re: [Xen-devel] Problems using xl migrate

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

On Mon, 24 Nov 2014, Wei Liu wrote:

> On Mon, Nov 24, 2014 at 01:13:25PM +0000, Andrew Cooper wrote:
> > On 24/11/14 11:50, 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?
> > The segfault for "--debug" has already been identified and a patch
> > posted by Wen Congyang
> >
> > The call to csum_page() incorrectly calculates the offset it is supposed
> > to checksum, and wanders beyond the mapping of guest space.
> >
> > Patch in 1409908261-18682-3-git-send-email-wency@xxxxxxxxxxxxxx
> And the said patch has been applied (3460eeb3fc2) so we're fine.
However that doesn't fix my crash. I tried with it applied and still saw 
the crash. I also tried 4.5-rc1 (without XSM to avoid my other issue) and 
that crashed as well.
        Michael Young

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