Re: [Xen-devel] Problems using xl migrate

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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

~Andrew

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