Re: Linux 4.1 reports wrong number of pages to toolstack

[David Vrabel <david.vrabel@citrix.com>]

On 04/09/15 15:53, Wei Liu wrote:
> On Fri, Sep 04, 2015 at 03:42:06PM +0100, David Vrabel wrote:
>> On 04/09/15 09:53, Ian Campbell wrote:
>>> On Fri, 2015-09-04 at 01:40 +0100, Wei Liu wrote:
>>>> Hi David
>>>>
>>>> This issue is exposed by the introduction of migration v2. The symptom is that
>>>> a guest with 32 bit 4.1 kernel can't be restored because it's asking for too
>>>> many pages.
>>>
>>> FWIW my adhoc tests overnight gave me:
>>>
>>> 37858: b953c0d234bc72e8489d3bf51a276c5c4ec85345 v4.1		Fail
>>> 37862: 39a8804455fb23f09157341d3ba7db6d7ae6ee76 v4.0		Fail
>>> 37860: bfa76d49576599a4b9f9b7a71f23d73d6dcff735 v3.19		Fail
>>>
>>> 37872: e36f014edff70fc02b3d3d79cead1d58f289332e v3.19-rc7	Fail
>>> 37866: 26bc420b59a38e4e6685a73345a0def461136dce v3.19-rc6	Fail
>>> 37868: ec6f34e5b552fb0a52e6aae1a5afbbb1605cc6cc v3.19-rc5	Fail
>>> 37864: eaa27f34e91a14cdceed26ed6c6793ec1d186115 v3.19-rc4	Fail *
>>> 37867: b1940cd21c0f4abdce101253e860feff547291b0 v3.19-rc3	Pass *
>>> 37865: b7392d2247cfe6771f95d256374f1a8e6a6f48d6 v3.19-rc2	Pass
>>>
>>> 37863: 97bf6af1f928216fd6c5a66e8a57bfa95a659672 v3.19-rc1	Pass
>>>
>>> 37861: b2776bf7149bddd1f4161f14f79520f17fc1d71d v3.18		Pass
>>>
>>> I have set the adhoc bisector working on the ~200 commits between rc3 and
>>> rc4. It's running in the Citrix instance (which is quieter) so the interim
>>> results are only visible within our network at http://osstest.xs.citrite.ne
>>> t/~osstest/testlogs/results-adhoc/bisect/xen-unstable/test-amd64-i386
>>> -xl..html.
>>>
>>> So far it has confirmed the basis fail and it is now rechecking the basis
>>> pass.
>>>
>>> Slightly strange though is:
>>> $ git log --oneline v3.19-rc3..v3.19-rc4 -- drivers/xen/ arch/x86/xen/ include/xen/
>>> $
>>>
>>> i.e. there are no relevant seeming xen commits in that range. Maybe the
>>> last one of this is more relevant?
>>
>> Since this bisect attempt appears to have disappeared into the weeds I
>> did my own and it fingered:
>>
>> 633d6f17cd91ad5bf2370265946f716e42d388c6 (x86/xen: prepare p2m list for
>> memory hotplug) which was introduced in 4.0-rc7.
>>
>> This looks a lot more plausible as the Linux change triggering the
>> migration failures.
>>
> 
> FWIW. Same 32bit kernel, 128MB memory, migration is OK.

This commit is only bad with 64-bit guests -- with a 32-bit guest the
maximum p2m size covers only 64 GiB.  It will also requires
XEN_BALLOON_MEMORY_HOTPLUG to be enabled.

This commit is exposing a toolstack bug.

David