Re: [Qemu-devel] broken incoming migration

[Peter Lieven <pl@kamp.de>]

On 03.06.2013 12:04, Alexey Kardashevskiy wrote:
> On 05/31/2013 12:38 AM, Peter Lieven wrote:
>>
>>
>> Am 30.05.2013 um 15:41 schrieb "Paolo Bonzini" <pbonzini@redhat.com>:
>>
>>> Il 30/05/2013 11:08, Peter Lieven ha scritto:
>>>> Am 30.05.2013 10:18, schrieb Alexey Kardashevskiy:
>>>>> On 05/30/2013 05:49 PM, Paolo Bonzini wrote:
>>>>>> Il 30/05/2013 09:44, Alexey Kardashevskiy ha scritto:
>>>>>>> Hi!
>>>>>>>
>>>>>>> I found the migration broken on pseries platform, specifically, this patch
>>>>>>> broke it:
>>>>>>>
>>>>>>> f1c72795af573b24a7da5eb52375c9aba8a37972
>>>>>>> migration: do not sent zero pages in bulk stage
>>>>>>>
>>>>>>> The idea is not to send zero pages to the destination guest which is
>>>>>>> expected to have 100% empty RAM.
>>>>>>>
>>>>>>> However on pseries plaftorm the guest always has some stuff in the RAM as a
>>>>>>> part of initialization (device tree, system firmware and rtas (?)) so it is
>>>>>>> not completely empty. As the source guest cannot detect this, it skips some
>>>>>>> pages during migration and we get a broken destination guest. Bug.
>>>>>>>
>>>>>>> While the idea is ok in general, I do not see any easy way to fix it as
>>>>>>> neither QEMUMachine::init nor QEMUMachine::reset callbacks has information
>>>>>>> about whether we are about to receive a migration or not (-incoming
>>>>>>> parameter) and we cannot move device-tree and system firmware
>>>>>>> initialization anywhere else.
>>>>>>>
>>>>>>> ram_bulk_stage is static and cannot be disabled from the platform
>>>>>>> initialization code.
>>>>>>>
>>>>>>> So what would the community suggest?
>>>>>> Revert the patch. :)
>>>>> I'll wait for 24 hours (forgot to cc: the author) and then post a revert
>>>>> patch :)
>>>> does this problem only occur on pseries emulation?
>>> Probably not.  On a PC, it would occur if you had 4K of zeros in the
>>> source BIOS but not in the destination BIOS.  When you reboot, the BIOS
>>> image is wrong.
>>>
>>>> not sending zero pages is not only a performance benefit it also makes
>>>> overcomitted memory usable. the madv_dontneed seems to kick in asynchronously
>>>> and memory is not available immediately.
>>> You could also scan the page for nonzero values before writing it.
>> i had this in mind, but then choosed the other approach.... turned out to be a bad idea.
>>
>> alexey: i will prepare a patch later today, could you then please verify it fixes your problem.
>
> Yes I can, where is the patch? :)

its on my todo for today. sorry, have been a bit busy lately.

Peter