Re: [Qemu-devel] broken incoming migration

[Paolo Bonzini]

Il 30/05/2013 15:38, Alexey Kardashevskiy ha scritto:
>>> 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.
> 
> Scan where? On the source? It is there already. On the destination? Won't
> just trying to read a page kill all effect from MADV_DONTNEED?

No, zero pages are handled with special copy-on-write magic by the
kernel. :)  So, reading works but writing doesn't.

Paolo