Re: [Qemu-devel] [PATCH 1/2] Postcopy: Force allocation of all-zero precopy pages

[Christian Borntraeger]

On 04/26/2017 09:52 PM, Christian Borntraeger wrote:
> On 04/26/2017 09:04 PM, Dr. David Alan Gilbert wrote:
>> * Christian Borntraeger (address@hidden) wrote:
>>> On 04/26/2017 08:37 PM, Dr. David Alan Gilbert (git) wrote:
>>>> From: "Dr. David Alan Gilbert" <address@hidden>
>>>>
>>>> When an all-zero page is received during the precopy
>>>> phase of a postcopy-enabled migration we must force
>>>> allocation otherwise accesses to the page will still
>>>> get blocked by userfault.
>>>>
>>>> Symptom:
>>>>   a) If the page is accessed by a device during device-load
>>>>     then we get a deadlock as the source finishes sending
>>>>     all its pages but the destination device-load is still
>>>>     paused and so doesn't clean up.
>>>>
>>>>   b) If the page is accessed later, then the thread will stay
>>>>     paused until the end of migration rather than carrying on
>>>>     running, until we release userfault at the end.
>>>>
>>>> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
>>>> Reported-by: Christian Borntraeger <address@hidden>
>>>
>>> CC stable? after all the guest hangs on both sides
>>>
>>> Has survived 40 migrations (usually failed at the 2nd)
>>> Tested-by: Christian Borntraeger <address@hidden>
>>
>> Great...but.....
>> Andrea (added to the mail) says this shouldn't be necessary.
>> The read we were doing in the is_zero_range() should have been sufficient
>> to get the page mapped and that zero page should have survived.
> 
> We do not do is_zero_range if ch==0 because of lazy evaluation, no?

Sorry misread that code. We do.