Re: [Xen-devel] Migration memory corruption - PV backends need to quiesce

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 02/07/14 11:02, Ian Campbell wrote:
> On Tue, 2014-07-01 at 11:29 +0100, David Vrabel wrote:
>> On 30/06/14 10:53, Ian Campbell wrote:
>>> On Mon, 2014-06-30 at 10:47 +0100, David Vrabel wrote:
>>>
>>>> Using persistent grants means that backend writes may be lost since the
>>>> memcpy in the backend does not update the dirty bitmap and guest writes
>>>> to the shared ring may make the backends response writes visible.  I
>>>> think we may need to disable persistent grant support from blkback until
>>>> a mechanism for updating the dirty log is in place.
>>>
>>> Yes, sadly I think you might be right here.
>>
>> The copy in the frontend from the persistently granted pages to the
>> pages in the bio dirties those pages so this is actually safe.
> 
> What about if the pause+copy happens after the I/O has completed
> (response on the ring) and the copy out to the bio pages? Meaning that
> on resume the copy would pick up the stale data from the persistent
> grant.
> 
> I think the answer is that the frontend won't consider things actually
> complete until after the copy and therefore that read will be reissued.

Yes.

David

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