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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

Ian.


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