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Re: [Xen-devel] Problem with PV disk and iSCSI

To: Kurt Hackel <kurt.hackel@xxxxxxxxxx>, Gary Grebus <ggrebus@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Problem with PV disk and iSCSI
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Sat, 09 Feb 2008 08:07:34 +0000
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 9/2/08 06:15, "Kurt Hackel" <kurt.hackel@xxxxxxxxxx> wrote:

>> I'm thinking blkback will have to make a dom0 copy of the page before
>> doing the unmap if there are still extra references?
> Can the unmap be deferred, handled by the last reference holder?  Or
> does this open up a potential security hole?

netback already does this kind of reference counting. It oughtn't to be hard
to check the page reference count in the blkback I/O completion handler and,
if non-zero, set up a callback for when the count does fall to zero. And
defer responding to the frontend until that time. Netback is even more
sophisticated in that it also sets a time out and if the page languishes for
too long with non-zero count, it's able to forcibly copy-and-release the
page. I don't think we need to go that far for blkback however.

 -- Keir

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