[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Problem with PV disk and iSCSI
On Sat, 2008-02-09 at 08:07 +0000, Keir Fraser wrote: > 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. In the failure I'm seeing, the skb could sit on a socket queue indefinitely. The application reading the socket could be blocked for some other reason. blkback can't defer responding to blkfront (completing the guest I/O). I think blkback needs to assume that a completion with a non-zero page reference count means it needs to make a copy, or implement a timeout like netback. Gary -- Gary Grebus Virtual Iron Software, Inc. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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