Re: [Xen-devel] Fatal crash on xen4.2 HVM + qemu-xen dm + NFS

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-01-21 at 15:15 +0000, Alex Bligh wrote:
> Surely before Xen removes the grant on the page, unmapping it from dom0's
> memory, it should check to see if there are any existing references
> to the page and if there are, given the kernel its own COW copy, rather
> than unmap it totally which is going to lead to problems.

Unfortunately each page only has one reference count, so you cannot
distinguish between references from this particular NFS write from other
references (other writes, the ref held by the process itself, etc).

My old series added a reference count to the SKB itself exactly so that
it would be possible to know when the network stack was truly finished
with the page in the context of a specific operation.

Unfortunately due to lack of time I've not been able to finish those
off.

Ian.



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