Joe,

Do you know if have a fix for above? so far we also suspected the
grant page be unmapped earlier, we using 4.1 stable during our test.

A true fix? No, but I posted a patch set (see later email message
for a link) that you could forward port. The workaround is:

quoted

A workaround is to turn off O_DIRECT use by Xen as that ensures
the pages are copied. Xen 4.3 does this by default.

I believe fixes for this are in 4.3 and 4.2.2 if using the
qemu upstream DM. Note these aren't real fixes, just a workaround
of a kernel bug.

The guest is pvm, and disk model is xvbd, guest config file as below:

...

I think this only for pvhvm/hvm?

I don't have much experience outside pvhvm/hvm, but I believe it
should work for any device.

Testing was simple - just find all (*) the references to O_DIRECT
in your device model and remove them!

(*)=you could be less lazy than me and find the right ones.

I am guessing it will be the same ones though.

-- 
Alex Bligh