Comment 18 for bug 1007082

So there is one thing which may or may not be related. It would be good to gather information about the Xen version this happens on. This can be found in dmesg (grep Xen). While rebooting an instance keeps you on the same version, starting new instances (even within the same region) can cause it to be run on a different Xen version.
Even if it is not related, at least it would give bring a bit more confidence whether this is guest kernel related when seeing various versions showing the issue. And it would be a waste not to take the chance while this happens every few days.

It is very strange to see this only happening on pulling a page from the freelist. Either things get corrupted while on it (since there seem to be the same tests done when giving a page back into the list) or by some weird luck those pages end up on the list wrongly.