I'm going to test setting this parameter (although I don't use Xen):
Googling around there seems to be a relation between Oops, drbd active/active and this setting.
DRBD source code says this which sounds like it could be related:
/* e.g. XFS meta- & log-data is in slab pages, which have a
* page_count of 0 and/or have PageSlab() set.
* we cannot use send_page for those, as that does get_page();
* put_page(); and would cause either a VM_BUG directly, or
* __page_cache_release a page that would actually still be referenced
* by someone, leading to some obscure delayed Oops somewhere else. */
if (disable_sendpage || (page_count(page) < 1) || PageSlab(page))
return _drbd_no_send_page(mdev, page, offset, size, msg_flags);
--
"Judge me by my size, do you? And well you should not"
Master Yoda