Hello Nick,
KVM is currently broken upstream, starting with commit
db64fe02258f1507e13fe5212a989922323685ce (rewrite vmap layer)
The symptoms are some vmallocs from kvm fail.
cat /proc/meminfo shows this at the time of the fail:
VmallocTotal: 122880 kB
VmallocUsed: 15184 kB
VmallocChunk: 83764 kB
so there's plenty of space.
The allocation that fails seem to be 512 bytes long. I'm not
sure it always fails in there, but it seems so.