With THP 'always' journald's mmapped journal files sit in 2 MiB folios, so each small
journal write dirties and writes back 2 MiB; on the control node that saturated the
root mirror holding the bus store and the store, and the controller lost its lease.
The module sets the mode from a per-machine setting (default always, the kernel's own,
so assigning it changes nothing until a machine sets madvise), verifies it took, rotates
the journal once so new files are mapped under the new mode, gives back what it found
when stopped, and serves kernel_huge_pages to measure file_thp per unit.
