memfd_secret() in 5.14

Yup. And as I understand it, that's the difference between sleep and hibernate. Hibernate also means ram-refresh is OFF.

Which, when you're off-grid and battery life is critical, is important.

And I don't know which, in general, uses more battery. Rebooting? or restoring from hibernate? My gut feel is hibernate is likely to use less battery, because it doesn''t go through the (possibly) long start up sequence before getting to a usable system.

Cheers,
Wol