Pagemap: security fixes vs. ABI compatibility

Fortunately, from our PoV, whilst the change breaks our preferred operating mode we can use our fallback on the kernels that have this behaviour. If our patching work is accepted then hopefully it won't present overly widespread problems for us.

In terms of rowhammer, it's an interesting point about hugepages - presumably as there aren't as many of them you're much more likely to get one that's contiguous with something you'd like to meddle with.

More generally, my understanding is that hiding PFNs is explicitly intended as an obstacle to slow attackers rather than an outright fix. I can also see the argument that exposing PFNs in the first place was probably not the best plan, in hindsight...