On Thu, Feb 11, 2021 at 11:28:09AM -0800, Axel Rasmussen wrote:

Ah, I had added this just after VM_UFFD_WP, without noticing that this
would be sharing a bit with VM_LOCKED. That seems like not such a
great idea.

I don't see another unused bit, and I don't see some other obvious
candidate to share with. So, the solution that comes to mind is

it'd be even better if you didn't use the last unused bit for UFFD_WP.
not sure how feasible that is, but you can see we're really short on
bits here.