On Mon, Oct 12, 2020 at 09:28:29AM -0700, Dave Hansen wrote:

kmap_atomic() is always preferred over kmap()/kmap_thread().
kmap_atomic() is _much_ more lightweight since its TLB invalidation is
always CPU-local and never broadcast.

So, basically, unless you *must* sleep while the mapping is in place,
kmap_atomic() is preferred.

But kmap_atomic() disables preemption, so the _ideal_ interface would map
it only locally, then on preemption make it global.  I don't even know
if that _can_ be done.  But this email makes it seem like kmap_atomic()
has no downsides.