On 10/30/18 10:58 AM, Matthew Wilcox wrote:

Does this satisfy Igor's requirements?  We wouldn't be able to
copy_to/from_user() while rare_mm was active.  I think that's a feature
though!  It certainly satisfies my interests (kernel code be able to
mark things as dynamically-allocated-and-read-only-after-initialisation)

It has to be more than copy_to/from_user(), though, I think.

rare_modify(q) either has to preempt_disable(), or we need to teach the
context-switching code when and how to switch in/out of the rare_mm.
preempt_disable() would also keep us from sleeping.