mm, x86, powerpc: pkey semantics for key-0 ?

[Ram Pai]

Dave,

Is there a reason why the default key; key-0, is not allowed to be
explicitly associated with pages using pkey_mprotect()?

I see valid usecases where an application may initially want to
associate an address-range with some key and latter choose to revert to
its initial state, by associating key-0.  However our implementation
(both x86 and power) do not allow pkey_mprotect() to be called with
key-0.

I do not see a reason why it must be blocked.

Thoughts?
RP