Clean up a couple of MD warts in vm_fault_populate():

[jah]

Summary

--Eliminate a big ifdef that encompassed all currently-supported
architectures except mips and powerpc32. This applied to the case
in which we've allocated a superpage but the pager-populated range
is insufficient for a superpage mapping. For platforms that don't
support superpages the check should be inexpensive as we shouldn't
get a superpage in the first place. Make the normal-page fallback
logic identical for all platforms and provide a simple implementation
of pmap_ps_enabled() for MIPS and Book-E/AIM32 powerpc.

--Apply the logic for handling pmap_enter() failure if a superpage
mapping can't be supported due to higher-level protection policy.
Use KERN_PROTECTION_FAILURE instead of KERN_FAILURE for this case,
and note Intel PKU on amd64 as the first example of such protection
policy.

[jah]

sys/powerpc/aim/mmu_oea.c

1128

For PowerPC I believe pmap_nomethod() dispatch should work for the purpose of this change without needing these added functions. In fact it seems that it already is for 64-bit Book E MMUs. But it seemed less fragile to provide the real dispatch entries since we already allocate them anyway.

[kib]

sys/amd64/amd64/pmap.c

7150

This is fine and makes things consistent with pmap_enter_largepage(), but now I see that there are some issues with KERN_PROTECTION_FAILURE.

Since they are mine and pre-existing, I propose D29442.

[kib]

sys/vm/vm_fault.c

562

I would say 'lower-level', since PKRU is somewhere below pmap. Or remove the 'higher-level' words.

578–579

IMO it would be cleaner to assert rv value right after pmap_enter(). Something like
MPASS(rv == KERN_SUCCESS || (psind > 0 && rv == KERN_PROTECTION_FAILURE);

[jah]

Adjust wording in comment, rearrange return value assertion

[bdragon]

OK PowerPC