lkpi_vmf_insert_pfn_prot_locked(): detect uninitialized pages

[kib]

Summary

This would not detect an unitialized page at the physical address zero,
but otherwise should be reliable.

[kib]

I understand that the immediate objection would be that memset's on the vm_page_array[] are slow and somewhat against the lazy initialization of vm_pages. But this is plain memset instead of the CPU-intensive calculations of proper vm_pages.

This should work against the class of drivers bugs where vma is created against a region that is not registered with the fictitious managed range. I was bitten by drm 6.12 on my new (but quite old) KabyLake notebook.

[bz]

What about powerpc?