s390/mm: use invalid asce instead of kernel asce
Create a region 3 page table which contains only invalid entries, and use that via "s390_invalid_asce" instead of the kernel ASCE whenever there is either - no user address space available, e.g. during early startup - as an intermediate ASCE when address spaces are switched This makes sure that user space accesses in such situations are guaranteed to fail. Reviewed-by: NSven Schnelle <svens@linux.ibm.com> Reviewed-by: NAlexander Gordeev <agordeev@linux.ibm.com> Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>
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