sparc64 MM regression...

[David Miller]

So I bisected a userspace corruption regression down to commit:

commit a8e654f01cb725d0bfd741ebca1bf4c9337969cc                                                                                    
Author: Nitin Gupta <nitin.m.gupta@xxxxxxxxxx>                                                                                     
Date:   Wed Jan 31 16:18:09 2018 -0800                                                                                             
                                                                                                                                   
    sparc64: update pmdp_invalidate() to return old pmd value                                                                      

The transformation is basically from a set_pte_at() call into
an atomic cmpxchg64() loop to set the pmd.

The problem is that set_pmd_at() does more than just assign
the pmd entry.

It also does some accounting and also queues up a batch TLB
flush entry.

So the side effect of this change is that the TLB is never
flushed for these changed PMDs, and thus the userland memory
corruption I was seeing.