sparc64: Do not insert non-valid PTEs into the TSB hash table.

[ Upstream commit 18f38132 ]

The assumption was that update_mmu_cache() (and the equivalent for PMDs) would
only be called when the PTE being installed will be accessible by the user.

This is not true for code paths originating from remove_migration_pte().

There are dire consequences for placing a non-valid PTE into the TSB.  The TLB
miss frramework assumes thatwhen a TSB entry matches we can just load it into
the TLB and return from the TLB miss trap.

So if a non-valid PTE is in there, we will deadlock taking the TLB miss over
and over, never satisfying the miss.

Just exit early from update_mmu_cache() and friends in this situation.

Based upon a report and patch from Christopher Alexander Tobias Schulze.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

arch/sparc/mm/init_64.c

@@ -350,6 +350,10 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long address, pte_t *
 
 	mm = vma->vm_mm;
 
+	/* Don't insert a non-valid PTE into the TSB, we'll deadlock.  */
+	if (!pte_accessible(mm, pte))
+		return;
+
 	spin_lock_irqsave(&mm->context.lock, flags);
 
 #if defined(CONFIG_HUGETLB_PAGE) || defined(CONFIG_TRANSPARENT_HUGEPAGE)