powerpc/mm/mce: Keep irqs disabled during lockless page table walk

__find_linux_mm_pte() returns a page table entry pointer after walking
the page table without holding locks. To make it safe against a THP
split and/or collapse, we disable interrupts around the lockless page
table walk. However we need to keep interrupts disabled as long as we
use the page table entry pointer that is returned.

Fix addr_to_pfn() to do that.

Fixes: ba41e1e1 ("powerpc/mce: Hookup derror (load/store) UE errors")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
[mpe: Rearrange code slightly and tweak change log wording]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190918145328.28602-1-aneesh.kumar@linux.ibm.com

arch/powerpc/kernel/mce_power.c

@@ -29,7 +29,7 @@ unsigned long addr_to_pfn(struct pt_regs *regs, unsigned long addr)
 {
 	pte_t *ptep;
 	unsigned int shift;
-	unsigned long flags;
+	unsigned long pfn, flags;
 	struct mm_struct *mm;
 
 	if (user_mode(regs))
@@ -39,18 +39,22 @@ unsigned long addr_to_pfn(struct pt_regs *regs, unsigned long addr)
 
 	local_irq_save(flags);
 	ptep = __find_linux_pte(mm->pgd, addr, NULL, &shift);
-	local_irq_restore(flags);
 
-	if (!ptep || pte_special(*ptep))
-		return ULONG_MAX;
+	if (!ptep || pte_special(*ptep)) {
+		pfn = ULONG_MAX;
+		goto out;
+	}
 
-	if (shift > PAGE_SHIFT) {
+	if (shift <= PAGE_SHIFT)
+		pfn = pte_pfn(*ptep);
+	else {
 		unsigned long rpnmask = (1ul << shift) - PAGE_SIZE;
-
-		return pte_pfn(__pte(pte_val(*ptep) | (addr & rpnmask)));
+		pfn = pte_pfn(__pte(pte_val(*ptep) | (addr & rpnmask)));
 	}
 
-	return pte_pfn(*ptep);
+out:
+	local_irq_restore(flags);
+	return pfn;
 }
 
 /* flush SLBs and reload */