FRV: Handle update_mmu_cache() being called when current->mm is NULL [try #2]

Handle update_mmu_cache() being called when current->mm is NULL.

We cache static TLB mappings for the current page table in DAMPR4 and DAMPR5
on the theory that the next data lookup is likely to be in the same general
region, and thus is likely to be mapped by the same page table.  However, we
can't get this information if we can't access the appropriate mm_struct.

If current->mm is NULL, we just clear the cache in the knowledge that the TLB
miss handlers will load it.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-frv/pgtable.h

@@ -507,13 +507,22 @@ static inline int pte_file(pte_t pte)
  */
 static inline void update_mmu_cache(struct vm_area_struct *vma, unsigned long address, pte_t pte)
 {
+	struct mm_struct *mm;
 	unsigned long ampr;
-	pgd_t *pge = pgd_offset(current->mm, address);
-	pud_t *pue = pud_offset(pge, address);
-	pmd_t *pme = pmd_offset(pue, address);
 
-	ampr = pme->ste[0] & 0xffffff00;
-	ampr |= xAMPRx_L | xAMPRx_SS_16Kb | xAMPRx_S | xAMPRx_C | xAMPRx_V;
+	mm = current->mm;
+	if (mm) {
+		pgd_t *pge = pgd_offset(mm, address);
+		pud_t *pue = pud_offset(pge, address);
+		pmd_t *pme = pmd_offset(pue, address);
+
+		ampr = pme->ste[0] & 0xffffff00;
+		ampr |= xAMPRx_L | xAMPRx_SS_16Kb | xAMPRx_S | xAMPRx_C |
+			xAMPRx_V;
+	} else {
+		address = ULONG_MAX;
+		ampr = 0;
+	}
 
 	asm volatile("movgs %0,scr0\n"
 		     "movgs %0,scr1\n"