s390/mm: cleanup init_new_context() callback

The set of values asce_limit may be assigned with is TASK_SIZE_MAX,
_REGION1_SIZE, _REGION2_SIZE and 0 as a special case if the callback
was called from execve().
Do VM_BUG_ON() if asce_limit is something else.

Save few CPU cycles by removing unnecessary asce_limit re-assignment
in case of 3-level task and redundant PGD entry type reconstruction.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/mmu_context.h

@@ -18,6 +18,8 @@
 static inline int init_new_context(struct task_struct *tsk,
 				   struct mm_struct *mm)
 {
+	unsigned long asce_type, init_entry;
+
 	spin_lock_init(&mm->context.lock);
 	INIT_LIST_HEAD(&mm->context.pgtable_list);
 	INIT_LIST_HEAD(&mm->context.gmap_list);
@@ -35,29 +37,34 @@ static inline int init_new_context(struct task_struct *tsk,
 	mm->context.allow_gmap_hpage_1m = 0;
 #endif
 	switch (mm->context.asce_limit) {
-	case _REGION2_SIZE:
+	default:
 		/*
-		 * forked 3-level task, fall through to set new asce with new
-		 * mm->pgd
+		 * context created by exec, the value of asce_limit can
+		 * only be zero in this case
 		 */
-	case 0:
-		/* context created by exec, set asce limit to 4TB */
+		VM_BUG_ON(mm->context.asce_limit);
+		/* continue as 3-level task */
 		mm->context.asce_limit = _REGION2_SIZE;
-		mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
-				   _ASCE_USER_BITS | _ASCE_TYPE_REGION3;
+		fallthrough;
+	case _REGION2_SIZE:
+		/* forked 3-level task */
+		init_entry = _REGION3_ENTRY_EMPTY;
+		asce_type = _ASCE_TYPE_REGION3;
 		break;
 	case TASK_SIZE_MAX:
-		/* forked 5-level task, set new asce with new_mm->pgd */
-		mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
-			_ASCE_USER_BITS | _ASCE_TYPE_REGION1;
+		/* forked 5-level task */
+		init_entry = _REGION1_ENTRY_EMPTY;
+		asce_type = _ASCE_TYPE_REGION1;
 		break;
 	case _REGION1_SIZE:
-		/* forked 4-level task, set new asce with new mm->pgd */
-		mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
-				   _ASCE_USER_BITS | _ASCE_TYPE_REGION2;
+		/* forked 4-level task */
+		init_entry = _REGION2_ENTRY_EMPTY;
+		asce_type = _ASCE_TYPE_REGION2;
 		break;
 	}
-	crst_table_init((unsigned long *) mm->pgd, pgd_entry_type(mm));
+	mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
+			   _ASCE_USER_BITS | asce_type;
+	crst_table_init((unsigned long *) mm->pgd, init_entry);
 	return 0;
 }
 

arch/s390/include/asm/pgalloc.h

@@ -34,17 +34,6 @@ static inline void crst_table_init(unsigned long *crst, unsigned long entry)
 	memset64((u64 *)crst, entry, _CRST_ENTRIES);
 }
 
-static inline unsigned long pgd_entry_type(struct mm_struct *mm)
-{
-	if (mm_pmd_folded(mm))
-		return _SEGMENT_ENTRY_EMPTY;
-	if (mm_pud_folded(mm))
-		return _REGION3_ENTRY_EMPTY;
-	if (mm_p4d_folded(mm))
-		return _REGION2_ENTRY_EMPTY;
-	return _REGION1_ENTRY_EMPTY;
-}
-
 int crst_table_upgrade(struct mm_struct *mm, unsigned long limit);
 
 static inline unsigned long check_asce_limit(struct mm_struct *mm, unsigned long addr,