s390/boot: cleanup number of page table levels setup

The separate vmalloc area size check against _REGION2_SIZE
is needed in case user provided insanely large value using
vmalloc= kernel command line parameter. That could lead to
overflow and selecting 3 page table levels instead of 4.

Use size_add() for the overflow check and get rid of the
extra vmalloc area check.

With the current values of CONFIG_MAX_PHYSMEM_BITS and
PAGES_PER_SECTION the sum of maximal possible size of
identity mapping and vmemmap area (derived from these
macros) plus modules area size MODULES_LEN can not
overflow. Thus, that sum is used as first addend while
vmalloc area size is second addend for size_add().

Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/boot/startup.c

@@ -177,6 +177,7 @@ static unsigned long setup_kernel_memory_layout(void)
 	unsigned long asce_limit;
 	unsigned long rte_size;
 	unsigned long pages;
+	unsigned long vsize;
 	unsigned long vmax;
 
 	pages = ident_map_size / PAGE_SIZE;
@@ -184,11 +185,9 @@ static unsigned long setup_kernel_memory_layout(void)
 	vmemmap_size = SECTION_ALIGN_UP(pages) * sizeof(struct page);
 
 	/* choose kernel address space layout: 4 or 3 levels. */
-	vmemmap_start = round_up(ident_map_size, _REGION3_SIZE);
-	if (IS_ENABLED(CONFIG_KASAN) ||
-	    vmalloc_size > _REGION2_SIZE ||
-	    vmemmap_start + vmemmap_size + vmalloc_size + MODULES_LEN >
-		    _REGION2_SIZE) {
+	vsize = round_up(ident_map_size, _REGION3_SIZE) + vmemmap_size + MODULES_LEN;
+	vsize = size_add(vsize, vmalloc_size);
+	if (IS_ENABLED(CONFIG_KASAN) || (vsize > _REGION2_SIZE)) {
 		asce_limit = _REGION1_SIZE;
 		rte_size = _REGION2_SIZE;
 	} else {