s390/mm: zero page cache synonyms for zEC12

To avoid cache synonyms on System zEC12 32 independent zero pages are
required, one for each combination for bits 2**12 to 2**16 of the virtual
address. To avoid wasting too much memory on small virtual systems the
number of zero pages is limited to 4 if the memory size is less or equal
to 64MB.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/init.c

@@ -63,10 +63,18 @@ static unsigned long __init setup_zero_pages(void)
 		break;
 	case 0x2097:	/* z10 */
 	case 0x2098:	/* z10 */
-	default:
+	case 0x2817:	/* z196 */
+	case 0x2818:	/* z196 */
 		order = 2;
 		break;
+	case 0x2827:	/* zEC12 */
+	default:
+		order = 5;
+		break;
 	}
+	/* Limit number of empty zero pages for small memory sizes */
+	if (order > 2 && totalram_pages <= 16384)
+		order = 2;
 
 	empty_zero_page = __get_free_pages(GFP_KERNEL | __GFP_ZERO, order);
 	if (!empty_zero_page)