s390/mm: split lowcore pages with set_memory_4k()

Use set_memory_4k() to split lowcore pages within the kernel mapping
instead of using the quite subtle !addr check within modify_pmd_table()
and modify_pud_table() to prevent large pages for address zero.

With this lowcore might be mapped with 1MB / 2GB frames and only later
will be split. This way this mapping is handled like every other.
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/mm/vmem.c

@@ -240,7 +240,7 @@ static int __ref modify_pmd_table(pud_t *pud, unsigned long addr,
 		} else if (pmd_none(*pmd)) {
 			if (IS_ALIGNED(addr, PMD_SIZE) &&
 			    IS_ALIGNED(next, PMD_SIZE) &&
-			    MACHINE_HAS_EDAT1 && addr && direct &&
+			    MACHINE_HAS_EDAT1 && direct &&
 			    !debug_pagealloc_enabled()) {
 				set_pmd(pmd, __pmd(__pa(addr) | prot));
 				pages++;
@@ -336,7 +336,7 @@ static int modify_pud_table(p4d_t *p4d, unsigned long addr, unsigned long end,
 		} else if (pud_none(*pud)) {
 			if (IS_ALIGNED(addr, PUD_SIZE) &&
 			    IS_ALIGNED(next, PUD_SIZE) &&
-			    MACHINE_HAS_EDAT2 && addr && direct &&
+			    MACHINE_HAS_EDAT2 && direct &&
 			    !debug_pagealloc_enabled()) {
 				set_pud(pud, __pud(__pa(addr) | prot));
 				pages++;
@@ -584,6 +584,9 @@ void __init vmem_map_init(void)
 	__set_memory(__stext_amode31, (__etext_amode31 - __stext_amode31) >> PAGE_SHIFT,
 		     SET_MEMORY_RO | SET_MEMORY_X);
 
+	/* lowcore requires 4k mapping for real addresses / prefixing */
+	set_memory_4k(0, LC_PAGES);
+
 	/* lowcore must be executable for LPSWE */
 	if (!static_key_enabled(&cpu_has_bear))
 		set_memory_x(0, 1);