revert "s390/mm: make hugepages_supported a boot time decision"

Heiko noticed that the current check for hugepage support on s390 is a
little bit too harsh as systems which do not support will crash.

The reason is that pageblock_order can now get negative when we set
HPAGE_SHIFT to 0.  To avoid all this and to avoid opening another can of
worms with enabling HUGETLB_PAGE_SIZE_VARIABLE I think it would be best
to simply allow architectures to define their own hugepages_supported().

Revert bea41197ead3 ("s390/mm: make hugepages_supported a boot time
decision") in preparation.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/s390/include/asm/page.h

@@ -17,10 +17,7 @@
 #define PAGE_DEFAULT_ACC	0
 #define PAGE_DEFAULT_KEY	(PAGE_DEFAULT_ACC << 4)
 
-#include <asm/setup.h>
-#ifndef __ASSEMBLY__
-
-extern unsigned int HPAGE_SHIFT;
+#define HPAGE_SHIFT	20
 #define HPAGE_SIZE	(1UL << HPAGE_SHIFT)
 #define HPAGE_MASK	(~(HPAGE_SIZE - 1))
 #define HUGETLB_PAGE_ORDER	(HPAGE_SHIFT - PAGE_SHIFT)
@@ -30,6 +27,9 @@ extern unsigned int HPAGE_SHIFT;
 #define ARCH_HAS_PREPARE_HUGEPAGE
 #define ARCH_HAS_HUGEPAGE_CLEAR_FLUSH
 
+#include <asm/setup.h>
+#ifndef __ASSEMBLY__
+
 static inline void storage_key_init_range(unsigned long start, unsigned long end)
 {
 #if PAGE_DEFAULT_KEY

arch/s390/kernel/setup.c

@@ -885,8 +885,6 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	setup_hwcaps();
 
-	HPAGE_SHIFT = MACHINE_HAS_HPAGE ? 20 : 0;
-
 	/*
 	 * Create kernel page tables and switch to virtual addressing.
 	 */

arch/s390/mm/pgtable.c

@@ -31,8 +31,6 @@
 #define ALLOC_ORDER	2
 #define FRAG_MASK	0x03
 
-unsigned int HPAGE_SHIFT;
-
 unsigned long *crst_table_alloc(struct mm_struct *mm)
 {
 	struct page *page = alloc_pages(GFP_KERNEL, ALLOC_ORDER);