s390: implement pte special bit

Convert XIP to support non-struct page backed memory, using VM_MIXEDMAP for
the user mappings.

This requires the get_xip_page API to be changed to an address based one.
Improve the API layering a little bit too, while we're here.

This is required in order to support XIP filesystems on memory that isn't
backed with struct page (but memory with struct page is still supported too).
Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Carsten Otte <cotte@de.ibm.com>
Cc: Jared Hulbert <jaredeh@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-s390/pgtable.h

@@ -220,6 +220,8 @@ extern char empty_zero_page[PAGE_SIZE];
 /* Software bits in the page table entry */
 #define _PAGE_SWT	0x001		/* SW pte type bit t */
 #define _PAGE_SWX	0x002		/* SW pte type bit x */
+#define _PAGE_SPECIAL	0x004		/* SW associated with special page */
+#define __HAVE_ARCH_PTE_SPECIAL
 
 /* Six different types of pages. */
 #define _PAGE_TYPE_EMPTY	0x400
@@ -520,7 +522,7 @@ static inline int pte_file(pte_t pte)
 
 static inline int pte_special(pte_t pte)
 {
-	return 0;
+	return (pte_val(pte) & _PAGE_SPECIAL);
 }
 
 #define __HAVE_ARCH_PTE_SAME
@@ -722,6 +724,7 @@ static inline pte_t pte_mkyoung(pte_t pte)
 
 static inline pte_t pte_mkspecial(pte_t pte)
 {
+	pte_val(pte) |= _PAGE_SPECIAL;
 	return pte;
 }