sh: Provide _PAGE_SPECIAL for 32-bit.

Allocate one of the unused PTE bits for _PAGE_SPECIAL directly. This is
prep work for fast gup and the zero page revival.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/include/asm/pgtable_32.h

@@ -20,7 +20,7 @@
  * - Bit 9 is reserved by everyone and used by _PAGE_PROTNONE.
  *
  * - Bits 10 and 11 are low bits of the PPN that are reserved on >= 4K pages.
- *   Bit 10 is used for _PAGE_ACCESSED, bit 11 remains unused.
+ *   Bit 10 is used for _PAGE_ACCESSED, and bit 11 is used for _PAGE_SPECIAL.
  *
  * - On 29 bit platforms, bits 31 to 29 are used for the space attributes
  *   and timing control which (together with bit 0) are moved into the
@@ -52,6 +52,7 @@
 #define _PAGE_PROTNONE	0x200		/* software: if not present  */
 #define _PAGE_ACCESSED	0x400		/* software: page referenced */
 #define _PAGE_FILE	_PAGE_WT	/* software: pagecache or swap? */
+#define _PAGE_SPECIAL	0x800		/* software: special page */
 
 #define _PAGE_SZ_MASK	(_PAGE_SZ0 | _PAGE_SZ1)
 #define _PAGE_PR_MASK	(_PAGE_RW | _PAGE_USER)
@@ -148,8 +149,12 @@
 # define _PAGE_SZHUGE	(_PAGE_FLAGS_HARD)
 #endif
 
+/*
+ * Mask of bits that are to be preserved accross pgprot changes.
+ */
 #define _PAGE_CHG_MASK \
-	(PTE_MASK | _PAGE_ACCESSED | _PAGE_CACHABLE | _PAGE_DIRTY)
+	(PTE_MASK | _PAGE_ACCESSED | _PAGE_CACHABLE | \
+	 _PAGE_DIRTY | _PAGE_SPECIAL)
 
 #ifndef __ASSEMBLY__
 
@@ -328,7 +333,7 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
 #define pte_dirty(pte)		((pte).pte_low & _PAGE_DIRTY)
 #define pte_young(pte)		((pte).pte_low & _PAGE_ACCESSED)
 #define pte_file(pte)		((pte).pte_low & _PAGE_FILE)
-#define pte_special(pte)	(0)
+#define pte_special(pte)	((pte).pte_low & _PAGE_SPECIAL)
 
 #ifdef CONFIG_X2TLB
 #define pte_write(pte)		((pte).pte_high & _PAGE_EXT_USER_WRITE)
@@ -358,8 +363,9 @@ PTE_BIT_FUNC(low, mkclean, &= ~_PAGE_DIRTY);
 PTE_BIT_FUNC(low, mkdirty, |= _PAGE_DIRTY);
 PTE_BIT_FUNC(low, mkold, &= ~_PAGE_ACCESSED);
 PTE_BIT_FUNC(low, mkyoung, |= _PAGE_ACCESSED);
+PTE_BIT_FUNC(low, mkspecial, |= _PAGE_SPECIAL);
 
-static inline pte_t pte_mkspecial(pte_t pte) { return pte; }
+#define __HAVE_ARCH_PTE_SPECIAL
 
 /*
  * Macro and implementation to make a page protection as uncachable.