powerpc/mm: Fix _PAGE_CHG_MASK to protect _PAGE_SPECIAL

Fix _PAGE_CHG_MASK so that pte_modify() does not affect the _PAGE_SPECIAL bit.
Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/pgtable-4k.h

@@ -60,7 +60,7 @@
 /* It should be preserving the high 48 bits and then specifically */
 /* preserving _PAGE_SECONDARY | _PAGE_GROUP_IX */
 #define _PAGE_CHG_MASK	(PAGE_MASK | _PAGE_ACCESSED | _PAGE_DIRTY | \
-                         _PAGE_HPTEFLAGS)
+                         _PAGE_HPTEFLAGS | _PAGE_SPECIAL)
 
 /* Bits to mask out from a PMD to get to the PTE page */
 #define PMD_MASKED_BITS		0

arch/powerpc/include/asm/pgtable-64k.h

@@ -114,7 +114,7 @@ static inline struct subpage_prot_table *pgd_subpage_prot(pgd_t *pgd)
  * pgprot changes
  */
 #define _PAGE_CHG_MASK	(PTE_RPN_MASK | _PAGE_HPTEFLAGS | _PAGE_DIRTY | \
-                         _PAGE_ACCESSED)
+                         _PAGE_ACCESSED | _PAGE_SPECIAL)
 
 /* Bits to mask out from a PMD to get to the PTE page */
 #define PMD_MASKED_BITS		0x1ff

arch/powerpc/include/asm/pgtable-ppc32.h

@@ -429,7 +429,8 @@ extern int icache_44x_need_flush;
 #define PMD_PAGE_SIZE(pmd)	bad_call_to_PMD_PAGE_SIZE()
 #endif
 
-#define _PAGE_CHG_MASK	(PAGE_MASK | _PAGE_ACCESSED | _PAGE_DIRTY)
+#define _PAGE_CHG_MASK	(PAGE_MASK | _PAGE_ACCESSED | _PAGE_DIRTY | \
+			 _PAGE_SPECIAL)
 
 
 #define PAGE_PROT_BITS	(_PAGE_GUARDED | _PAGE_COHERENT | _PAGE_NO_CACHE | \