powerpc/mm: fix _PAGE_SWP_SOFT_DIRTY breaking swapoff

Swapoff after swapping hangs on the G5, when CONFIG_CHECKPOINT_RESTORE=y
but CONFIG_MEM_SOFT_DIRTY is not set.  That's because the non-zero
_PAGE_SWP_SOFT_DIRTY bit, added by CONFIG_HAVE_ARCH_SOFT_DIRTY=y, is not
discounted when CONFIG_MEM_SOFT_DIRTY is not set: so swap ptes cannot be
recognized.

(I suspect that the peculiar dependence of HAVE_ARCH_SOFT_DIRTY on
CHECKPOINT_RESTORE in arch/powerpc/Kconfig comes from an incomplete
attempt to solve this problem.)

It's true that the relationship between CONFIG_HAVE_ARCH_SOFT_DIRTY and
and CONFIG_MEM_SOFT_DIRTY is too confusing, and it's true that swapoff
should be made more robust; but nevertheless, fix up the powerpc ifdefs
as x86_64 and s390 (which met the same problem) have them, defining the
bits as 0 if CONFIG_MEM_SOFT_DIRTY is not set.

Fixes: 7207f43665b8 ("powerpc/mm: Add page soft dirty tracking")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/hash.h

@@ -33,7 +33,12 @@
 #define _PAGE_F_GIX_SHIFT	12
 #define _PAGE_F_SECOND		0x08000 /* Whether to use secondary hash or not */
 #define _PAGE_SPECIAL		0x10000 /* software: special page */
+
+#ifdef CONFIG_MEM_SOFT_DIRTY
 #define _PAGE_SOFT_DIRTY	0x20000 /* software: software dirty tracking */
+#else
+#define _PAGE_SOFT_DIRTY	0x00000
+#endif
 
 /*
  * THP pages can't be special. So use the _PAGE_SPECIAL

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -167,8 +167,13 @@ static inline void pgd_set(pgd_t *pgdp, unsigned long val)
 #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val((pte)) & ~_PAGE_PTE })
 #define __swp_entry_to_pte(x)	__pte((x).val | _PAGE_PTE)
 
-#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
+#ifdef CONFIG_MEM_SOFT_DIRTY
 #define _PAGE_SWP_SOFT_DIRTY   (1UL << (SWP_TYPE_BITS + _PAGE_BIT_SWAP_TYPE))
+#else
+#define _PAGE_SWP_SOFT_DIRTY	0UL
+#endif /* CONFIG_MEM_SOFT_DIRTY */
+
+#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
 static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
 {
 	return __pte(pte_val(pte) | _PAGE_SWP_SOFT_DIRTY);
@@ -181,8 +186,6 @@ static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
 {
 	return __pte(pte_val(pte) & ~_PAGE_SWP_SOFT_DIRTY);
 }
-#else
-#define _PAGE_SWP_SOFT_DIRTY	0
 #endif /* CONFIG_HAVE_ARCH_SOFT_DIRTY */
 
 void pgtable_cache_add(unsigned shift, void (*ctor)(void *));