[PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 6

[Naoya Horiguchi]

pmd_present() checks _PAGE_PSE along with _PAGE_PRESENT to avoid false negative
return when it races with thp spilt (during which _PAGE_PRESENT is temporary
cleared.) I don't think that dropping _PAGE_PSE check in pmd_present() works
well because it can hurt optimization of tlb handling in thp split.
In the current kernel, bit 6 is not used in non-present format because nonlinear
file mapping is obsolete, so let's move _PAGE_SWP_SOFT_DIRTY to that bit.
Bit 7 is used as reserved (always clear), so please don't use it for other
purpose.

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/pgtable_types.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git v4.9-rc2-mmotm-2016-10-27-18-27/arch/x86/include/asm/pgtable_types.h v4.9-rc2-mmotm-2016-10-27-18-27_patched/arch/x86/include/asm/pgtable_types.h
index 8b4de22..89e88f1 100644
--- v4.9-rc2-mmotm-2016-10-27-18-27/arch/x86/include/asm/pgtable_types.h
+++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/arch/x86/include/asm/pgtable_types.h
@@ -98,14 +98,14 @@
  * Tracking soft dirty bit when a page goes to a swap is tricky.
  * We need a bit which can be stored in pte _and_ not conflict
  * with swap entry format. On x86 bits 6 and 7 are *not* involved
- * into swap entry computation, but bit 6 is used for nonlinear
- * file mapping, so we borrow bit 7 for soft dirty tracking.
+ * into swap entry computation, but bit 7 is used for thp migration,
+ * so we borrow bit 6 for soft dirty tracking.
  *
  * Please note that this bit must be treated as swap dirty page
- * mark if and only if the PTE has present bit clear!
+ * mark if and only if the PTE/PMD has present bit clear!
  */
 #ifdef CONFIG_MEM_SOFT_DIRTY
-#define _PAGE_SWP_SOFT_DIRTY	_PAGE_PSE
+#define _PAGE_SWP_SOFT_DIRTY	_PAGE_DIRTY
 #else
 #define _PAGE_SWP_SOFT_DIRTY	(_AT(pteval_t, 0))
 #endif
-- 
2.7.0