mm: make sure _PAGE_SWP_SOFT_DIRTY bit is not set on present pte

_PAGE_SOFT_DIRTY bit should never be set on present pte so add VM_BUG_ON
to catch any potential future abuse.

Also add a comment on _PAGE_SWP_SOFT_DIRTY definition explaining scope of
its usage.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/pgtable.h

@@ -315,21 +315,6 @@ static inline pmd_t pmd_mksoft_dirty(pmd_t pmd)
 	return pmd_set_flags(pmd, _PAGE_SOFT_DIRTY);
 }
 
-static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
-{
-	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
-}
-
-static inline int pte_swp_soft_dirty(pte_t pte)
-{
-	return pte_flags(pte) & _PAGE_SWP_SOFT_DIRTY;
-}
-
-static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
-{
-	return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);
-}
-
 static inline pte_t pte_file_clear_soft_dirty(pte_t pte)
 {
 	return pte_clear_flags(pte, _PAGE_SOFT_DIRTY);
@@ -446,6 +431,7 @@ pte_t *populate_extra_pte(unsigned long vaddr);
 
 #ifndef __ASSEMBLY__
 #include <linux/mm_types.h>
+#include <linux/mmdebug.h>
 #include <linux/log2.h>
 
 static inline int pte_none(pte_t pte)
@@ -864,6 +850,24 @@ static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
 {
 }
 
+static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
+{
+	VM_BUG_ON(pte_present(pte));
+	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
+}
+
+static inline int pte_swp_soft_dirty(pte_t pte)
+{
+	VM_BUG_ON(pte_present(pte));
+	return pte_flags(pte) & _PAGE_SWP_SOFT_DIRTY;
+}
+
+static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
+{
+	VM_BUG_ON(pte_present(pte));
+	return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);
+}
+
 #include <asm-generic/pgtable.h>
 #endif	/* __ASSEMBLY__ */
 

arch/x86/include/asm/pgtable_types.h

@@ -75,6 +75,9 @@
  * 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.
+ *
+ * Please note that this bit must be treated as swap dirty page
+ * mark if and only if the PTE has present bit clear!
  */
 #ifdef CONFIG_MEM_SOFT_DIRTY
 #define _PAGE_SWP_SOFT_DIRTY	_PAGE_PSE