-/* Mask and four different kinds of invalid pages. */
-#define _PAGE_INVALID_MASK 0x601
-#define _PAGE_INVALID_EMPTY 0x400
-#define _PAGE_INVALID_NONE 0x401
-#define _PAGE_INVALID_SWAP 0x600
-#define _PAGE_INVALID_FILE 0x601
+/*
+ * PTE type bits are rather complicated. handle_pte_fault uses pte_present,
+ * pte_none and pte_file to find out the pte type WITHOUT holding the page
+ * table lock. ptep_clear_flush on the other hand uses ptep_clear_flush to
+ * invalidate a given pte. ipte sets the hw invalid bit and clears all tlbs
+ * for the page. The page table entry is set to _PAGE_TYPE_EMPTY afterwards.
+ * This change is done while holding the lock, but the intermediate step
+ * of a previously valid pte with the hw invalid bit set can be observed by
+ * handle_pte_fault. That makes it necessary that all valid pte types with
+ * the hw invalid bit set must be distinguishable from the four pte types
+ * empty, none, swap and file.
+ *
+ * irxt ipte irxt
+ * _PAGE_TYPE_EMPTY 1000 -> 1000
+ * _PAGE_TYPE_NONE 1001 -> 1001
+ * _PAGE_TYPE_SWAP 1011 -> 1011
+ * _PAGE_TYPE_FILE 11?1 -> 11?1
+ * _PAGE_TYPE_RO 0100 -> 1100
+ * _PAGE_TYPE_RW 0000 -> 1000
+ *
+ * pte_none is true for bits combinations 1000, 1100
+ * pte_present is true for bits combinations 0000, 0010, 0100, 0110, 1001
+ * pte_file is true for bits combinations 1101, 1111
+ * swap pte is 1011 and 0001, 0011, 0101, 0111, 1010 and 1110 are invalid.
+ */