nios2/mm: refactor swap PTE layout

nios2 disables swap for a good reason: it doesn't even provide sufficient
type bits as required by core MM.  However, swap entries are nowadays also
used for other purposes (migration entries, PTE markers, HWPoison, ...),
and accidential use could be problematic.

Let's properly use 5 bits for the swap type and document the layout.  Bits
26--31 should get ignored by hardware completely, so they can be used.

Link: https://lkml.kernel.org/r/20230113171026.582290-14-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/nios2/include/asm/pgtable.h

@@ -232,19 +232,21 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
 		__FILE__, __LINE__, pgd_val(e))
 
 /*
- * Encode and decode a swap entry (must be !pte_none(pte) && !pte_present(pte):
+ * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
+ * are !pte_none() && !pte_present().
  *
- * 31 30 29 28 27 26 25 24 23 22 21 20 19 18 ...  1  0
- *  0  0  0  0 type.  0  0  0  0  0  0 offset.........
+ * Format of swap PTEs:
  *
- * This gives us up to 2**2 = 4 swap files and 2**20 * 4K = 4G per swap file.
+ *   3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1
+ *   1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
+ *   0 < type -> 0 0 0 0 0 0 <-------------- offset --------------->
  *
- * Note that the offset field is always non-zero, thus !pte_none(pte) is always
- * true.
+ * Note that the offset field is always non-zero if the swap type is 0, thus
+ * !pte_none() is always true.
  */
-#define __swp_type(swp)		(((swp).val >> 26) & 0x3)
+#define __swp_type(swp)		(((swp).val >> 26) & 0x1f)
 #define __swp_offset(swp)	((swp).val & 0xfffff)
-#define __swp_entry(type, off)	((swp_entry_t) { (((type) & 0x3) << 26) \
+#define __swp_entry(type, off)	((swp_entry_t) { (((type) & 0x1f) << 26) \
 						 | ((off) & 0xfffff) })
 #define __swp_entry_to_pte(swp)	((pte_t) { (swp).val })
 #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) })