extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
-/* Encode and decode a swap entry.
+/*
+ * Encode and decode a swap entry. Swap entries are stored in the Linux
+ * page tables as follows:
+ *
+ * 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
+ * <--------------- offset --------------------> <--- type --> 0 0
*
- * We support up to 32GB of swap on 4k machines
+ * This gives us up to 127 swap files and 32GB per swap file. Note that
+ * the offset field is always non-zero.
*/
-#define __swp_type(x) (((x).val >> 2) & 0x7f)
-#define __swp_offset(x) ((x).val >> 9)
-#define __swp_entry(type,offset) ((swp_entry_t) { ((type) << 2) | ((offset) << 9) })
+#define __SWP_TYPE_SHIFT 2
+#define __SWP_TYPE_BITS 7
+#define __SWP_TYPE_MASK ((1 << __SWP_TYPE_BITS) - 1)
+#define __SWP_OFFSET_SHIFT (__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)
+
+#define __swp_type(x) (((x).val >> __SWP_TYPE_SHIFT) & __SWP_TYPE_MASK)
+#define __swp_offset(x) ((x).val >> __SWP_OFFSET_SHIFT)
+#define __swp_entry(type,offset) ((swp_entry_t) { ((type) << __SWP_TYPE_SHIFT) | ((offset) << __SWP_OFFSET_SHIFT) })
+
#define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val(pte) })
#define __swp_entry_to_pte(swp) ((pte_t) { (swp).val })
+/*
+ * It is an error for the kernel to have more swap files than we can
+ * encode in the PTEs. This ensures that we know when MAX_SWAPFILES
+ * is increased beyond what we presently support.
+ */
+#define MAX_SWAPFILES_CHECK() BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > __SWP_TYPE_BITS)
+
/* Needs to be defined here and not in linux/mm.h, as it is arch dependent */
/* FIXME: this is not correct */
#define kern_addr_valid(addr) (1)