[ARM] pgtable: swp pte layout documentation, definitions, and check

Document the layout of our swp PTE entries, adding definitions for
the bit masks/shifts/sizes, and implement MAX_SWAPFILES_CHECK()
such that we fail to build if we are unable to properly encode the
swp type field.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/pgtable.h

@@ -384,16 +384,36 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 
 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:
  *
- * We support up to 32GB of swap on 4k machines
+ *   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
+ *
+ * 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)