parisc/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE by using the yet-unused
_PAGE_ACCESSED location in the swap PTE.  Looking at pte_present() and
pte_none() checks, there seems to be no actual reason why we cannot use
it: we only have to make sure we're not using _PAGE_PRESENT.

Reusing this bit avoids having to steal one bit from the swap offset.

Link: https://lkml.kernel.org/r/20230113171026.582290-17-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/parisc/include/asm/pgtable.h

@@ -218,6 +218,9 @@ extern void __update_cache(pte_t pte);
 #define _PAGE_KERNEL_RWX	(_PAGE_KERNEL_EXEC | _PAGE_WRITE)
 #define _PAGE_KERNEL		(_PAGE_KERNEL_RO | _PAGE_WRITE)
 
+/* We borrow bit 23 to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE	_PAGE_ACCESSED
+
 /* The pgd/pmd contains a ptr (in phys addr space); since all pgds/pmds
  * are page-aligned, we don't care about the PAGE_OFFSET bits, except
  * for a few meta-information bits, so we shift the address to be
@@ -394,17 +397,49 @@ extern void paging_init (void);
 
 #define update_mmu_cache(vms,addr,ptep) __update_cache(*ptep)
 
-/* Encode and de-code a swap entry */
-
+/*
+ * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
+ * are !pte_none() && !pte_present().
+ *
+ * Format of swap PTEs (32bit):
+ *
+ *                         1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
+ *   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ *   <---------------- offset -----------------> P E <ofs> < type ->
+ *
+ *   E is the exclusive marker that is not stored in swap entries.
+ *   _PAGE_PRESENT (P) must be 0.
+ *
+ *   For the 64bit version, the offset is extended by 32bit.
+ */
 #define __swp_type(x)                     ((x).val & 0x1f)
 #define __swp_offset(x)                   ( (((x).val >> 6) &  0x7) | \
 					  (((x).val >> 8) & ~0x7) )
-#define __swp_entry(type, offset)         ((swp_entry_t) { (type) | \
+#define __swp_entry(type, offset)         ((swp_entry_t) { \
+					    ((type) & 0x1f) | \
 					    ((offset &  0x7) << 6) | \
 					    ((offset & ~0x7) << 8) })
 #define __pte_to_swp_entry(pte)		((swp_entry_t) { pte_val(pte) })
 #define __swp_entry_to_pte(x)		((pte_t) { (x).val })
 
+#define __HAVE_ARCH_PTE_SWP_EXCLUSIVE
+static inline int pte_swp_exclusive(pte_t pte)
+{
+	return pte_val(pte) & _PAGE_SWP_EXCLUSIVE;
+}
+
+static inline pte_t pte_swp_mkexclusive(pte_t pte)
+{
+	pte_val(pte) |= _PAGE_SWP_EXCLUSIVE;
+	return pte;
+}
+
+static inline pte_t pte_swp_clear_exclusive(pte_t pte)
+{
+	pte_val(pte) &= ~_PAGE_SWP_EXCLUSIVE;
+	return pte;
+}
+
 static inline int ptep_test_and_clear_young(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
 {
 	pte_t pte;