openrisc/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE by stealing one bit from the
type.  Generic MM currently only uses 5 bits for the type
(MAX_SWAPFILES_SHIFT), so the stolen bit is effectively unused.

While at it, mask the type in __swp_entry().

Link: https://lkml.kernel.org/r/20230113171026.582290-16-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/openrisc/include/asm/pgtable.h

@@ -154,6 +154,9 @@ extern void paging_init(void);
 #define _KERNPG_TABLE \
 	(_PAGE_BASE | _PAGE_SRE | _PAGE_SWE | _PAGE_ACCESSED | _PAGE_DIRTY)
 
+/* We borrow bit 11 to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE	_PAGE_U_SHARED
+
 #define PAGE_NONE       __pgprot(_PAGE_ALL)
 #define PAGE_READONLY   __pgprot(_PAGE_ALL | _PAGE_URE | _PAGE_SRE)
 #define PAGE_READONLY_X __pgprot(_PAGE_ALL | _PAGE_URE | _PAGE_SRE | _PAGE_EXEC)
@@ -385,16 +388,44 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
 
 /* __PHX__ FIXME, SWAP, this probably doesn't work */
 
-/* Encode and de-code a swap entry (must be !pte_none(e) && !pte_present(e)) */
-/* Since the PAGE_PRESENT bit is bit 4, we can use the bits above */
-
-#define __swp_type(x)			(((x).val >> 5) & 0x7f)
+/*
+ * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
+ * are !pte_none() && !pte_present().
+ *
+ * Format of swap PTEs:
+ *
+ *   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 ---------------> E <- type --> 0 0 0 0 0
+ *
+ *   E is the exclusive marker that is not stored in swap entries.
+ *   The zero'ed bits include _PAGE_PRESENT.
+ */
+#define __swp_type(x)			(((x).val >> 5) & 0x3f)
 #define __swp_offset(x)			((x).val >> 12)
 #define __swp_entry(type, offset) \
-	((swp_entry_t) { ((type) << 5) | ((offset) << 12) })
+	((swp_entry_t) { (((type) & 0x3f) << 5) | ((offset) << 12) })
 #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;
+}
+
 typedef pte_t *pte_addr_t;
 
 #endif /* __ASSEMBLY__ */