loongarch/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, also mask the type in mk_swap_pte().

Note that this bit does not conflict with swap PMDs and could also be used
in swap PMD context later.

Link: https://lkml.kernel.org/r/20230113171026.582290-9-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/loongarch/include/asm/pgtable-bits.h

@@ -20,6 +20,7 @@
 #define	_PAGE_SPECIAL_SHIFT	11
 #define	_PAGE_HGLOBAL_SHIFT	12 /* HGlobal is a PMD bit */
 #define	_PAGE_PFN_SHIFT		12
+#define	_PAGE_SWP_EXCLUSIVE_SHIFT 23
 #define	_PAGE_PFN_END_SHIFT	48
 #define	_PAGE_NO_READ_SHIFT	61
 #define	_PAGE_NO_EXEC_SHIFT	62
@@ -33,6 +34,9 @@
 #define _PAGE_PROTNONE		(_ULCAST_(1) << _PAGE_PROTNONE_SHIFT)
 #define _PAGE_SPECIAL		(_ULCAST_(1) << _PAGE_SPECIAL_SHIFT)
 
+/* We borrow bit 23 to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE	(_ULCAST_(1) << _PAGE_SWP_EXCLUSIVE_SHIFT)
+
 /* Used by TLB hardware (placed in EntryLo*) */
 #define _PAGE_VALID		(_ULCAST_(1) << _PAGE_VALID_SHIFT)
 #define _PAGE_DIRTY		(_ULCAST_(1) << _PAGE_DIRTY_SHIFT)

arch/loongarch/include/asm/pgtable.h

@@ -249,13 +249,26 @@ extern void pud_init(void *addr);
 extern void pmd_init(void *addr);
 
 /*
- * Non-present pages:  high 40 bits are offset, next 8 bits type,
- * low 16 bits zero.
+ * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
+ * are !pte_none() && !pte_present().
+ *
+ * Format of swap PTEs:
+ *
+ *   6 6 6 6 5 5 5 5 5 5 5 5 5 5 4 4 4 4 4 4 4 4 4 4 3 3 3 3 3 3 3 3
+ *   3 2 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
+ *   <--------------------------- offset ---------------------------
+ *
+ *   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
+ *   --------------> E <--- type ---> <---------- zeroes ---------->
+ *
+ *   E is the exclusive marker that is not stored in swap entries.
+ *   The zero'ed bits include _PAGE_PRESENT and _PAGE_PROTNONE.
  */
 static inline pte_t mk_swap_pte(unsigned long type, unsigned long offset)
-{ pte_t pte; pte_val(pte) = (type << 16) | (offset << 24); return pte; }
+{ pte_t pte; pte_val(pte) = ((type & 0x7f) << 16) | (offset << 24); return pte; }
 
-#define __swp_type(x)		(((x).val >> 16) & 0xff)
+#define __swp_type(x)		(((x).val >> 16) & 0x7f)
 #define __swp_offset(x)		((x).val >> 24)
 #define __swp_entry(type, offset) ((swp_entry_t) { pte_val(mk_swap_pte((type), (offset))) })
 #define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val(pte) })
@@ -263,6 +276,24 @@ static inline pte_t mk_swap_pte(unsigned long type, unsigned long offset)
 #define __pmd_to_swp_entry(pmd) ((swp_entry_t) { pmd_val(pmd) })
 #define __swp_entry_to_pmd(x)	((pmd_t) { (x).val | _PAGE_HUGE })
 
+#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;
+}
+
 extern void paging_init(void);
 
 #define pte_none(pte)		(!(pte_val(pte) & ~_PAGE_GLOBAL))