[PATCH] alpha: Add ARCH_HAS_PTE_SPECIAL support

[Matt Turner <mattst88@gmail.com>]

Add _PAGE_SPECIAL using bit 19 (0x80000), which is unused in the Alpha
PTE layout. This enables pte_special()/pte_mkspecial() for marking PTEs
that are not backed by struct page (VDSO mappings, zero page,
io_remap_pfn_range, etc.).

Include _PAGE_SPECIAL in _PAGE_CHG_MASK so it is preserved across
pte_modify() calls.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Matt Turner <mattst88@gmail.com>
---
 arch/alpha/Kconfig               | 1 +
 arch/alpha/include/asm/pgtable.h | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git ./arch/alpha/Kconfig ./arch/alpha/Kconfig
index a5feb98cc088..a29788e588ee 100644
--- ./arch/alpha/Kconfig
+++ ./arch/alpha/Kconfig
@@ -8,6 +8,7 @@ config ALPHA
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_GCOV_PROFILE_ALL
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
+	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_UBSAN
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_MIGHT_HAVE_PC_SERIO
diff --git ./arch/alpha/include/asm/pgtable.h ./arch/alpha/include/asm/pgtable.h
index 849abe2a1f98..832f29340ec6 100644
--- ./arch/alpha/include/asm/pgtable.h
+++ ./arch/alpha/include/asm/pgtable.h
@@ -73,6 +73,7 @@ struct vm_area_struct;
 /* .. and these are ours ... */
 #define _PAGE_DIRTY	0x20000
 #define _PAGE_ACCESSED	0x40000
+#define _PAGE_SPECIAL	0x80000
 
 /* We borrow bit 39 to store the exclusive marker in swap PTEs. */
 #define _PAGE_SWP_EXCLUSIVE	0x8000000000UL
@@ -94,7 +95,7 @@ struct vm_area_struct;
 #define _PFN_MASK	0xFFFFFFFF00000000UL
 
 #define _PAGE_TABLE	(_PAGE_VALID | __DIRTY_BITS | __ACCESS_BITS)
-#define _PAGE_CHG_MASK	(_PFN_MASK | __DIRTY_BITS | __ACCESS_BITS)
+#define _PAGE_CHG_MASK	(_PFN_MASK | __DIRTY_BITS | __ACCESS_BITS | _PAGE_SPECIAL)
 
 /*
  * All the normal masks have the "page accessed" bits on, as any time they are used,
@@ -260,6 +261,8 @@ extern inline pte_t pte_mkold(pte_t pte)	{ pte_val(pte) &= ~(__ACCESS_BITS); ret
 extern inline pte_t pte_mkwrite_novma(pte_t pte){ pte_val(pte) &= ~_PAGE_FOW; return pte; }
 extern inline pte_t pte_mkdirty(pte_t pte)	{ pte_val(pte) |= __DIRTY_BITS; return pte; }
 extern inline pte_t pte_mkyoung(pte_t pte)	{ pte_val(pte) |= __ACCESS_BITS; return pte; }
+extern inline int pte_special(pte_t pte)	{ return pte_val(pte) & _PAGE_SPECIAL; }
+extern inline pte_t pte_mkspecial(pte_t pte)	{ pte_val(pte) |= _PAGE_SPECIAL; return pte; }
 
 /*
  * The smp_rmb() in the following functions are required to order the load of
-- 
2.52.0