[PATCH v3 1/9] mm: introduce hw_pte_t for PTE table storage

[Muhammad Usama Anjum <usama.anjum@arm.com>]

pte_t is used both for software PTE values and for entries stored in a PTE
table, so pte_t * does not distinguish a pointer to a software PTE
value from a pointer to table storage.

Introduce hw_pte_t as the generic name for a PTE table element. Define it
as a macro alias of pte_t by default. When an architecture selects
ARCH_HAS_HW_PTE_T, define it as a structure containing a pte_t instead.
This preserves the representation while allowing converted architectures
to enforce the distinction at compile time.

Name the generic wrapper structure __hw_pte_t so architectures can
forward-declare it when pgtable_t must be defined before the generic
hw_pte_t typedef is visible. This avoids header-order dependencies.

Keep the C type definitions behind an __ASSEMBLY__ check because
architecture assembly sources can include this header indirectly. Include
asm/page.h so consumers such as linux/vmalloc.h retain the page definitions
they previously obtained from that header.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
---
Changes since v1:
- Name the generic wrapper structure __hw_pte_t so architectures can
  forward-declare it, and explain why this is required.
- Use software PTE value terminology.

Changes since RFC v1:
- Add the ARCH_HAS_HW_PTE_T opt-in and generic wrapper definition.
- Exclude the C type definitions from assembly sources.
- Update the description for the new opt-in model.
---
 MAINTAINERS                   |  1 +
 include/linux/pgtable_types.h | 17 +++++++++++++++++
 mm/Kconfig                    |  3 +++
 3 files changed, 21 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index 2133aec4a2004..da60a8bdcddb5 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17140,6 +17140,7 @@ F:	include/linux/mmu_notifier.h
 F:	include/linux/pagewalk.h
 F:	include/linux/pgalloc.h
 F:	include/linux/pgtable.h
+F:	include/linux/pgtable_types.h
 F:	include/linux/ptdump.h
 F:	include/linux/vmpressure.h
 F:	include/linux/vmstat.h
diff --git a/include/linux/pgtable_types.h b/include/linux/pgtable_types.h
new file mode 100644
index 0000000000000..07da05d375c2c
--- /dev/null
+++ b/include/linux/pgtable_types.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_PGTABLE_TYPES_H
+#define _LINUX_PGTABLE_TYPES_H
+
+#include <asm/page.h>
+
+#ifndef __ASSEMBLY__
+
+#ifdef CONFIG_ARCH_HAS_HW_PTE_T
+typedef struct __hw_pte_t { pte_t __pte; } hw_pte_t;
+#else
+#define hw_pte_t pte_t
+#endif
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* _LINUX_PGTABLE_TYPES_H */
diff --git a/mm/Kconfig b/mm/Kconfig
index c1ddf59c0d71a..5f462ec6fa5e5 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1312,6 +1312,9 @@ comment "GUP_TEST needs to have DEBUG_FS enabled"
 config GUP_GET_PXX_LOW_HIGH
 	bool
 
+config ARCH_HAS_HW_PTE_T
+	bool
+
 config DMAPOOL_TEST
 	tristate "Enable a module to run time tests on dma_pool"
 	depends on HAS_DMA

-- 
2.47.3