riscv/mm: Adjust PAGE_PROT_NONE to comply with THP semantics

This is a preparation for enabling THP migration.
As the commit b65399f6("arm64/mm: Change THP helpers
to comply with generic MM semantics") mentioned, pmd_present()
and pmd_trans_huge() are expected to behave in the following
manner:
-------------------------------------------------------------------------
|	PMD states	|	pmd_present	|	pmd_trans_huge	|
-------------------------------------------------------------------------
|	Mapped		|	Yes		|	Yes		|
-------------------------------------------------------------------------
|	Splitting	|	Yes		|	Yes		|
-------------------------------------------------------------------------
|	Migration/Swap	|	No		|	No		|
-------------------------------------------------------------------------

At present the PROT_NONE bit reuses the READ bit could not comply with
above semantics with two problems:
1. When splitting a PMD THP, PMD is first invalidated with
pmdp_invalidate()->pmd_mkinvalid(), which clears the PRESENT bit
and PROT_NONE bit/READ bit, if the PMD is read-only, then the PAGE_LEAF
property is also cleared, which results in pmd_present() return false.
2. When migrating, the swap entry only clear the PRESENT bit
and PROT_NONE bit/READ bit, the W/X bit may be set, so _PAGE_LEAF may be
true which results in pmd_present() return true.

Solution:
Adjust PROT_NONE bit from READ to GLOBAL bit can satisfy the above rules:
1. GLOBAL bit has no other meanings, not like the R/W/X bit, which is
also relative with _PAGE_LEAF property.
2. GLOBAL bit is at bit 5, making swap entry start from bit 6, bit 0-5
are zero, which means the PRESENT, PROT_NONE, and PAGE_LEAF are
all false, then the pmd_present() and pmd_trans_huge() return false when
in migration/swap.
Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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

@@ -31,7 +31,7 @@
  * _PAGE_PROT_NONE is set on not-present pages (and ignored by the hardware) to
  * distinguish them from swapped out pages
  */
-#define _PAGE_PROT_NONE _PAGE_READ
+#define _PAGE_PROT_NONE _PAGE_GLOBAL
 
 #define _PAGE_PFN_SHIFT 10
 

arch/riscv/include/asm/pgtable.h

@@ -119,7 +119,7 @@
 /* Page protection bits */
 #define _PAGE_BASE	(_PAGE_PRESENT | _PAGE_ACCESSED | _PAGE_USER)
 
-#define PAGE_NONE		__pgprot(_PAGE_PROT_NONE)
+#define PAGE_NONE		__pgprot(_PAGE_PROT_NONE | _PAGE_READ)
 #define PAGE_READ		__pgprot(_PAGE_BASE | _PAGE_READ)
 #define PAGE_WRITE		__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_WRITE)
 #define PAGE_EXEC		__pgprot(_PAGE_BASE | _PAGE_EXEC)
@@ -628,11 +628,12 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
  *
  * Format of swap PTE:
  *	bit            0:	_PAGE_PRESENT (zero)
- *	bit            1:	_PAGE_PROT_NONE (zero)
- *	bits      2 to 6:	swap type
- *	bits 7 to XLEN-1:	swap offset
+ *	bit       1 to 3:       _PAGE_LEAF (zero)
+ *	bit            5:	_PAGE_PROT_NONE (zero)
+ *	bits      6 to 10:	swap type
+ *	bits 10 to XLEN-1:	swap offset
  */
-#define __SWP_TYPE_SHIFT	2
+#define __SWP_TYPE_SHIFT	6
 #define __SWP_TYPE_BITS		5
 #define __SWP_TYPE_MASK		((1UL << __SWP_TYPE_BITS) - 1)
 #define __SWP_OFFSET_SHIFT	(__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)