[PATCH mm-unstable RFC 20/26] sh/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

David Hildenbrand
2022-12-06T15:08:09Z

Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE by using bit 6 in the PTE,
reducing the swap type in the !CONFIG_X2TLB case to 5 bits. Generic MM
currently only uses 5 bits for the type (MAX_SWAPFILES_SHIFT), so the
stolen bit is effectively unused.

Interrestingly, the swap type in the !CONFIG_X2TLB case could currently
overlap with the _PAGE_PRESENT bit, because there is a sneaky shift by 1 in
__pte_to_swp_entry() and __swp_entry_to_pte(). Bit 0-7 in the architecture
specific swap PTE would get shifted to bit 1-8 in the PTE. As generic MM
uses 5 bits only, this didn't matter so far.

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

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
arch/sh/include/asm/pgtable_32.h | 54 +++++++++++++++++++++++++-------
1 file changed, 42 insertions(+), 12 deletions(-)

diff --git a/arch/sh/include/asm/pgtable_32.h b/arch/sh/include/asm/pgtable_32.h
index d0240decacca..090940aadbcc 100644
--- a/arch/sh/include/asm/pgtable_32.h
+++ b/arch/sh/include/asm/pgtable_32.h
@@ -423,40 +423,70 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
#endif

/*
- * Encode and de-code a swap entry
+ * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
+ * are !pte_none() && !pte_present().
*
* Constraints:
* _PAGE_PRESENT at bit 8
* _PAGE_PROTNONE at bit 9
*
- * For the normal case, we encode the swap type into bits 0:7 and the
- * swap offset into bits 10:30. For the 64-bit PTE case, we keep the
- * preserved bits in the low 32-bits and use the upper 32 as the swap
- * offset (along with a 5-bit type), following the same approach as x86
- * PAE. This keeps the logic quite simple.
+ * For the normal case, we encode the swap type and offset into the swap PTE
+ * such that bits 8 and 9 stay zero. For the 64-bit PTE case, we use the
+ * upper 32 for the swap offset and swap type, following the same approach as
+ * x86 PAE. This keeps the logic quite simple.
*
* As is evident by the Alpha code, if we ever get a 64-bit unsigned
* long (swp_entry_t) to match up with the 64-bit PTEs, this all becomes
* much cleaner..
- *
- * NOTE: We should set ZEROs at the position of _PAGE_PRESENT
- * and _PAGE_PROTNONE bits
*/
+
#ifdef CONFIG_X2TLB
+/*
+ * 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 ----------------------> < type ->
+ *
+ * 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
+ * <------------------- zeroes --------------------> E 0 0 0 0 0 0
+ */
#define __swp_type(x) ((x).val & 0x1f)
#define __swp_offset(x) ((x).val >> 5)
-#define __swp_entry(type, offset) ((swp_entry_t){ (type) | (offset) << 5})
+#define __swp_entry(type, offset) ((swp_entry_t){ ((type) & 0x1f) | (offset) << 5})
#define __pte_to_swp_entry(pte) ((swp_entry_t){ (pte).pte_high })
#define __swp_entry_to_pte(x) ((pte_t){ 0, (x).val })

#else
-#define __swp_type(x) ((x).val & 0xff)
+/*
+ * 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 ----------------> 0 0 0 0 E < type -> 0
+ *
+ * E is the exclusive marker that is not stored in swap entries.
+ */
+#define __swp_type(x) ((x).val & 0x1f)
#define __swp_offset(x) ((x).val >> 10)
-#define __swp_entry(type, offset) ((swp_entry_t){(type) | (offset) <<10})
+#define __swp_entry(type, offset) ((swp_entry_t){((type) & 0x1f) | (offset) <<10})

#define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val(pte) >> 1 })
#define __swp_entry_to_pte(x) ((pte_t) { (x).val << 1 })
#endif

+/* In both cases, we borrow bit 6 to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE _PAGE_USER
+
+#define __HAVE_ARCH_PTE_SWP_EXCLUSIVE
+static inline int pte_swp_exclusive(pte_t pte)
+{
+ return pte.pte_low & _PAGE_SWP_EXCLUSIVE;
+}
+
+PTE_BIT_FUNC(low, swp_mkexclusive, |= _PAGE_SWP_EXCLUSIVE);
+PTE_BIT_FUNC(low, swp_clear_exclusive, &= ~_PAGE_SWP_EXCLUSIVE);
+
#endif /* __ASSEMBLY__ */
#endif /* __ASM_SH_PGTABLE_32_H */