[PATCH v3 2/4] mm: Default implementation of arch_wants_pte_order()

[Ryan Roberts <ryan.roberts@arm.com>]

arch_wants_pte_order() can be overridden by the arch to return the
preferred folio order for pte-mapped memory. This is useful as some
architectures (e.g. arm64) can coalesce TLB entries when the physical
memory is suitably contiguous.

The first user for this hint will be FLEXIBLE_THP, which aims to
allocate large folios for anonymous memory to reduce page faults and
other per-page operation costs.

Here we add the default implementation of the function, used when the
architecture does not define it, which returns -1, implying that the HW
has no preference. In this case, mm will choose it's own default order.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 include/linux/pgtable.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 5063b482e34f..2a1d83775837 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -313,6 +313,19 @@ static inline bool arch_has_hw_pte_young(void)
 }
 #endif
 
+#ifndef arch_wants_pte_order
+/*
+ * Returns preferred folio order for pte-mapped memory. Must be in range [0,
+ * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
+ * to be at least order-2. Negative value implies that the HW has no preference
+ * and mm will choose it's own default order.
+ */
+static inline int arch_wants_pte_order(void)
+{
+	return -1;
+}
+#endif
+
 #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
 static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 				       unsigned long address,
-- 
2.25.1