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

On 17/07/2023 14:01, David Hildenbrand wrote:

On 14.07.23 18:17, Ryan Roberts wrote:

quoted

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,

What is the reason to have this into a separate patch? That should simply be
squashed into the actual user -- patch #3.

There was a lot more in this at v1 IIRC, so made more sense as standalone. I
agree it can be squashed into the next patch now. Will do for next version.

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