[PATCH 1/1] mm: move FAULT_AROUND_ORDER to arch/

[Madhavan Srinivasan]

Kirill A. Shutemov with the commit 96bacfe542 introduced
vm_ops->map_pages() for mapping easy accessible pages around
fault address in hope to reduce number of minor page faults.
Based on his workload runs, suggested FAULT_AROUND_ORDER
(knob to control the numbers of pages to map) is 4.

This patch moves the FAULT_AROUND_ORDER macro to arch/ for
architecture maintainers to decide on suitable FAULT_AROUND_ORDER
value based on performance data for that architecture.

Signed-off-by: Madhavan Srinivasan <maddy@xxxxxxxxxxxxxxxxxx>
---
 arch/powerpc/include/asm/pgtable.h |    6 ++++++
 arch/x86/include/asm/pgtable.h     |    5 +++++
 include/asm-generic/pgtable.h      |   10 ++++++++++
 mm/memory.c                        |    2 --
 4 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
index 3ebb188..9fcbd48 100644
--- a/arch/powerpc/include/asm/pgtable.h
+++ b/arch/powerpc/include/asm/pgtable.h
@@ -19,6 +19,12 @@ struct mm_struct;
 #endif
 
 /*
+ * With a few real world workloads that were run,
+ * the performance data showed that a value of 3 is more advantageous.
+ */
+#define FAULT_AROUND_ORDER	3
+
+/*
  * We save the slot number & secondary bit in the second half of the
  * PTE page. We use the 8 bytes per each pte entry.
  */
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 938ef1d..8387a65 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -7,6 +7,11 @@
 #include <asm/pgtable_types.h>
 
 /*
+ * Based on Kirill's test results, fault around order is set to 4
+ */
+#define FAULT_AROUND_ORDER 4
+
+/*
  * Macro to mark a page protection value as UC-
  */
 #define pgprot_noncached(prot)					\
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 1ec08c1..62f7f07 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -7,6 +7,16 @@
 #include <linux/mm_types.h>
 #include <linux/bug.h>
 
+
+/*
+ * Fault around order is a control knob to decide the fault around pages.
+ * Default value is set to 0UL (disabled), but the arch can override it as
+ * desired.
+ */
+#ifndef FAULT_AROUND_ORDER
+#define FAULT_AROUND_ORDER	0UL
+#endif
+
 /*
  * On almost all architectures and configurations, 0 can be used as the
  * upper ceiling to free_pgtables(): on many architectures it has the same
diff --git a/mm/memory.c b/mm/memory.c
index b02c584..fd79ffc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3358,8 +3358,6 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 	update_mmu_cache(vma, address, pte);
 }
 
-#define FAULT_AROUND_ORDER 4
-
 #ifdef CONFIG_DEBUG_FS
 static unsigned int fault_around_order = FAULT_AROUND_ORDER;
 
-- 
1.7.10.4

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