[PATCH V3 1/2] mm: move FAULT_AROUND_ORDER to arch/

[Madhavan Srinivasan]

Kirill A. Shutemov with 8c6e50b029 commit introduced
vm_ops->map_pages() for mapping easy accessible pages around
fault address in hope to reduce number of minor page faults.

This patch creates infrastructure to modify the FAULT_AROUND_ORDER
value using mm/Kconfig. This will enable architecture maintainers
to decide on suitable FAULT_AROUND_ORDER value based on
performance data for that architecture. Patch also defaults
FAULT_AROUND_ORDER Kconfig element to 4.

Signed-off-by: Madhavan Srinivasan <maddy at linux.vnet.ibm.com>
---
 mm/Kconfig  |    8 ++++++++
 mm/memory.c |   11 ++++-------
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index ebe5880..c7fc4f1 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -176,6 +176,14 @@ config MOVABLE_NODE
 config HAVE_BOOTMEM_INFO_NODE
 	def_bool n
 
+#
+# Fault around order is a control knob to decide the fault around pages.
+# Default value is set to 4 , but the arch can override it as desired.
+#
+config FAULT_AROUND_ORDER
+	int
+	default	4
+
 # eventually, we can have this option just 'select SPARSEMEM'
 config MEMORY_HOTPLUG
 	bool "Allow for memory hot-add"
diff --git a/mm/memory.c b/mm/memory.c
index d0f0bef..457436d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3382,11 +3382,9 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 	update_mmu_cache(vma, address, pte);
 }
 
-#define FAULT_AROUND_ORDER 4
+unsigned int fault_around_order = CONFIG_FAULT_AROUND_ORDER;
 
 #ifdef CONFIG_DEBUG_FS
-static unsigned int fault_around_order = FAULT_AROUND_ORDER;
-
 static int fault_around_order_get(void *data, u64 *val)
 {
 	*val = fault_around_order;
@@ -3395,7 +3393,6 @@ static int fault_around_order_get(void *data, u64 *val)
 
 static int fault_around_order_set(void *data, u64 val)
 {
-	BUILD_BUG_ON((1UL << FAULT_AROUND_ORDER) > PTRS_PER_PTE);
 	if (1UL << val > PTRS_PER_PTE)
 		return -EINVAL;
 	fault_around_order = val;
@@ -3430,14 +3427,14 @@ static inline unsigned long fault_around_pages(void)
 {
 	unsigned long nr_pages;
 
-	nr_pages = 1UL << FAULT_AROUND_ORDER;
+	nr_pages = 1UL << fault_around_order;
 	BUILD_BUG_ON(nr_pages > PTRS_PER_PTE);
 	return nr_pages;
 }
 
 static inline unsigned long fault_around_mask(void)
 {
-	return ~((1UL << (PAGE_SHIFT + FAULT_AROUND_ORDER)) - 1);
+	return ~((1UL << (PAGE_SHIFT + fault_around_order)) - 1);
 }
 #endif
 
@@ -3495,7 +3492,7 @@ static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * if page by the offset is not ready to be mapped (cold cache or
 	 * something).
 	 */
-	if (vma->vm_ops->map_pages) {
+	if ((vma->vm_ops->map_pages) && (fault_around_order)) {
 		pte = pte_offset_map_lock(mm, pmd, address, &ptl);
 		do_fault_around(vma, address, pte, pgoff, flags);
 		if (!pte_same(*pte, orig_pte))
-- 
1.7.10.4