mm: add debugfs tunable for fault_around_order

Let's allow people to tweak faultaround at runtime.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Ning Qu <quning@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -60,6 +60,7 @@
 #include <linux/migrate.h>
 #include <linux/string.h>
 #include <linux/dma-debug.h>
+#include <linux/debugfs.h>
 
 #include <asm/io.h>
 #include <asm/pgalloc.h>
@@ -3382,8 +3383,63 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 }
 
 #define FAULT_AROUND_ORDER 4
-#define FAULT_AROUND_PAGES (1UL << FAULT_AROUND_ORDER)
-#define FAULT_AROUND_MASK ~((1UL << (PAGE_SHIFT + FAULT_AROUND_ORDER)) - 1)
+
+#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;
+	return 0;
+}
+
+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;
+	return 0;
+}
+DEFINE_SIMPLE_ATTRIBUTE(fault_around_order_fops,
+		fault_around_order_get, fault_around_order_set, "%llu\n");
+
+static int __init fault_around_debugfs(void)
+{
+	void *ret;
+
+	ret = debugfs_create_file("fault_around_order",	0644, NULL, NULL,
+			&fault_around_order_fops);
+	if (!ret)
+		pr_warn("Failed to create fault_around_order in debugfs");
+	return 0;
+}
+late_initcall(fault_around_debugfs);
+
+static inline unsigned long fault_around_pages(void)
+{
+	return 1UL << fault_around_order;
+}
+
+static inline unsigned long fault_around_mask(void)
+{
+	return ~((1UL << (PAGE_SHIFT + fault_around_order)) - 1);
+}
+#else
+static inline unsigned long fault_around_pages(void)
+{
+	unsigned long nr_pages;
+
+	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);
+}
+#endif
 
 static void do_fault_around(struct vm_area_struct *vma, unsigned long address,
 		pte_t *pte, pgoff_t pgoff, unsigned int flags)
@@ -3393,21 +3449,19 @@ static void do_fault_around(struct vm_area_struct *vma, unsigned long address,
 	struct vm_fault vmf;
 	int off;
 
-	BUILD_BUG_ON(FAULT_AROUND_PAGES > PTRS_PER_PTE);
-
-	start_addr = max(address & FAULT_AROUND_MASK, vma->vm_start);
+	start_addr = max(address & fault_around_mask(), vma->vm_start);
 	off = ((address - start_addr) >> PAGE_SHIFT) & (PTRS_PER_PTE - 1);
 	pte -= off;
 	pgoff -= off;
 
 	/*
 	 *  max_pgoff is either end of page table or end of vma
-	 *  or FAULT_AROUND_PAGES from pgoff, depending what is neast.
+	 *  or fault_around_pages() from pgoff, depending what is neast.
 	 */
 	max_pgoff = pgoff - ((start_addr >> PAGE_SHIFT) & (PTRS_PER_PTE - 1)) +
 		PTRS_PER_PTE - 1;
 	max_pgoff = min3(max_pgoff, vma_pages(vma) + vma->vm_pgoff - 1,
-			pgoff + FAULT_AROUND_PAGES - 1);
+			pgoff + fault_around_pages() - 1);
 
 	/* Check if it makes any sense to call ->map_pages */
 	while (!pte_none(*pte)) {