Implement a new DAMON sysfs file named 'refresh_ms' under each kdamond
directory.  The file will be used as a control knob of automatic refresh
of a few DAMON internal status files.  This commit implements only
minimum file operations, though.  The automatic refresh feature will be
implemented by the following commit.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index cce2c8a296e2..4296dc201f4d 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1155,6 +1155,7 @@ struct damon_sysfs_kdamond {
 	struct kobject kobj;
 	struct damon_sysfs_contexts *contexts;
 	struct damon_ctx *damon_ctx;
+	unsigned int refresh_ms;
 };
 
 static struct damon_sysfs_kdamond *damon_sysfs_kdamond_alloc(void)
@@ -1690,6 +1691,30 @@ static ssize_t pid_show(struct kobject *kobj,
 	return sysfs_emit(buf, "%d\n", pid);
 }
 
+static ssize_t refresh_ms_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
+			struct damon_sysfs_kdamond, kobj);
+
+	return sysfs_emit(buf, "%u\n", kdamond->refresh_ms);
+}
+
+static ssize_t refresh_ms_store(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
+			struct damon_sysfs_kdamond, kobj);
+	unsigned int nr;
+	int err = kstrtouint(buf, 0, &nr);
+
+	if (err)
+		return err;
+
+	kdamond->refresh_ms = nr;
+	return count;
+}
+
 static void damon_sysfs_kdamond_release(struct kobject *kobj)
 {
 	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
@@ -1706,9 +1731,13 @@ static struct kobj_attribute damon_sysfs_kdamond_state_attr =
 static struct kobj_attribute damon_sysfs_kdamond_pid_attr =
 		__ATTR_RO_MODE(pid, 0400);
 
+static struct kobj_attribute damon_sysfs_kdamond_refresh_ms_attr =
+		__ATTR_RW_MODE(refresh_ms, 0600);
+
 static struct attribute *damon_sysfs_kdamond_attrs[] = {
 	&damon_sysfs_kdamond_state_attr.attr,
 	&damon_sysfs_kdamond_pid_attr.attr,
+	&damon_sysfs_kdamond_refresh_ms_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(damon_sysfs_kdamond);
-- 
2.39.5