Implement a sysfs directory and a file under the directory that will be
used for setting DAMON probe preps.  Implement only preparatory data
structures and functions.  The code is not really generating the
directories at the moment.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 376c6ecbd4bf3..7b971bfd1169f 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -750,6 +750,74 @@ static const struct kobj_type damon_sysfs_intervals_ktype = {
 	.default_groups = damon_sysfs_intervals_groups,
 };
 
+/*
+ * preps directory
+ */
+
+struct damon_sysfs_preps {
+	struct kobject kobj;
+	int nr;
+};
+
+static int damon_sysfs_preps_add_dirs(struct damon_sysfs_preps *preps,
+		int nr_preps)
+{
+	preps->nr = nr_preps;
+	return 0;
+}
+
+static ssize_t nr_preps_show(struct kobject *kobj, struct kobj_attribute *attr,
+		char *buf)
+{
+	struct damon_sysfs_preps *preps = container_of(kobj,
+			struct damon_sysfs_preps, kobj);
+
+	return sysfs_emit(buf, "%d\n", preps->nr);
+}
+
+static ssize_t nr_preps_store(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	struct damon_sysfs_preps *preps;
+	int nr, err = kstrtoint(buf, 0, &nr);
+
+	if (err)
+		return err;
+	if (nr < 0)
+		return -EINVAL;
+
+	preps = container_of(kobj, struct damon_sysfs_preps, kobj);
+
+	if (!mutex_trylock(&damon_sysfs_lock))
+		return -EBUSY;
+	err = damon_sysfs_preps_add_dirs(preps, nr);
+	mutex_unlock(&damon_sysfs_lock);
+	if (err)
+		return err;
+
+	return count;
+}
+
+static void damon_sysfs_preps_release(struct kobject *kobj)
+{
+	kfree(container_of(kobj, struct damon_sysfs_preps, kobj));
+}
+
+static struct kobj_attribute damon_sysfs_preps_nr_attr =
+		__ATTR_RW_MODE(nr_preps, 0600);
+
+static struct attribute *damon_sysfs_preps_attrs[] = {
+	&damon_sysfs_preps_nr_attr.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(damon_sysfs_preps);
+
+static const struct kobj_type damon_sysfs_preps_ktype = {
+	.release = damon_sysfs_preps_release,
+	.sysfs_ops = &kobj_sysfs_ops,
+	.default_groups = damon_sysfs_preps_groups,
+};
+
 /*
  * filter directory
  */
-- 
2.47.3