md: Add new_level sysfs interface

Now reshape supports two ways: with backup file or without backup file.
For the situation without backup file, it needs to change data offset.
It doesn't need systemd service mdadm-grow-continue. So it can finish
the reshape job in one process environment. It can know the new level
from mdadm --grow command and can change to new level after reshape
finishes.

For the situation with backup file, it needs systemd service
mdadm-grow-continue to monitor reshape progress. So there are two process
envolved. One is mdadm --grow command whick kicks off reshape and wakes
up mdadm-grow-continue service. The second process is the service, which
doesn't know the new level from the first process.

In kernel space mddev->new_level is used to record the new level when
doing reshape. This patch adds a new interface to help mdadm update
new_level and sync it to metadata. Then mdadm-grow-continue can read the
right new_level.

Commit log revised by Song Liu. Please refer to the link for more details.
Signed-off-by: Xiao Ni <xni@redhat.com>
Link: https://lore.kernel.org/r/20240904235453.99120-1-xni@redhat.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/md.c

@@ -4052,6 +4052,34 @@ level_store(struct mddev *mddev, const char *buf, size_t len)
 static struct md_sysfs_entry md_level =
 __ATTR(level, S_IRUGO|S_IWUSR, level_show, level_store);
 
+static ssize_t
+new_level_show(struct mddev *mddev, char *page)
+{
+	return sprintf(page, "%d\n", mddev->new_level);
+}
+
+static ssize_t
+new_level_store(struct mddev *mddev, const char *buf, size_t len)
+{
+	unsigned int n;
+	int err;
+
+	err = kstrtouint(buf, 10, &n);
+	if (err < 0)
+		return err;
+	err = mddev_lock(mddev);
+	if (err)
+		return err;
+
+	mddev->new_level = n;
+	md_update_sb(mddev, 1);
+
+	mddev_unlock(mddev);
+	return len;
+}
+static struct md_sysfs_entry md_new_level =
+__ATTR(new_level, 0664, new_level_show, new_level_store);
+
 static ssize_t
 layout_show(struct mddev *mddev, char *page)
 {
@@ -5583,6 +5611,7 @@ __ATTR(serialize_policy, S_IRUGO | S_IWUSR, serialize_policy_show,
 
 static struct attribute *md_default_attrs[] = {
 	&md_level.attr,
+	&md_new_level.attr,
 	&md_layout.attr,
 	&md_raid_disks.attr,
 	&md_uuid.attr,