md: register new md sysfs file 'uuid' read-only

Report the UUID of the MD array in the following format:
xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

This is useful if you don't want to wait for udev to identify array.
And it is also easy for script to monitor it with the format.
Signed-off-by: Sebastian Parschauer <s.parschauer@gmx.de>
[Guoqing: mention the change in md.rst]
Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

Documentation/admin-guide/md.rst

@@ -426,6 +426,10 @@ All md devices contain:
      The accepted values when writing to this file are ``ppl`` and ``resync``,
      used to enable and disable PPL.
 
+  uuid
+     This indicates the UUID of the array in the following format:
+     xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
+
 
 As component devices are added to an md array, they appear in the ``md``
 directory as new directories named::

drivers/md/md.c

@@ -4252,6 +4252,14 @@ raid_disks_store(struct mddev *mddev, const char *buf, size_t len)
 static struct md_sysfs_entry md_raid_disks =
 __ATTR(raid_disks, S_IRUGO|S_IWUSR, raid_disks_show, raid_disks_store);
 
+static ssize_t
+uuid_show(struct mddev *mddev, char *page)
+{
+	return sprintf(page, "%pU\n", mddev->uuid);
+}
+static struct md_sysfs_entry md_uuid =
+__ATTR(uuid, S_IRUGO, uuid_show, NULL);
+
 static ssize_t
 chunk_size_show(struct mddev *mddev, char *page)
 {
@@ -5508,6 +5516,7 @@ static struct attribute *md_default_attrs[] = {
 	&md_level.attr,
 	&md_layout.attr,
 	&md_raid_disks.attr,
+	&md_uuid.attr,
 	&md_chunk_size.attr,
 	&md_size.attr,
 	&md_resync_start.attr,