[PATCH 4/4] btrfs: show temp_fsid feature in sysfs

[Anand Jain <anand.jain@oracle.com>]

This adds sysfs objects to indicate temp_fsid feature support and
its status.

  /sys/fs/btrfs/features/temp_fsid
  /sys/fs/btrfs/<UUID>/temp_fsid

  For example:

     Consider two cloned and mounted devices.

	$ blkid /dev/sdc[1-2]
	/dev/sdc1: UUID="509ad44b-ad2a-4a8a-bc8d-fe69db7220d5" ..
	/dev/sdc2: UUID="509ad44b-ad2a-4a8a-bc8d-fe69db7220d5" ..

     One gets actual fsid, and the other gets the temp_fsid when
     mounted.

	$ btrfs filesystem show -m
	Label: none  uuid: 509ad44b-ad2a-4a8a-bc8d-fe69db7220d5
		Total devices 1 FS bytes used 54.14MiB
		devid    1 size 300.00MiB used 144.00MiB path /dev/sdc1

	Label: none  uuid: 33bad74e-c91b-43a5-aef8-b3cab97ae63a
		Total devices 1 FS bytes used 54.14MiB
		devid    1 size 300.00MiB used 144.00MiB path /dev/sdc2

     Their sysfs as below.

	$ cat /sys/fs/btrfs/features/temp_fsid
	0

	$ cat /sys/fs/btrfs/509ad44b-ad2a-4a8a-bc8d-fe69db7220d5/temp_fsid
	0

	$ cat /sys/fs/btrfs/33bad74e-c91b-43a5-aef8-b3cab97ae63a/temp_fsid
	1

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/sysfs.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index e07be193323a..7f9a4790e013 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -425,6 +425,15 @@ static ssize_t acl_show(struct kobject *kobj, struct kobj_attribute *a, char *bu
 }
 BTRFS_ATTR(static_feature, acl, acl_show);
 
+static ssize_t temp_fsid_supported_show(struct kobject *kobj,
+					struct kobj_attribute *a, char *buf)
+{
+	int ret = 0;
+
+	return sysfs_emit(buf, "%d\n", ret);
+}
+BTRFS_ATTR(static_feature, temp_fsid, temp_fsid_supported_show);
+
 /*
  * Features which only depend on kernel version.
  *
@@ -438,6 +447,7 @@ static struct attribute *btrfs_supported_static_feature_attrs[] = {
 	BTRFS_ATTR_PTR(static_feature, send_stream_version),
 	BTRFS_ATTR_PTR(static_feature, supported_rescue_options),
 	BTRFS_ATTR_PTR(static_feature, supported_sectorsizes),
+	BTRFS_ATTR_PTR(static_feature, temp_fsid),
 	NULL
 };
 
@@ -1205,6 +1215,15 @@ static ssize_t btrfs_generation_show(struct kobject *kobj,
 }
 BTRFS_ATTR(, generation, btrfs_generation_show);
 
+static ssize_t btrfs_temp_fsid_show(struct kobject *kobj,
+				    struct kobj_attribute *a, char *buf)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(kobj);
+
+	return sysfs_emit(buf, "%d\n", fs_info->fs_devices->temp_fsid);
+}
+BTRFS_ATTR(, temp_fsid, btrfs_temp_fsid_show);
+
 static const char * const btrfs_read_policy_name[] = { "pid" };
 
 static ssize_t btrfs_read_policy_show(struct kobject *kobj,
@@ -1307,6 +1326,7 @@ static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, read_policy),
 	BTRFS_ATTR_PTR(, bg_reclaim_threshold),
 	BTRFS_ATTR_PTR(, commit_stats),
+	BTRFS_ATTR_PTR(, temp_fsid),
 	NULL,
 };
 
-- 
2.38.1