[PATCH][v2] btrfs: don't show full path of bind mounts in subvol=

[Josef Bacik <josef@toxicpanda.com>]

Chris Murphy reported a problem where rpm ostree will bind mount a bunch
of things for whatever voodoo it's doing.  But when it does this
/proc/mounts shows something like

/dev/mapper/vg0-lv0 /mnt/test btrfs rw,seclabel,relatime,ssd,space_cache,subvolid=256,subvol=/foo 0 0
/dev/mapper/vg0-lv0 /mnt/test/baz btrfs rw,seclabel,relatime,ssd,space_cache,subvolid=256,subvol=/foo/bar 0 0

Despite subvolid=256 being subvol=/roo.  This is because we're just
spitting out the dentry of the mount point, which in the case of bind
mounts is the source path for the mountpoint.  Instead we should spit
out the path to the actual subvol.  Fix this by looking up the name for
the subvolid we have mounted.  With this fix the same test looks like
this

/dev/mapper/vg0-lv0 /mnt/test btrfs rw,seclabel,relatime,ssd,space_cache,subvolid=256,subvol=/foo 0 0
/dev/mapper/vg0-lv0 /mnt/test/baz btrfs rw,seclabel,relatime,ssd,space_cache,subvolid=256,subvol=/foo 0 0

Reported-by: Chris Murphy <chris@colorremedies.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
v1->v2:
- Dropped the RFC.
- Added examples of before and after.

 fs/btrfs/super.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 58f890f73650..0e1647c08610 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1367,6 +1367,7 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
 {
 	struct btrfs_fs_info *info = btrfs_sb(dentry->d_sb);
 	const char *compress_type;
+	const char *subvol_name;
 
 	if (btrfs_test_opt(info, DEGRADED))
 		seq_puts(seq, ",degraded");
@@ -1453,8 +1454,12 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
 		seq_puts(seq, ",ref_verify");
 	seq_printf(seq, ",subvolid=%llu",
 		  BTRFS_I(d_inode(dentry))->root->root_key.objectid);
-	seq_puts(seq, ",subvol=");
-	seq_dentry(seq, dentry, " \t\n\\");
+	subvol_name = btrfs_get_subvol_name_from_objectid(info,
+			BTRFS_I(d_inode(dentry))->root->root_key.objectid);
+	if (subvol_name) {
+		seq_printf(seq, ",subvol=%s", subvol_name);
+		kfree(subvol_name);
+	}
 	return 0;
 }
 
-- 
2.24.1