[PATCH][RFC] 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/vda4 on /usr type btrfs (ro,relatime,seclabel,space_cache,subvolid=256,subvol=/root/ostree/deploy/fedora/deploy/610b0f9be3141c79f19a65800f89746c70183cc7f14f3cfba29d695d49128075.0/usr)

Despite subvolid=256 being subvol=/root.  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.

Fixes: c8d3fe028f64 ("Btrfs: show subvol= and subvolid= in /proc/mounts")
Reported-by: Chris Murphy <chris@colorremedies.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---

I talked with Omar about this and his other suggestion is we simply don't spit
out subvol=<path> if we're not the actual subvolume dentry.  I'm ok with that
option too, and it avoids a memory alloc for show_options, however it does mean
that we'll stop spitting out subvol= in some cases, which may cause problems?
If we prefer that option I can code that up instead.

 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