FAILED: patch "[PATCH] Btrfs: allow empty subvol= again" failed to apply to 4.16-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.16-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 37becec95ac31b209eb1c8e096f1093a7db00f32 Mon Sep 17 00:00:00 2001
From: Omar Sandoval <osandov@fb.com>
Date: Mon, 21 May 2018 17:07:19 -0700
Subject: [PATCH] Btrfs: allow empty subvol= again

I got a report that after upgrading to 4.16, someone's filesystems
weren't mounting:

[   23.845852] BTRFS info (device loop0): unrecognized mount option 'subvol='

Before 4.16, this mounted the default subvolume. It turns out that this
empty "subvol=" is actually an application bug, but it was causing the
application to fail, so it's an ABI break if you squint.

The generic parsing code we use for mount options (match_token())
doesn't match an empty string as "%s". Previously, setup_root_args()
removed the "subvol=" string, but the mount path was cleaned up to not
need that. Add a dummy Opt_subvol_empty to fix this.

The simple workaround is to use / or . for the value of 'subvol=' .

Fixes: 312c89fbca06 ("btrfs: cleanup btrfs_mount() using btrfs_mount_root()")
CC: stable@vger.kernel.org # 4.16+
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index c67fafaa2fe7..81107ad49f3a 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -323,6 +323,7 @@ enum {
 	Opt_ssd, Opt_nossd,
 	Opt_ssd_spread, Opt_nossd_spread,
 	Opt_subvol,
+	Opt_subvol_empty,
 	Opt_subvolid,
 	Opt_thread_pool,
 	Opt_treelog, Opt_notreelog,
@@ -388,6 +389,7 @@ static const match_table_t tokens = {
 	{Opt_ssd_spread, "ssd_spread"},
 	{Opt_nossd_spread, "nossd_spread"},
 	{Opt_subvol, "subvol=%s"},
+	{Opt_subvol_empty, "subvol="},
 	{Opt_subvolid, "subvolid=%s"},
 	{Opt_thread_pool, "thread_pool=%u"},
 	{Opt_treelog, "treelog"},
@@ -461,6 +463,7 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 			btrfs_set_opt(info->mount_opt, DEGRADED);
 			break;
 		case Opt_subvol:
+		case Opt_subvol_empty:
 		case Opt_subvolid:
 		case Opt_subvolrootid:
 		case Opt_device: