btrfs: allow mounting btrfs subvolumes with different ro/rw options

Given the following /etc/fstab entries:

/dev/sda3 /mnt/foo btrfs subvol=foo,ro 0 0
/dev/sda3 /mnt/bar btrfs subvol=bar,rw 0 0

you can't issue:

$ mount /mnt/foo
$ mount /mnt/bar

You would have to do:

$ mount /mnt/foo
$ mount -o remount,rw /mnt/foo
$ mount --bind -o remount,ro /mnt/foo
$ mount /mnt/bar

or

$ mount /mnt/bar
$ mount --rw /mnt/foo
$ mount --bind -o remount,ro /mnt/foo

With this patch you can do

$ mount /mnt/foo
$ mount /mnt/bar

$ cat /proc/self/mountinfo
49 33 0:41 /foo /mnt/foo ro,relatime shared:36 - btrfs /dev/sda3 rw,ssd,space_cache
87 33 0:41 /bar /mnt/bar rw,relatime shared:74 - btrfs /dev/sda3 rw,ssd,space_cache

Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index d4878ddba87a..994c40955315 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -66,6 +66,8 @@
 static const struct super_operations btrfs_super_ops;
 static struct file_system_type btrfs_fs_type;
 
+static int btrfs_remount(struct super_block *sb, int *flags, char *data);
+
 static const char *btrfs_decode_error(int errno)
 {
         char *errstr = "unknown";
@@ -1185,6 +1187,26 @@ static struct dentry *mount_subvol(const char *subvol_name, int flags,
         mnt = vfs_kern_mount(&btrfs_fs_type, flags, device_name,
                              newargs);
         kfree(newargs);
+
+        if (PTR_RET(mnt) == -EBUSY) {
+                if (flags & MS_RDONLY) {
+                        mnt = vfs_kern_mount(&btrfs_fs_type, flags & ~MS_RDONLY, device_name,
+                                             newargs);
+                } else {
+                        int r;
+                        mnt = vfs_kern_mount(&btrfs_fs_type, flags | MS_RDONLY, device_name,
+                                             newargs);
+                        if (IS_ERR(mnt))
+                                return ERR_CAST(mnt);
+
+                        r = btrfs_remount(mnt->mnt_sb, &flags, NULL);
+                        if (r < 0) {
+                                /* FIXME: release vfsmount mnt ??*/
+                                return ERR_PTR(r);
+                        }
+                }
+        }
+
         if (IS_ERR(mnt))
                 return ERR_CAST(mnt);