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>

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index d4878ddba87acdd4ceb927ce71d8c015b5085fc8..994c409553150eac353bc32ba6bcc5bc4bbdf465 100644 (file)
--- 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);