Re: [PATCH] btrfs: allow mounting btrfs subvolumes with different ro/rw options

[Harald Hoyer <harald@redhat.com>]

On 06/18/2013 05:29 PM, harald@redhat.com wrote:
> From: Harald Hoyer <harald@redhat.com>
> 
> Given the following /etc/fstab entries:
> 
> /dev/sda3 /mnt/foo btrfs subvol=foo,ro 0
> /dev/sda3 /mnt/bar btrfs subvol=bar,rw 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 /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
> ---
>  fs/btrfs/super.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index f0857e0..80ed3e2 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -64,6 +64,8 @@
>  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";
> @@ -1036,6 +1038,26 @@ 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);
>  
> 

Any comments?