[PATCH v3 01/39] fs: introduce a ->s_cdev field into struct super_block

[Christoph Hellwig]

On Tue, Sep 15, 2015 at 05:01:56PM +0800, Dongsheng Yang wrote:
> There are some filesystems are running on char devs, such
> as ubifs. So we need a field in super_block to hold a
> reference to the char device.

We only care about the block device to get a device number to return
in st.st_dev.  And while filesystem literally run on top of a block
device that is not the case of a character device - you might use a
chardev to identify a device to mount but it'll never use the actual
char device.

So please don't bloat the superblock with this.