Paul Moore [off-list ref] wrote:

I guess my question is this: for inodes inside the superblock, does
their superblock pointer point to the submount's superblock, or the
parent filesystem's superblock?

They have to point to the submount superblock.  Too many things would break, I
think, if inode->i_sb pointed to the wrong place.  As far as the VFS is
concerned, apart from the way it is mounted, it's a perfectly normal
superblock.

David