Re: Is a NULL check missing in nfs_lookup?

[Al Viro <viro@ftp.linux.org.uk>]

On Fri, Jan 05, 2007 at 12:11:17PM -0500, Shaya Potter wrote:
> ok, I guess what I don't understand is when are
> 
> dentry->d_sb->s_root and nd->mnt->mnt_root not equivalent.
> 
> I guess it would be "when crossing a mountpoint on the server", so I
> look at nfs_follow_mountpoint() and basically see that you create a new
> mountpoint and a new nd for that.  And since superblock objects are only
> per "real" file system not per mount point, they will be different.
> 
> I guess the question is, why shouldn't a dentry object know what
> vfsmount it belongs to is?  Can it belong to multiple vfsmount objects?

Yes.  dentry belongs to superblock.  vfsmount refers to a subtree of some
superblock's dentry tree.  There can be any number of vfsmounts refering
to subtrees of the same fs.  Some might refer to the entire tree, some -
to its arbitrary subtrees (possibly as small as a single file).  There
can be any number of vfsmounts refering to any given subtree.

Think what happens when you create a binding.  Or when you clone an entire
namespace.  (Pieces of) the same filesystem might be mounted in a lot
of places.  vfsmount describes a part of unified tree delimited by
mountpoints; if the same fs (or its parts) are seen under several
mountpoints, you get vfsmounts that refer to the same fs instance, i.e.
the same superblock and dentry tree.