[PATCH 0/1] [RFC] New mode for path_lookup (V1)

[Josef Sipek <jsipek@cs.sunysb.edu>]

Stackable file systems frequently need to lookup paths or path components
starting from an arbitrary point in the namespace (identified by a dentry
and a vfsmount).  Currently, such file systems use lookup_one_len, which is
frowned upon [1] as it does not pass the lookup intent along; not passing a
lookup intent, for example, can trigger BUG_ON's when stacking on top of
NFSv4.

The following patch introduces a new mode to path_lookup to allow lookup to
start from an arbitrary point in the namespace.  This approach has been
suggested by Christoph Hellwig at the Linux Storage & Filesystem workshop in
February of this year.

One indicates that the lookup should be relative to a dentry-vfsmnt pair by
using the LOOKUP_ONE flag.  For example, the following snippet of code,
looks up "pathcomponent" in a directory pointed to by
parent_{dentry,vfsmnt}:

nd.dentry = parent_dentry;
nd.mnt = parent_vfsmnt;

err = path_lookup("pathcomponent", LOOKUP_ONE, &nd);
if (!err) {
	/* exits */

	...

	/* once done, release the references */
	path_release(&nd);
} else if (err == -ENOENT) {
	/* doesn't exits */
} else {
	/* other error */
}

VFS functions such as lookup_create can be used on the nameidata structure
to pass the create intent to the file system.

Currently, there is no easy way to pass the LOOKUP_OPEN intent.  The proper
way would be to call open_namei.

We'd like to get comments about what's necessary to make stackable file
systems do lookups right - this includes potential changes to open_namei. 

Josef 'Jeff' Sipek.

[1] http://marc.info/?l=linux-kernel&m=117343337823760&w=2