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