RFC: New VOP to translate vnode to its component name

[Kostik Belousov]

On Wed, Dec 10, 2008 at 02:32:24AM -0500, Joe Marcus Clarke wrote:
> On Tue, 2008-12-09 at 11:22 +0000, Robert Watson wrote:
> > On Mon, 8 Dec 2008, Joe Marcus Clarke wrote:
> > 
> > >> Just to give a general vote of "we need to do something here, whether the 
> > >> details are exactly these or not" -- having better object->path resolution 
> > >> is quite important for audit, as well as if we want to adopt a file system 
> > >> notification services along the lines of Apple's fsevents (which is 
> > >> path-centric and operates from close() events rather than open() events). 
> > >> I don't think we should run in the Linux 'dentry' direction, but having a 
> > >> more robust translation service would be quite valuable.  One comment: I 
> > >> think we should continue to have a KPI which does a sleep-free translation 
> > >> to call, but with weaker semantics than one that is sleepable and can query 
> > >> for more robust reverse lookup.
> > >
> > > Okay, what about a name?
> > 
> > Oh, I do love a good bikeshed.  I'm actually fine with any of these, but 
> > vn_fullpath_cache() sounds good to me.  One of the cases I have in mind is 
> > path-based MAC policies, which will convert from a vnode to a path while 
> > holding the vnode lock -- if something is going to run around locking vnodes 
> > and doing VOP_READDIR's, that is not the time to do it.
> 
> I've duplicated vn_fullpath, vn_fullpath_global, and textvp_fullpath in
> this latest version.  The new (well, old) functions are named *_cache.
> I even updated the vn_fullpath.9 man page.
> 
> http://www.marcuscom.com/downloads/vop_vptocnp_7.diff

Main reason for vn_fullpath_cache() is to not sleep inside the function.
I think this shall be reflected in the man page changes you did.

I do not like having the old code pasted into the vfs_cache.c together
with new implementation. I think that vn_fullpath1 can take a flag
specifying whether to call the vop, and return ENOENT when call is
disabled. This shall give the same effect without code bloat.
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