Re: when should the client request a directory delegation?

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2024-02-07 at 14:21 +0000, Trond Myklebust wrote:
> On Wed, 2024-02-07 at 08:34 -0500, Jeff Layton wrote:
> > I've started work on a patchset to add support for directory
> > delegations
> > to the Linux kernel client and server. It's still too rough to post
> > at
> > this point, and for now, I'm just cobbling in a ioctl to drive it.
> > 
> > As I started working on some of the client bits, however, I realized
> > that I don't really have a clear picture as to when the client should
> > request a delegation on a directory. It seems like there are a lot of
> > things we could do:
> > 
> > One idea: request one on an initial directory readdir. So maybe when
> > the
> > offset is 0 and we don't have a dir delegation already, do:
> > 
> > 	PUTFH:GET_DIR_DELEGATION:READDIR
> > 
> > Or, maybe just do it on any readdir when we haven't requested one in
> > a
> > little while?
> > 
> > We could also do one on every lookup, when we expect that the result
> > will be a directory. I'm not sure if LOOKUP_DIRECTORY would be a
> > sufficient indicator or if we'd need the vfs to indicate that with a
> > new
> > flag.
> > 
> > Would we also want to request one after a mkdir?
> > 
> > 	PUTFH:CREATE:GET_DIR_DELEGATION:GETFH:GET_DIR_DELEGATION:...
> > 
> > Assuming we can get this all working, what should drive the client to
> > issues GET_DIR_DELEGATION ops?
> 
> As far as I'm concerned, the main case to be made for directory
> delegations in the client is for reducing the number of revalidations
> on said directory, particularly during path lookups.
> i.e. the goal is to eliminate the need to constantly poll the directory
> change attribute, and to eliminate the need to constantly revalidate
> the dentries (and negative dentries!) contained in the directory after
> a change.
> 
> Perhaps that means we should focus on adding a request for a directory
> delegation to the function nfs_lookup_revalidate() since that would
> seem to indicate that we're going through the same directory multiple
> times? The other call site to consider would be nfs_check_verifier().
> 

Sounds good. I'm not nearly at the point where I'm modifying client
behavior yet, but I'll plan to try wiring it up in the revalidate
codepaths first.
-- 
Jeff Layton <jlayton@kernel.org>