Re: when should the client request a directory delegation?

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2024-02-07 at 15:02 +0000, Trond Myklebust wrote:
> On Wed, 2024-02-07 at 09:56 -0500, Jeff Layton wrote:
> > 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.
> 
> Understood, but you appeared to be asking which COMPOUNDs to modify. I
> think a discussion around the goals of introducing directory
> delegations needs to inform that choice.
> 

The goal is to improve lookup performance, and reduce the GETATTR load
on directories. What sort of userland behavior should trigger the client
to get a dir_deleg? Trying to improve repeated lookups in the same dir
does seem like the biggest initial win for this.

Plumbing one into readdir might also be reasonable. Someone doing a
readdir is probably interested in the contents. If we get a deleg first,
then that might allow the client to mark the directory "complete" once
it has read every entry.
-- 
Jeff Layton <jlayton@kernel.org>