On Wed, 2024-02-07 at 14:56 +0000, Trond Myklebust wrote:

On Wed, 2024-02-07 at 14:21 +0000, Trond Myklebust wrote:

quoted

On Wed, 2024-02-07 at 08:34 -0500, Jeff Layton wrote:

quoted

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().

Note: if you disagree with the above argument, and think that improving
that caching READDIR of results is more important, then consider the
whole discussion we had in the thread started by Tigran here:
https://lore.kernel.org/all/CAGue13pe_ZH_Eto-jL3mLjTNGFK26izTarnZdjs3eL82A2Z37w@mail.gmail.com/ (local)

I think you're correct about the main use-case. I certainly don't plan
to try for readdir caching in the initial stages (there's enough to do
just to get to a minimum-viable feature).  Long term though, it might be
possible to do something:

The ceph protocol has had directory caps for a long time. The client can
mark directories as "complete" when it has every dentry in the
directory, and also as "ordered" when they are correctly ordered in the
dcache.

So if it has the right caps, "complete" allows them to satisfy any
lookup in the dir w/o going to the MDS, and "ordered" allows them to
satisfy readdir using dcache_readdir() (also w/o going to the MDS).

It may be possible for us to leverage dir delegations to do something
like that in NFS too.

-- 
Jeff Layton [off-list ref]