linux-kernel - Re: If not readdir() then what?

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Date:	Wed, 11 Apr 2007 07:26:43 +1000
From:	Neil Brown <neilb@...e.de>
To:	Theodore Tso <tytso@....edu>
Cc:	Trond Myklebust <trond.myklebust@....uio.no>,
	Jörn Engel <joern@...ybastard.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ulrich Drepper <drepper@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: If not readdir() then what?

On Monday April 9, tytso@....edu wrote:
> On Mon, Apr 09, 2007 at 08:31:37AM -0400, Trond Myklebust wrote:
> > That is a protocol limitation, not a client limitation.
> 
> <Groan>
> 
> And after quickly checking RFC 3010, I see this limitation hasn't been
> lifted in NFSv4.
> 
> Speaking of which, right now ext3 doesn't know whether it's talking to
> an NFSv2 or NFS v3/v4 server, so it's always passing a 32-bit cookie.
> If NFSv3/v4 could use an explicit interface to request a 64-bit
> cookie, instead of just relying on the f_pos field in the file handle,
> we can reduce the chance of hash collisions when reading an ext3
> directory significantly.   
> 

We don't use f_pos (any more), we call llseek.

I think it would make a lot of sense - as Trond suggests - to not pass
O_LARGEFILE to dentry_open for an NFSv2 request.  Then ext3 could
trigger off that and return 64bits of cookie ... and I think nfsd will
actually pass them all back to the client now.  There is a
truncate-to-32bits bug that has only just been fixed.

But if a separate call is wanted, we have the export_operations struct
to put it in.  All we need is a good case an useful specification.

> If there are 2 or 3 directory entries that have a hash collision,
> would the NFS protocol allow the server to juggle things so that those
> 2-3 directory entries with the hash collision are sent back in a
> single readdir RPC reply?  Is it aceptable/legal to have multiple
> entries in the same READDIR reply packet have the same cookie value?

I think Trond has answered this, but I think it is also worth noting
that every entry returned in a READDIR reply includes a cookie, the
NFS client may use any of those cookies in a subsequent READDIR.  One
might hope the client will only ever use the last, but one can never
be sure....

NeilBrown
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