linux-ext4 - RE: regressions due to 64-bit ext4 directory cookies

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Date:	Thu, 14 Feb 2013 03:59:17 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	"Theodore Ts'o" <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"sandeen@...hat.com" <sandeen@...hat.com>,
	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>,
	"gluster-devel@...gnu.org" <gluster-devel@...gnu.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: RE: regressions due to 64-bit ext4 directory cookies

> -----Original Message-----
> From: J. Bruce Fields [mailto:bfields@...ldses.org]
> Sent: Wednesday, February 13, 2013 4:34 PM
> To: Myklebust, Trond
> Cc: Theodore Ts'o; linux-ext4@...r.kernel.org; sandeen@...hat.com;
> Bernd Schubert; gluster-devel@...gnu.org; linux-nfs@...r.kernel.org
> Subject: Re: regressions due to 64-bit ext4 directory cookies
> 
> On Wed, Feb 13, 2013 at 04:43:05PM +0000, Myklebust, Trond wrote:
> > On Wed, 2013-02-13 at 11:20 -0500, J. Bruce Fields wrote:
> > > Oops, probably should have cc'd linux-nfs.
> > >
> > > On Wed, Feb 13, 2013 at 10:36:54AM -0500, Theodore Ts'o wrote:
> > > > The other thing that I'd note is that the readdir cookie has been
> > > > 64-bit since NFSv3, which was released in June ***1995***.  And
> > > > the explicit, stated purpose of making it be a 64-bit value (as
> > > > stated in RFC 1813) was to reduce interoperability problems.  If
> > > > that were the case, are you telling me that Sun (who has
> > > > traditionally been pretty good worrying about interoperability
> > > > concerns, and in fact employed the editors of RFC 1813) didn't get
> > > > this right?  This seems quite.... surprising to me.
> > > >
> > > > I thought this was the whole point of the various NFS
> > > > interoperability testing done at Connectathon, for which Sun was a
> > > > major sponsor?!?  No one noticed?!?
> > >
> > > Beats me.  But it's not necessarily easy to replace clients running
> > > legacy applications, so we're stuck working with the clients we have....
> > >
> > > The linux client does remap the server-provided cookies to small
> > > integers, I believe exactly because older applications had trouble
> > > with servers returning "large" cookies.  So presumably
> > > ext4-exporting-Linux servers aren't the first to do this.
> > >
> > > I don't know which client versions are affected--Connectathon's next
> > > week and I'll talk to people and make sure there's an ext4 export
> > > with this turned on to test against.
> >
> > Actually, one of the main reasons for the Linux client not exporting
> > raw readdir cookies is because the glibc-2 folks in their infinite
> > wisdom declared that telldir()/seekdir() use an off_t. They then went
> > yet one further and decided to declare negative offsets to be illegal
> > so that they could use the negative values internally in their syscall
> wrappers.
> >
> > The POSIX definition has none of the above rubbish
> > (http://pubs.opengroup.org/onlinepubs/009695399/functions/telldir.html
> > ) and so glibc brilliantly saddled Linux with a crippled readdir
> > implementation that is _not_ POSIX compatible.
> >
> > No, I'm not at all bitter...
> 
> Oh, right, I knew I'd forgotten part of the story....
> 
> But then you must have actually been testing against servers that were using
> that 32nd bit?
> 
> I think ext4 actually only uses 31 bits even in the 32-bit case.  And for a server
> that was literally using an offset inside a directory file, that would be a
> colossal directory.
> 
> So I'm wondering how you ran across it.
> 
> Partly just pure curiosity.

IIRC, XFS on IRIX used 0xFFFFF as the readdir eof marker, which caused us to generate an EIO...

Cheers
  Trond
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