linux-kernel - Re: [NFS] rpc.mountd crashes when extensively using netgroups

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Date:	Thu, 2 Aug 2007 12:05:36 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	nfs@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	Stefan Walter <stefan.walter@....ethz.ch>
Subject: Re: [NFS] rpc.mountd crashes when extensively using netgroups

On Thu, Aug 02, 2007 at 11:32:55AM -0400, Jeff Layton wrote:
> On Tue, 31 Jul 2007 10:48:24 -0400
> "J. Bruce Fields" <bfields@...ldses.org> wrote:
> > That's an interesting problem.  Thanks for the report!
> > 
> > I don't believe that long comma-delimited string actually has any
> > meaning to the kernel--as far as the kernel is concerned, it's just an
> > opaque object that will be passed back to mountd later (along with a
> > path name) to get export options.
> > 
> > So I suppose that string could be replaced by a hash, or maybe even just
> > by the ip address of the particular host--the disadvantage to the latter
> > being that it would require the kernel to keep a separate export for
> > each client address.
> 
> I started having a look at this today. The original patches that I
> proposed to clean up the rmtab a few months ago also eliminated this
> comma-delimited string. Neil had valid objections to it at the time,
> but if we switched to using the IP address as a cache key like Bruce
> describes then doing that becomes more reasonable.
> 
> The only downside I see is the one Bruce points out -- the size of
> the kernel export cache would increase. I don't have a feel for whether
> this is a show stopper, however.

Yeah, there might be some risk of solving that problem at the expense of
people with tons of clients all matching *.example.com.  The actual
export objects are actually pretty small--68 bytes, last I checked?
You'd also need two upcalls to mountd per client (for ip address and
export, as opposed to just ip address).  I don't know what other costs
there might be.

What about the idea of hashing the comma-delimited list and passing
that?  You'd want hash large enough to be collision-free--might as well
use some cryptographic hash, I guess, though it may be mild overkill.
Is there any reason why that wouldn't work?

Could you remind me what problems you were trying to fix with your rmtab
cleanup?

--b.
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