NFS problem

Jim Nance (jlnance@avanticorp.com)
Fri, 1 Nov 1996 22:55:04 -0500 (EST)

Hello Kernel Gurus,
I have found an inconsistency in the handling of NFS mounting
directories over the top of other directories. I am not sure if this shows
up under Linux/x86 or not, but I found it on the alpha, so I am posting
it here. I am also not sure what the correct behavior is, but I am almost
positive that I am not seeing it.

First some background. I wanted to rebuild one of my companies products
under Linux/Alpha. All the files are on AFS, and since there is no AFS for
Linux/Alpha (yet), I am using transarcs AFS to NFS translator, and the amd
automounter. My Linux distribution is Redhat 4.0. One complicating factor
is that because of the AFS->NFS translator, root can not look into the
AFS tree, even though normal users can.

In order to be able to use the standard scripts to build the product,
I needed to be able to write into the following directories:

/afs/rtp/product/verification/build/arch/alpha-dec-linux2.0
/afs/rtp/product/lib/build/arch/alpha-dec-linux2.0

Now the problem is that these directories do not exist. The arch
directories do exist though, and their path after the translator/automounter
is:

/.automounter/triton1/root/afs/rtp/product/verification/build/arch
/.automounter/triton1/root/afs/rtp/product/lib/build/arch

My solution was to create the directory /home/jlnance/arch/alpha-dec-linux2.0,
and to export the arch directory via NFS and mount it on top of the arch
directories in the automounter path. So, as root I tried something like:

archdir=/home/jlnance/arch
autopth=/.automounter/triton1/root/afs/rtp/product
mount -t nfs localhost:$archdir $autopath/verification/build/arch
mount -t nfs localhost:$archdir $autopath/lib/build/arch

Now when you execute these commands an interesting thing happens. It gives
2 warning messages. The first says that the operation is not permitted,
the second says that the mount was done read only. It will mount the first
directory, but not the second. It is also impossible to unmount this
directory.

At this point I thought that perhaps it did not like mounting the same
directory in 2 places, so I made /home/jlnance/arch2/alpha-dec-linux
and tried to mount a different directory on each arch directory. This
made almost no difference. One of the mounts works, the other one does
not, and each command generates 2 warning message. It is not possible to
unmount the directory.

I just realized that the automounter path I mounter over contains a symlink.
I don't know if it matters, but /afs/rtp is a symlink to something like
/afs/rtp.avanticorp.

At this point I decided I did not know enough about what was supposed to
happen to investigate much further. I can repeat these experiments and
take more detailed notes if anyone is interested. I guess the first question
that comes to my mind is whether it should have allowed me to mount
both directories, or forbidden me to mount both of them. Anyone know?

Thanks,

Jim

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