Arch Linux

NeoZelux

Member

Registered: 2012-10-28

Posts: 45

NFS server unmount error w/ bind-mount

Hello, please let me know if I should change the thread title.

so I'm sharing a folder through NFS between two arch-linux pc's.
Host1 is my desktop, Host2 is my laptop.

• I boot up my computer, rpcbind is started on boot-up

• I launch the server on Host2

• I launch the client on Host1

• on Host2, I do a bind-mount from some directory to the one I'm exporting

• I then unmount it, and it unmounts w/o error

• then I redo the bind-mount

• then, on the client, I mount the server's NFS share to some directory

• then, on the client, I unmount it w/o error

• then, on the server, I try to unmount the bind-mounted directory, but it won't, says it's "busy"

if I restart the server, then I can unmount the bind-mount w/o error, but I believe it should work without me having to restart the server every time
I want to bind a different directory to the exported directory.

server
     

 $ systemctl start nfs-server 

client
     

 $ systemctl start nfs-client.target 

server
     

 $ mount test/ nfs/ --bind 

client

 
 $ mount Host1:/srv/nfs /mnt/nfs -t nfs 
 $ umount /mnt/nfs

server

 $ umount /srv/nfs
 umount: /srv/nfs: target is busy
         (In some cases useful info about processes that
          use the device is found by lsof(8) or fuser(1).)

the output of

 $ lsof | grep /srv/nfs
 $ lsof | grep /srv

did not show anything that had either the substring "nfs" or "rpc" in it...

below is the status of both hosts rpcbind @startup (they were identical)

$ systemctl status rpcbind
    rpcbind.service - RPC bind service
   Loaded: loaded (/usr/lib/systemd/system/rpcbind.service; static)
   Active: active (running) since Sat 2014-12-06 15:51:38 PST; 46s ago
  Process: 262 ExecStart=/usr/bin/rpcbind -w ${RPCBIND_ARGS} (code=exited, status=0/SUCCESS)
 Main PID: 264 (rpcbind)
   CGroup: /system.slice/rpcbind.service
           └─264 /usr/bin/rpcbind -w 

Dec 06 15:51:38 BabaLinux systemd[1]: Started RPC bind service.

below is the status of the nfs-server @ startup then after it was launched

$ systemctl status nfs-server
    nfs-server.service - NFS server and services
   Loaded: loaded (/usr/lib/systemd/system/nfs-server.service; disabled)
   Active: inactive (dead)
$ systemctl status nfs-server
    nfs-server.service - NFS server and services
   Loaded: loaded (/usr/lib/systemd/system/nfs-server.service; disabled)
   Active: active (exited) since Sat 2014-12-06 15:53:44 PST; 21s ago
  Process: 483 ExecStart=/usr/sbin/rpc.nfsd $RPCNFSDARGS (code=exited, status=0/SUCCESS)
  Process: 480 ExecStartPre=/usr/sbin/exportfs -r (code=exited, status=0/SUCCESS)
 Main PID: 483 (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/nfs-server.service

Dec 06 15:53:44 BabaLinux exportfs[480]: exportfs: /etc/exports [1]: Neither 'subtree_check' or 'no_subtree_check' specified for export "NeoZelux:/srv/nfs".
Dec 06 15:53:44 BabaLinux exportfs[480]: Assuming default behaviour ('no_subtree_check').
Dec 06 15:53:44 BabaLinux exportfs[480]: NOTE: this default has changed since nfs-utils version 1.0.x
Dec 06 15:53:44 BabaLinux systemd[1]: Started NFS server and services.

below is the status of the nfs-client @ startup then after it was launched

$ systemctl status nfs-client.target
    nfs-client.target - NFS client services
   Loaded: loaded (/usr/lib/systemd/system/nfs-client.target; disabled)
   Active: inactive (dead)
$ systemctl status nfs-client.target
    nfs-client.target - NFS client services
   Loaded: loaded (/usr/lib/systemd/system/nfs-client.target; disabled)
   Active: active since Sat 2014-12-06 15:54:18 PST; 4s ago

Dec 06 15:54:18 NeoZelux systemd[1]: Starting NFS client services.
Dec 06 15:54:18 NeoZelux systemd[1]: Reached target NFS client services.

Spider.007

Member

Registered: 2004-06-20

Posts: 1,175

Re: NFS server unmount error w/ bind-mount

It's not unreasonable that your nfs-server keeps a lock on the directory you decided to share. I also fail to see why you are mount-binding so many times, could you explain the reason behind this? Maybe you're better of with a symlink to the nfs share instead of mounting it multiple times

TE

Member

Registered: 2014-06-21

Posts: 85

Re: NFS server unmount error w/ bind-mount

NFSv4 operates in "namespaces" and you're basically messing with it's brain by bind-mounting (and unmounting) within the exported directory while it's live, without using the exportfs command (either via add/remove in /etc/exports or just putting it all on the commandline). You'll want to read up on section 7 of the RFC:

https://tools.ietf.org/html/rfc5661

In essence you're confusing it's brain -- when you add a bind mount, re-export and vice versa -- when removing it, deconfigure and re-export. It still might not work but you can try -- like Spider.007 I question your technique and methodology. It just feels like you're trying to do something... "wrong" here that should be designed in a better way.

NeoZelux

Member

Registered: 2012-10-28

Posts: 45

Re: NFS server unmount error w/ bind-mount

so sorry for the super late reply, christmas shopping...

anyway sorry if it was confusing to show the double bind mount, I was trying to demonstrate that unmounting only fails after I start the client.

So are you trying to say that it's normal behaviour to be unable to unmount a directory while it was being shared ? I had thought that it would be bad practice
to terminate the server while the directory was still bound to another one, but it sounds like you're saying it's the other way around.

So what is the most elegant/convenient, least convoluted way to share a different directory w/ the client after I'm done doing stuff with the other directory ?