Re: nfsroot protocol version

[Mason <slash.tmp@free.fr>]

Hello,

On 01/04/2015 11:55, Mason wrote:

> My embedded system is supposed to mount the rootfs via NFS.
> https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt
>
> So I set these options in my kernel .config
>
> CONFIG_IP_PNP=y
> CONFIG_IP_PNP_DHCP=y
> CONFIG_ROOT_NFS=y
> CONFIG_NFS_V4=y
>
> But it didn't work, the system would panic after a long time out
> (around 95 seconds)
>
> [   98.567037] VFS: Unable to mount root fs via NFS, trying floppy.
> [   98.573238] Freeing unused kernel memory: 144K (c0332000 - c0356000)
> [   98.579698] Kernel panic - not syncing: No working init found.
>      Try passing init= option to kernel. See Linux Documentation/init.txt for guidance.
>
> I then enabled NFSv2 and NFSv3, and then it worked.
>
> When I was debugging the problem, I ran across NFS_DEF_OPTIONS
> in fs/nfs/nfsroot.c
>
> #define NFS_DEF_OPTIONS        "vers=2,udp,rsize=4096,wsize=4096"
>
> IIUC, we are explicitly requesting NFSv2?
>
> Would that explain why it fails when NFSv2 support is not compiled in?
>
> Would it make sense to use version 3?
> (So NFSv2 is not needed just to load the rootfs.)

I saw commit 4251c94833aa (NFS: Revert NFSROOT default mount options)
which states:

     Last fall we removed the ad hoc mount option parser in
     fs/nfs/nfsroot.c in favor of using the main parser in fs/nfs/super.c
     (see commit 56463e50 "NFS: Use super.c for NFSROOT mount option
     parsing").  That commit changed the default NFSROOT mount options to
     be the same as those employed by user space mounts.
     
     As it turns out, these new default mount options are not tolerated by
     many embedded systems.  So far these problems have been due to
     specific behavior of certain embedded NICs.  The NFS community does
     not have such hardware on hand for running tests.
     
     Commit 53d47375 recently introduced a clean way to specify default
     mount options for NFSROOT, so we can now easily restore the
     traditional defaults for NFSROOT:
     
        vers=2,udp,rsize=4096,wsize=4096
     
     This should revert the new default NFSROOT mount options introduced
     with commit 56463e50.

I guess this explains why vers=3 is not the default?

I'm now wondering how to override the mount options on the boot command
line. Looking more closely at commit 53d473758053 (NFS: NFSROOT should
default to "proto=udp") nothing jumps out at me.

My current boot command line is
"root=/dev/nfs ip=:::::eth0:dhcp rdinit=/none console=ttyS0,115200 mem=512M earlyprintk"

Looking at the documentation
https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt
I don't see where the protocol version is specified.

Where am I supposed to add the vers=3 option?

Regards.