linux-kernel - Re: Boot regression caused by commit 6829a048

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Date:	Fri, 06 Jan 2012 13:26:44 -0500
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	chuck.lever@...cle.com, linux@...ik.name,
	Pekka Enberg <penberg@...nel.org>,
	linux-nfs <linux-nfs@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Boot regression caused by commit 6829a048

On Fri, 2012-01-06 at 19:32 +0200, Sasha Levin wrote: 
> Hi all,
> 
> I've noticed a boot regression caused by commit 6829a048 ("NFS: Retry
> mounting NFSROOT") which has increased boot time by 95 seconds.
> 
> The scenario is as follows:
>  - A virtual guest running under the KVM tool.
>  - Guest is using kernel automatic IP DHCP configuration ("ip=dhcp").
>  - Guest is booting from a 9p device (which is not detected as block,
> and gets mounted after NFS tries to do its mounts).
>  - No NFS server at all, no NFS parameters passed to the guest kernel.
> 
> Under this scenario, theres an additional 95 second delay before NFS
> fails and tries to boot using 9p:
> 
> [...]
> [    6.505269] md: autorun ...
> [    6.506954] md: ... autorun DONE.
> [  101.522716] VFS: Unable to mount root fs via NFS, trying floppy.
> [  101.534499] VFS: Mounted root (9p filesystem) on device 0:18.
> [...]

Can't you avoid the whole NFS root mount attempt by setting "root=2:0"
directly instead of relying on 'mount_root' to do it for you?

> This probably happens since the NFS server isn't configured, so the
> bootserver is automatically assumed to be the DHCP server, and with the
> commit above we won't simply fail immediately when the NFS code fails
> connecting to it.
> 
> I'm not quite sure about the correct solution for this. While I can
> forcefully disable NFS, is it really the right solution? Should we be
> retrying a NFS server even if one wasn't specifically set?

One option might be to check the 'root_wait' flag. We could also add
nfsroot support for the 'retry=' mount option.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com

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