Re: recent nfs change causes autofs regression

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 30 Aug 2007 18:37:13 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> On Thu, 2007-08-30 at 14:07 -0700, Hua Zhong wrote:
> > I am re-sending this after help from Ian and git-bisect. To me it's a
> > show-stopper: I cannot find an acceptable workaround that I can implement.
> > 
> > The problem: upgrading to 2.6.23-rc4 from 2.6.22 causes several autofs
> > mounts to fail silently - they just not appear when they should.
> > 
> > I believe it's caused by the NFS change that forces multiple mounts from
> > different directories under the same server side filesystem to have the same
> > mount options by default, otherwise it returns EBUSY.
> > 
> > For example, if server has a filesystem /a, and it exports /a/x and /a/y
> > (maybe with rw or ro), and a client must mount /a/x and /a/y with the same
> > mount options now.
> 
> Which is better than having it fail silently, or giving you a mount with
> the wrong mount options.
> 
> If you need to mount the same filesystem with incompatible mount options
> on the same client, then there is a new mount option "nosharecache",
> which enables it.
> The new option is there in order to make it damned clear to sysadmins
> that this is a dangerous thing to do: mounts which don't share the same
> superblock also don't share the same data and attribute caches. Any file
> or directory which appears in both mounts had better only be used by one
> application at a time or be using an appropriate locking scheme.
> 

If we're going to send a message to sysadmins, we shouldn't force them to go
through a git bisection search and a lkml discussion to receive it!

Is there at least some way in which the kernel can detect this situation
and emit a friendly printk which guides people to a friendly document?