linux-kernel - Re: XFS noikeep remount in 2.6.27-rc1-next-20080730

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Message-ID: <20080805233956.GI21635@disturbed>
Date:	Wed, 6 Aug 2008 09:39:57 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Karel Zak <kzak@...hat.com>
Cc:	Christoph Hellwig <hch@....de>,
	Jasper Bryant-Greene <jasper@...ton.co.nz>,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	util-linux-ng@...r.kernel.org
Subject: Re: XFS noikeep remount in 2.6.27-rc1-next-20080730

On Tue, Aug 05, 2008 at 01:03:57PM +0200, Karel Zak wrote:
> On Fri, Aug 01, 2008 at 09:31:33PM +0200, Christoph Hellwig wrote:
> > I'ts most likely a fallout, but I wonder why.  To get this behaviour
> > moutn would have to add all the options it finds in /proc/self/mounts
> > to the command line.
> 
>  mount(8) does not read and use /proc/self/mounts at all.
> 
>     Karel
> 
> 
>  Man mount:
> 
>  remount
> 
>      Attempt  to remount an already-mounted file system.  This is commonly used
>      to change the mount flags for a file system, especially to make a readonly
>      file system writeable. It does not change device or mount point.
> 
>      The  remount  functionality follows the standard way how the mount command
>      works with options from fstab. It means the  mount  command  doesn’t  read
>      fstab (or mtab) only when a device and dir are fully specified.
> 
>      mount -o remount,rw /dev/foo /dir
> 
>      After  this  call  all  old mount options are replaced and arbitrary stuff
>      from fstab is ignored, except the loop= option which is internally  gener-
>      ated and maintained by the mount command.
> 
>      mount -o remount,rw  /dir
> 
>      After  this call mount reads fstab (or mtab) and merges these options with
>      options from command line ( -o ).

So, given the command at issue was:

luna ~ # mount -o remount,rw /usr

We're seeing the second case where mount is merging all the options in
/etc/fstab into the options passed into the remount command. How is
the filesystem expected to behave in these difference cases? The
first simply changes the ro/rw status, the second potentially
asks for the filesystem to change a bunch of other mount options
as well, which it may not be able to do.

So what is the correct behaviour? Should the filesystem *silently
ignore* unchangable options in the remount command, or should it
fail the remount and warn the user that certain options are not
allowed in remount?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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