Re: [PATCH 6/6] nfs(5): Document remount behavior

[Steve Dickson <SteveD@redhat.com>]

Chuck,

I'm a bit concern about this patch.... 

I'm asking myself who is going care how remounts update 
/etc/mtab and what mode is used. I just thinking that type of info 
adds a lot verbiage that nobody really care about. Plus why are
we documenting something (/etc/mtab) that will be going 
away as soon as humanly possible? 
 
Basically I'm saying the entire "Unmounting after a remount" 
section is not needed. Only the 3 paragraphs in "THE REMOUNT OPTION"
section are needed IMHO... 

I'm also thinking we need to add the/etc/nfsmount.conf FILES section,
but I guess I can do that later... 

 
steved.

On 10/26/2010 11:58 AM, Chuck Lever wrote:
> It appears that, for a long while, NFS "remount" mounts have
> completely wiped the existing mount options in /etc/mtab for a given
> mount point.  This is a problem for umount.nfs, since it reads its
> options out of /etc/mtab to find out how to do the unmount.
> 
> The mount(8) command provides the NFS mount subcommand with the mount
> options to perform the remount.  There are four cases to consider:
> 
>   1. Both the device and mount directory are specified on the
>      command line, and the target mount point is in /etc/fstab
> 
>   2. Only one of the device and mount directory is specified on
>      the command line, and the target mount point is in
>      /etc/fstab
> 
>   3. Both the device and mount directory are specified on the
>      command line, and the target mount point is not in /etc/fstab
> 
>   4. Only one of the device and mount directory is specified on
>      the command line, and the target mount point is not in
>      /etc/fstab
> 
> Currently only case 4 works correctly.  In that case, mount(8)
> provides the correct set of mount options to the mount.nfs
> subcommand and it can update /etc/mtab correctly.
> 
> Cases 1 and 3 replace all mount options in /etc/mtab with the options
> provided on the command line during a remount.  Case 2 replaces the
> mount options in /etc/mtab with a mix of options from /etc/fstab and
> /etc/mtab.
> 
> Cases 1 and 3 are historical behavior.  Basically this is a formal
> interface to allow administrators to replace the mount options in
> /etc/mtab completely, instead of merging in new ones.  The present
> patch documents that behavior in nfs(5), and provides best practice
> for remounting NFS mount points.
> 
> There are near-term plans to address case 2 by fixing mount(8)
> (provided by utils-linux-ng in most distributions).
> 
> This is a partial fix for:
> 
>   https://bugzilla.linux-nfs.org/show_bug.cgi?id=188
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
> 
>  utils/mount/nfs.man |   92 ++++++++++++++++++++++++++++++++++++++++++---------
>  1 files changed, 76 insertions(+), 16 deletions(-)
> 
> diff --git a/utils/mount/nfs.man b/utils/mount/nfs.man
> index 1b86768..a357000 100644
> --- a/utils/mount/nfs.man
> +++ b/utils/mount/nfs.man
> @@ -1523,32 +1523,92 @@ of Access Control Lists that are semantically richer than POSIX ACLs.
>  NFS version 4 ACLs are not fully compatible with POSIX ACLs; as such,
>  some translation between the two is required
>  in an environment that mixes POSIX ACLs and NFS version 4.
> -.SH FILES
> -.TP 1.5i
> -.I /etc/fstab
> -file system table
> -.SH BUGS
> -The generic
> -.B remount
> -option is not fully supported.
> -Generic options, such as
> -.BR rw " and " ro
> -can be modified using the
> -.B remount
> -option,
> -but NFS-specific options are not all supported.
> +.SH "THE REMOUNT OPTION"
> +Generic mount options such as
> +.BR rw " and " sync
> +can be modified on NFS mount points using the
> +.BR remount
> +option.
> +See
> +.BR mount (8)
> +for more information on generic mount options.
> +.P
> +With few exceptions, NFS-specific options
> +are not able to be modified during a remount.
>  The underlying transport or NFS version
>  cannot be changed by a remount, for example.
> +.P
>  Performing a remount on an NFS file system mounted with the
>  .B noac
>  option may have unintended consequences.
>  The
>  .B noac
> -option is a mixture of a generic option,
> +option is a combination of the generic option
>  .BR sync ,
> -and an NFS-specific option
> +and the NFS-specific option
>  .BR actimeo=0 .
> +.SS "Unmounting after a remount"
> +For NFS versions 2 and 3, the NFS umount subcommand
> +depends on
> +.I /etc/mtab
> +to contain the mount options needed to send an UMNT request
> +to the server.
> +The NFS mount subcommand stores these mount options in
> +.I /etc/mtab
> +after a successful mount operation is complete.
> +A remount can alter these stored mount options, however.
>  .P
> +When it comes to what is written back to
> +.I /etc/mtab
> +during a remount, the
> +.BR mount (8)
> +command has two different modes of operation.
> +One mode
> +.I merges
> +the command line mount options with the mount
> +options already in
> +.IR /etc/mtab ,
> +and the other
> +.I replaces
> +the mount options in
> +.I /etc/mtab
> +with the command line mount options.
> +.P
> +To
> +.I replace
> +the mount options that are in
> +.IR /etc/mtab ,
> +specify both the server hostname and export path, and the
> +local mount directory during a remount.
> +To
> +.I merge
> +the mount options on the command line with
> +the options already in
> +.IR /etc/mtab ,
> +specify either the local mount directory, or the server
> +hostname and export pathname, but not both.  For example,
> +.P
> +.NF
> +.TA 2.5i
> +	mount -o remount,ro /mnt
> +.FI
> +.P
> +merges the mount option
> +.B ro
> +with the mount options already in
> +.I /etc/mtab
> +for the NFS server mounted at /mnt.
> +Merging is almost always the desired outcome, as it preserves the
> +ability of the client to contact the server when it comes time
> +to send an UMNT request.
> +.SH FILES
> +.TP 1.5i
> +.I /etc/fstab
> +file system table
> +.TP 1.5i
> +.I /etc/mtab
> +table of mounted file systems
> +.SH BUGS
>  Before 2.4.7, the Linux NFS client did not support NFS over TCP.
>  .P
>  Before 2.4.20, the Linux NFS client used a heuristic
>