[Devel] Re: How to query mount propagation state?

[Karel Zak]

On Mon, Apr 16, 2007 at 10:39:46AM -0700, Ram Pai wrote:

> This patch disambiguates multiple mount-instances of the same
> filesystem (or part of the same filesystem), by introducing a new
> interface /proc/mounts_new. The interface has the following format.
            ^^^^^^^^^^^^^^
 ... odd name. What will be the name for a next generation?
 "/proc/mounts_new_new"? :-)

> 'cat /proc/mounts' shows the following:
> /dev/root /mnt ext2 rw 0 0
> /dev/root /tmp1 ext2 rw 0 0
> 
> NOTE: The above mount entries, do not indicate that /tmp1 contains the same
> directory tree as /var/tmp.
> 
> But 'cat /proc/mounts_new' shows us the following:
> 0x6200 /mnt /var ext2 rw 0 0
> 0x6200 /tmp1 /var/tmp ext2 rw 0 0

 Can't you purely and simply add the fsid= option to /proc/mounts?

 /dev/root /mnt ext2 rw,fsid=0x6200 0 0 
 /dev/root /mnt ext2 rw,fsid=0x6200 0 0

 I think you can do it without a negative impact to userspace.

> This patch introduces a new proc interface that exposes all the propagation 
> trees within the namespace.

 Good idea.

> It walks through each off the mounts in the namespace, and prints the following information.
> 
> mount-id: a unique mount identifier
> dev-id : the unique device used to identify the device containing the filesystem
  ^^^^
 Why not major:minor?

> path-from-root: mount point of the mount from /
> path-from-root-of-its-sb: path from its own root dentry.
> propagation-flag: SHARED, SLAVE, UNBINDABLE, PRIVATE
> peer-mount-id: the mount-id of its peer mount (if this mount is shared)
> master-mount-id: the mount-id of its master mount (if this mount is slave)

> Example:
> Here is a sample output of cat /proc/$$/mounts_propagation
> 
> 0xa917800 0x1 / / PRIVATE
> 0xa917200 0x6200 / / PRIVATE
> 0xa917180 0x3 /proc / PRIVATE
> 0xa917f80 0xa /dev/pts / PRIVATE
> 0xa917100 0x6210 /mnt / SHARED peer:0xa917100
> 0xa917f00 0x6210 /tmp /1 SLAVE master:0xa917100
> 0xa917900 0x6220 /mnt/2 / SHARED peer:0xa917900

 Same thing (although the mounts_propagation makes more sense than
 mount_new from my point of view). 
 
 cat /proc/mounts (or /proc/$$/mounts)

 /dev/root /mnt ext2 rw,mid=0xa917100,did=0x6210,prop=SHARED,peer=0xa917100


 my $0.02...

    Karel

-- 
 Karel Zak  <kzak at redhat.com>
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