linux-kernel - Re: Adding subroot information to /proc/mounts, or obtaining that through other means

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Message-Id: <1182497690.2812.54.camel@ram.us.ibm.com>
Date:	Fri, 22 Jun 2007 00:34:50 -0700
From:	Ram Pai <linuxram@...ibm.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Al Viro <viro@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, util-linux-ng@...r.kernel.org
Subject: Re: Adding subroot information to /proc/mounts, or obtaining that
	through other means

On Fri, 2007-06-22 at 00:06 -0700, H. Peter Anvin wrote:
> Ram Pai wrote:
> > 
> > the second patch made a /proc/propagation interface which had almost the
> > same fields, but also added fields to show the propagation type of the
> > mount as well as pointers to its peers and master depending on the type
> > of the mount. 
> > 
> > I think the consensus seems to have a new interface /proc/make-a-name
> > which extends the interface provided by /proc/mounts but provides the
> > propagation state of the mounts too as well as disambiguate bind mounts.
> > Which makes sense.
> > 
> 
> Why?  It seems a lot cleaner to have all the information in the same
> place.  It is highly unfriendly to userspace to have to gather
> information in a lot of places, plus it adds race conditions.
> 
> It would be another matter if the format that we have now couldn't be
> extended, but we need those fields (well, except the two zeros, but who
> cares) *anyway*, so we might as well stick to the existing file, and
> reduce the total amount of code and clutter.

Ok. so you think /proc/mounts can be extended easily without breaking
any userspace commands?

well lets see..
1. to disambiguate bind mounts, we have to add a field that displays the
	 path to the mount's root dentry from the filesystem's root
	 dentry. Agree?

2. For filesystems that do not have a backing store, it becomes hard
	to disambiguate bind mounts in (1). So we need to add a
	filesystem-id field.

3. if we need to add the propagation status of the mount we need a
	 propagation flag added in the output.

4. To be able to construct the propagation tree, we need a way to refer
	to the other mounts, since some mounts are peers and some other
	mounts are master. Which means we need a mount-id field.
	Agree?

If you agree to the above 4 new fields, it becomes challenging to
extend /proc/mounts to incorporate these new fields without
breaking any existing applications. 


> > 
> > BTW: what is the need for overmounted flag?  Do you mean two vfsmounts
> > mounted on the same dentry on the ***same vfsmount*** ?
> > 
> 
> Maybe I'm not following the uses of your flags well enough to figure out
>  if that information can already been deduced.

With the addition of the above 4 mentioned fields, I think one should be
easily able to decipher which mnt-id is mounted on which mnt-id. no?
maybe not. Well we will have to extend the mountpoint field to indicate
the mnt-id in which the mountpoint resides.  

RP

> 
> 	-hpa

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