Re: [RFC] shared subtrees

[Ram <linuxram@us.ibm.com>]

On Tue, 2005-02-01 at 13:15, Mike Waychison wrote:
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> (Hmm.. something is up with my quoting again..)
> 
> Ram wrote:
> > On Mon, 2005-01-31 at 23:02, Mike Waychison wrote:
> > 
> > Ram wrote:
> > 
> >>On Fri, 2005-01-28 at 14:31, Mike Waychison wrote:
> > 
> > 
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> >>>Al Viro wrote:
> > 
> > 
> > 
> >>>>OK, here comes the first draft of proposed semantics for subtree
> >>>>sharing.  What we want is being able to propagate events between
> >>>>the parts of mount trees.  Below is a description of what I think
> >>>>might be a workable semantics; it does *NOT* describe the data
> >>>>structures I would consider final and there are considerable
> >>>>areas where we still need to figure out the right behaviour.
> >>>>
> > 
> >>>Okay, I'm not convinced that shared subtrees as proposed will work well
> >>>with autofs.
> > 
> >>>The idea discussed off-line was this:
> > 
> >>>When you install an autofs mountpoint, on say /home, a daemon is started
> >>>to service the requests.  As far as the admin is concerned, an fs is
> >>>mounted in the current namespace, call it namespaceA.  The daemon
> >>>actually runs in it's one private namespace: call it namespaceB.
> >>>namespaceB receives a new autofs filesystem: call it autofsB.  autofsB
> >>>is in it's own p-node.  namespaceA gets an autofsA on /home as well, and
> >>>autofsA is 'owned' by autofsB's p-node.
> > 
> > 
> >>Mike, multiple parsing through the problem definition, still did not
> >>make the problem clear. What problem is autofs trying to solve using
> >>namespaces?
> > 
> >>My guess is you dont want to see a automount taking place in namespaceA,
> >>when a automount takes place in namespaceB, even though
> >>the automount-point is in a shared subtree?
> > 
> >>Sorry don't understand automount's requirement in the first place,
> >>RP
> > 
> > The major concern for automounting is that currently, if you start an
> > automount daemon in the primary namespace, and some process clones off
> > into a new namespace with clone(CLONE_NS), then there is no way for the
> > daemon running in the first namespace to automount (let alone expire)
> > any mounts in the second namespace.  There doesn't exist a way for the
> > daemon to mount(2) nor umount(2) across namespaces.
> > 
> > The proposed solution for this is to use shared and private subtrees to
> > have the daemon run in it's own namespace, with the primary and any
> > derivative namespaces inheriting the automounts.  I'm not convinced that
> > it'd work though.
> > 
> > Does this clarify?
> > 
> > 
> >> Yes it does clarify the problem and motivates the reason behind using
> >> shared subtree.
> > 
> >> However going back to your original problem 1:
> > 
> >> you have a daemon running in namespaceB, and a process running in
> >> namespaceA and it acceses a auto-mountpoint /home. 
> > 
> >> The expected behavior in this case should be: the autofs-daemon must
> >> mount the corresponding device at that mount point '/home' on all 
> >> existing namespaces(provided that part of the subtree is shared). Right?
> >> So in this case it should mount the device in both the namespaces, i.e
> >> namespaceA and namespaceB.  
> 
> Yes.  Sharing allows this to happen in a 'safe' way.  The daemon doesn't
> have to know how many instances of '/home' exist.
> 
> >> But you seem to be saying that you want to
> >> block the auto-mount in namespaceA?
> > 
> 
> No.  I want to allow the mount.  However, if there are several shared
> '/home' (through CLONE_NS or mount --bind), there remains the following
> two key problems:
> 
> - - How do you expire the mounts and umount them?  (undefined with shared
> subtrees thus far)

The same way we currently expire mounts. In the case of namespaces you
will expire a bunch of mounts simultaneously  and unmount them togather.

> - - How do you handle the case where '/home/mikew' is automounted in all
> instances of it, and then umounted in a single namespace.  Walking back
> into '/home/mikew' in that namespace will trigger the daemon to mount
> again, but the filesystem is already mounted in it's namespace. 

I think, mount and unmount are two different type of events. And any
event propogates down the propogation tree. My understanding of the
expected behavior is, the unmount should take place in all the
namespaces(provided all the umount-points belong to vfstructs belonging
to the same pnode or are owned by the pnode).  So in your example, I
imagine /home/mikew to be unmounted in all the namespaces. 


> 
> I guess a solution to ponder is what if we included the following rule:
> 
> "An attempt to umount a vfsmount X will induce the umounting of all
> vfsmounts in X's p-node as well as all vfsmounts/p-nodes 'owned' by said
> p-node."
> 
exactly. And I think this is what Al imagines it be too.

Well all this is my interpretation of Al's proposal. I would like to
hear from him though..... 

> I'm not sure that is a desirable solution or even nice to implement.
> 
> - --
> Mike Waychison
> Sun Microsystems, Inc.
> 1 (650) 352-5299 voice
> 1 (416) 202-8336 voice
> 
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