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Subject[RFC] shared subtrees
[apologies for delay - there'd been lots of unrelated crap lately]
======================================================================
NOTE: as far as I'm concerned, that's a beginning of VFS-2.7 branch.
All that work will stay in a separate tree, with gradual merge back
into 2.6 once the things start settling down.
======================================================================

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.

Let's start with introducing a notion of propagation node; I consider
it only as a convenient way to describe the desired behaviour - it
almost certainly won't be a data structure in the final variant.

1) each p-node corresponds to a group of 1 or more vfsmounts.
2) there is at most 1 p-node containing a given vfsmount.
3) each p-node owns a possibly empty set of p-nodes and vfsmounts
4) no p-node or vfsmount can be owned by more than one p-node
5) only vfsmounts that are not contained in any p-nodes might be owned.
6) no p-node can own (directly or via intermediates) itself (i.e. the
graph of p-node ownership is a forest).

These guys define propagation:
a) if vfsmounts A and B are contained in the same p-node, events
propagate from A to B
b) if vfsmount A is contained in p-node p, vfsmount B is contained
in p-node q and p owns q, events propagate from A to B
c) if vfsmount A is contained in p-node p and vfsmount B is owned
by p, events propagate from A to B
d) propagation is transitive: if events propagate from A to B and
from B to C, they propagate from A to C.

In other words, members of the same p-node are equivalent and events anywhere
in p-node are propagated to all its slaves. Note that not any transitive
relation can be represented that way; it has to satisfy the following
condition:
* A->C and B->C => A->B or B->A
All propagation setups we are going to deal with will satisfy that condition.


How do we set them up?

* we can mark a subtree sharable. Every vfsmount in the subtree
that is not already in some p-node gets a single-element p-node of its
own.
* we can mark a subtree slave. That removes all vfsmounts in
the subtree from their p-nodes and makes them owned by said p-nodes.
p-nodes that became empty will disappear and everything they used to
own will be repossessed by their owners (if any).
* we can mark a subtree private. Same as above, but followed
by taking all vfsmounts in our subtree and making them *not* owned
by anybody.


Of course, namespace operations (clone, mount, etc.) affect that structure
and are affected by it (that's what it's for, after all).

1. CLONE_NS

That one is simple - we copy vfsmounts as usual
* if vfsmount A is contained in p-node p, then copy of A goes into
the same p-node
* if A is owned by p, then copy of A is also owned by p
* no new p-nodes are created.

2. mount

We have a new vfsmount A and want to attach it to mountpoint somewhere in
vfsmount B. If B does not belong to any p-node, everything is as usual; A
doesn't become a member or slave of any p-node and is simply attached to B.

If B belongs to a p-node p, consider all vfsmounts B1,...,Bn that get events
propagated from B and all p-nodes p1,...,pk that contain them.
* A gets cloned into n copies and these copies (A1,...,An) are attached
to corresponding points in B1,...,Bn.
* k new p-nodes (q1,...,qk) are created
* Ai is contained in qj <=> Bi is contained in qj
* qi owns qj <=> pi owns pj
* qi owns Aj <=> pi owns Bj

In other words, mount is propagated and propagation among the new vfsmounts
mirrors the propagation between mountpoints.

3. bind

bind works almost identically to mount; new vfsmount is created for every
place that gets propagation from mountpoint and propagation is set up to
mirror that between the mountpoints. However, there is a difference: unlike
the case of mount, vfsmount we were going to attach (say it, A) has some
history - it was created as a copy of some pre-existing vfsmount V. And
that's where the things get interesting:
* if V is contained in some p-node p, A is placed into the same
p-node. That may require merging one of the p-nodes we'd just created
with p (that will be the counterpart of the p-node containing the mountpoint).
* if V is owned by some p-node p, then A (or p-node containing A)
becomes owned by p.

4. rbind
rbind is recursive bind, so we just do binds for everything we had in
a subtree we are binding in obvious order; everything is described
by previous case.

5. umount
umount everything that gets propagation from victim.

6. mount --move
prohibited if what we are moving is in some p-node, otherwise we move
as usual to intended mountpoint and create copies for everything that
gets propagation from there (as we would do for rbind).

7. pivot_root
similar to --move


How to use all that stuff?

Example 1:
mount --bind /floppy /floppy
mount --make-shared /floppy
mount --rbind / /jail
<finish setting the jail up, umount whatever doesn't belong there,
etc.>
mount --make-slave /jail/floppy
and we get /floppy in chroot jail slave to /floppy outside - if somebody
(u)mounts stuff on it, that will get propagated to jail.

Example 2:
same, but with the namespaces instead of chroots.

Example 3:
same subtree visible (and kept in sync) in several places - just
mark it shared and rbind; it will stay in sync

Example 4:
have some daemon control the stuff in a subtree sharable with many
namespaces, chroots, etc. without any magic:
mark that subtree sharable
clone with CLONE_NS
parent marks that subtree slave
child keeps working on the tree in its private namespace.

There's a lot more applications of the same idea, of course - AFS and its
ilk, autofs-like stuff (with proper handling of MNT_EXPIRE and traps - see
below), etc., etc.



Areas where we still have to figure things out:

* MNT_EXPIRE handling done right; there are some fun ideas in that area,
but they still need to be done in more details (basically, lazy expire -
mount in a slave expiring into a trap that would clone a copy from master
when stepped upon).

* traps and their sharing. What we want is an ability to use the master/slave
mechanisms for *all* cross-namespace/cross-chroot issues in autofs, so that
daemon would only need to work with the namespace of its own and no nothing
about other instances.

* implementation ;-) It certainly looks reasonably easy to do; memory
demands are linear by number of vfsmounts involved and locking appears
to be solvable.

* whatever issues that might come up from MVFS demands (and AFS, and...)
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