Attached reworked patches for shared subtree feature.
http://lwn.net/Articles/119232/ contains Al Viro's RFC explaining the
semantics of this feature.
These patches are against 2.6.13. The patches have incorporated comments
received in the last round.
If the shared-subtree functionality is not used, the footprint of this code is pretty small in the normal code path.
I have put in extensive documentation in the file
Documentation/sharedsubtree.txt . That file explains the semantics in detail
with couple of examples.
Overview
---------
How can a cloned namespace access the CD that just got mounted by the
automounter in the parent namespace? Shared subtree semantics provide the
necessary mechanism to accomplish the above.
Bind mount and namespaces are static in nature. They create a snapshot of the
current mounts. However any new mounts/unmounts within the original mount do
not reflect under the new one. Shared subtree semantics makes bind mounts and
namespaces dynamic in nature. The mount and unmount under any one replica
reflects in the other.
Shared subtree semantics also supports one-way-propagation. This is especially
handy for a cloned-namespace requiring all mounts in the parent namespace
propagated to itself, but still wants to keep its own mounts private.
Shared subtree provides powerful building blocks for features like
per-user-namespace and versioned filesystem.
I have documented the detailed semantics and some implementation detail in the
file Documentation/sharedsubtree.txt .
Who needs this?
--------------
The following projects have indicated potential use for this feature.
1. FUSE
2. SeLinux
3. MVFS
4. autofs
Patches
-------
I have broken down the implementation into 10 patches.
1. white_space.patch : conforms namespace.c file to coding style
2. shared_private_slave.patch : provides the ability to mark mounts as
shared or private or slave
3. unclone.patch : provides the ability to mark a mount unclonable.
This feature is not mentioned in the RFC though the need for it was
seen during the RFC review.
4. namespace_sem_removal.patch : this patch is written by Miklos
Szeredi. It gets rid of per namespace semaphore in favor of a global
semaphore. Given that mounts can propagate to multiple namespaces under
the new shared subtree semantics, the patch becomes relevant here.
5. rbind.patch : provides the ability to bind/rbind mounts of type
shared/private/slave to mountpoints residing in
shared/private/slave/unclonable mounts.
6. move.patch : provides the ability to move mount trees containing
shared/private/slave/unclonable mounts to mountpoints residing in
shared/private/slave/unclonable mounts. It handles the pivot_root
functionality too.
7. umount.patch : provides the ability to unmount mounts residing in
shared/private/unclonable/slave mounts.
8. namespace.patch : provides the ability to clone a namespace which
contains shared/private/slave/unclonable mounts.
9. automount.patch : provides the necessary auto-mounter helper
functions which are aware of shared subtree semantics.
10. documentation.patch: Contains all the documentation details of
shared subtree semantics and some implementation details too.
The diffstat of all the patches is:
Documentation/sharedsubtree.txt | 990 ++++++++++++++++++++++++++++++++++++++++
fs/Makefile | 2
fs/namespace.c | 854 +++++++++++++++++++++++++++-------
fs/pnode.c | 678 +++++++++++++++++++++++++++
include/linux/dcache.h | 1
include/linux/fs.h | 11
include/linux/mount.h | 30 -
include/linux/namespace.h | 1
include/linux/pnode.h | 68 ++
Tests:
-----
Avantika Mathur has developed about 90+ test cases that check various
scenarios. All these tests have passed. No regression was found. The
code survived some stress test too.
Bugs:
-----
There is one known umount bug that somebody will bump into and it has
to do with the semantics of multiple mounts on the same dentry of a
given mount. Its been documented in the sharedsubtree.txt file. The
effect of the bug is insignificant, but I will appreciate some
community-thought into the issue.
Thanks and Acknowledgement:
---------------------------
Miklos Szeredi for providing code snippets for traversing
the propagation tree, and in fact many __valuable__ review comments.
Mike Waychison, Bruce J Fields, Serge Hallyn, Pekka Enberg for their
valuable review comments.
Avantika Mathur for churning out many-many bugs.
Ram Pai
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