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docs: filesystems: convert sharedsubtree.txt to ReST 

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6692b8abc177130e9e53aace94117a2ad076cab5.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mauro Carvalho Chehab 2020-04-27 23:17:12 +02:00 committed by Jonathan Corbet
parent 53a41d3eec
commit cf06612c65
3 changed files with 228 additions and 171 deletions
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1
Documentation/filesystems/index.rst
View File

@ -33,6 +33,7 @@ algorithms work.
mount_api
quota
seq_file
sharedsubtree
automount-support

2
Documentation/filesystems/proc.rst
View File

@ -1870,7 +1870,7 @@ unbindable mount is unbindable
For more information on mount propagation see:
Documentation/filesystems/sharedsubtree.txt
Documentation/filesystems/sharedsubtree.rst
3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm

396
Documentation/filesystems/sharedsubtree.txt → Documentation/filesystems/sharedsubtree.rst
View File

@ -1,7 +1,10 @@
Shared Subtrees
---------------
.. SPDX-License-Identifier: GPL-2.0
Contents:
===============
Shared Subtrees
===============
.. Contents:
1) Overview
2) Features
3) Setting mount states
@ -41,31 +44,38 @@ replicas continue to be exactly same.
Here is an example:
Let's say /mnt has a mount that is shared.
mount --make-shared /mnt
Let's say /mnt has a mount that is shared::
mount --make-shared /mnt
Note: mount(8) command now supports the --make-shared flag,
so the sample 'smount' program is no longer needed and has been
removed.
# mount --bind /mnt /tmp
::
# mount --bind /mnt /tmp
The above command replicates the mount at /mnt to the mountpoint /tmp
and the contents of both the mounts remain identical.
#ls /mnt
a b c
::
#ls /tmp
a b c
#ls /mnt
a b c
Now let's say we mount a device at /tmp/a
# mount /dev/sd0 /tmp/a
#ls /tmp
a b c
#ls /tmp/a
t1 t2 t3
Now let's say we mount a device at /tmp/a::
#ls /mnt/a
t1 t2 t3
# mount /dev/sd0 /tmp/a
#ls /tmp/a
t1 t2 t3
#ls /mnt/a
t1 t2 t3
Note that the mount has propagated to the mount at /mnt as well.
@ -123,14 +133,15 @@ replicas continue to be exactly same.
2d) A unbindable mount is a unbindable private mount
let's say we have a mount at /mnt and we make it unbindable
let's say we have a mount at /mnt and we make it unbindable::
# mount --make-unbindable /mnt
# mount --make-unbindable /mnt
Let's try to bind mount this mount somewhere else.
# mount --bind /mnt /tmp
mount: wrong fs type, bad option, bad superblock on /mnt,
or too many mounted file systems
Let's try to bind mount this mount somewhere else::
# mount --bind /mnt /tmp
mount: wrong fs type, bad option, bad superblock on /mnt,
or too many mounted file systems
Binding a unbindable mount is a invalid operation.
@ -138,12 +149,12 @@ replicas continue to be exactly same.
3) Setting mount states
The mount command (util-linux package) can be used to set mount
states:
states::
mount --make-shared mountpoint
mount --make-slave mountpoint
mount --make-private mountpoint
mount --make-unbindable mountpoint
mount --make-shared mountpoint
mount --make-slave mountpoint
mount --make-private mountpoint
mount --make-unbindable mountpoint
4) Use cases
@ -154,9 +165,10 @@ replicas continue to be exactly same.
Solution:
The system administrator can make the mount at /cdrom shared
mount --bind /cdrom /cdrom
mount --make-shared /cdrom
The system administrator can make the mount at /cdrom shared::
mount --bind /cdrom /cdrom
mount --make-shared /cdrom
Now any process that clones off a new namespace will have a
mount at /cdrom which is a replica of the same mount in the
@ -172,14 +184,14 @@ replicas continue to be exactly same.
Solution:
To begin with, the administrator can mark the entire mount tree
as shareable.
as shareable::
mount --make-rshared /
mount --make-rshared /
A new process can clone off a new namespace. And mark some part
of its namespace as slave
of its namespace as slave::
mount --make-rslave /myprivatetree
mount --make-rslave /myprivatetree
Hence forth any mounts within the /myprivatetree done by the
process will not show up in any other namespace. However mounts
@ -206,13 +218,13 @@ replicas continue to be exactly same.
versions of the file depending on the path used to access that
file.
An example is:
An example is::
mount --make-shared /
mount --rbind / /view/v1
mount --rbind / /view/v2
mount --rbind / /view/v3
mount --rbind / /view/v4
mount --make-shared /
mount --rbind / /view/v1
mount --rbind / /view/v2
mount --rbind / /view/v3
mount --rbind / /view/v4
and if /usr has a versioning filesystem mounted, then that
mount appears at /view/v1/usr, /view/v2/usr, /view/v3/usr and
@ -224,8 +236,8 @@ replicas continue to be exactly same.
filesystem is being requested and return the corresponding
inode.
5) Detailed semantics:
-------------------
5) Detailed semantics
---------------------
The section below explains the detailed semantics of
bind, rbind, move, mount, umount and clone-namespace operations.
@ -235,6 +247,7 @@ replicas continue to be exactly same.
5a) Mount states
A given mount can be in one of the following states
1) shared
2) slave
3) shared and slave
@ -252,7 +265,8 @@ replicas continue to be exactly same.
A 'shared mount' is defined as a vfsmount that belongs to a
'peer group'.
For example:
For example::
mount --make-shared /mnt
mount --bind /mnt /tmp
@ -270,7 +284,7 @@ replicas continue to be exactly same.
A slave mount as the name implies has a master mount from which
mount/unmount events are received. Events do not propagate from
the slave mount to the master. Only a shared mount can be made
a slave by executing the following command
a slave by executing the following command::
mount --make-slave mount
@ -290,8 +304,10 @@ replicas continue to be exactly same.
peer group.
Only a slave vfsmount can be made as 'shared and slave' by
either executing the following command
either executing the following command::
mount --make-shared mount
or by moving the slave vfsmount under a shared vfsmount.
(4) Private mount
@ -307,30 +323,32 @@ replicas continue to be exactly same.
State diagram:
The state diagram below explains the state transition of a mount,
in response to various commands.
------------------------------------------------------------------------
| |make-shared | make-slave | make-private |make-unbindab|
--------------|------------|--------------|--------------|-------------|
|shared |shared |*slave/private| private | unbindable |
| | | | | |
|-------------|------------|--------------|--------------|-------------|
|slave |shared | **slave | private | unbindable |
| |and slave | | | |
|-------------|------------|--------------|--------------|-------------|
|shared |shared | slave | private | unbindable |
|and slave |and slave | | | |
|-------------|------------|--------------|--------------|-------------|
|private |shared | **private | private | unbindable |
|-------------|------------|--------------|--------------|-------------|
|unbindable |shared |**unbindable | private | unbindable |
------------------------------------------------------------------------
in response to various commands::
* if the shared mount is the only mount in its peer group, making it
slave, makes it private automatically. Note that there is no master to
which it can be slaved to.
-----------------------------------------------------------------------
| |make-shared | make-slave | make-private |make-unbindab|
--------------|------------|--------------|--------------|-------------|
|shared |shared |*slave/private| private | unbindable |
| | | | | |
|-------------|------------|--------------|--------------|-------------|
|slave |shared | **slave | private | unbindable |
| |and slave | | | |
|-------------|------------|--------------|--------------|-------------|
|shared |shared | slave | private | unbindable |
|and slave |and slave | | | |
|-------------|------------|--------------|--------------|-------------|
|private |shared | **private | private | unbindable |
|-------------|------------|--------------|--------------|-------------|
|unbindable |shared |**unbindable | private | unbindable |
------------------------------------------------------------------------
** slaving a non-shared mount has no effect on the mount.
* if the shared mount is the only mount in its peer group, making it
slave, makes it private automatically. Note that there is no master to
which it can be slaved to.
** slaving a non-shared mount has no effect on the mount.
Apart from the commands listed below, the 'move' operation also changes
the state of a mount depending on type of the destination mount. Its
@ -338,31 +356,32 @@ replicas continue to be exactly same.
5b) Bind semantics
Consider the following command
Consider the following command::
mount --bind A/a B/b
mount --bind A/a B/b
where 'A' is the source mount, 'a' is the dentry in the mount 'A', 'B'
is the destination mount and 'b' is the dentry in the destination mount.
The outcome depends on the type of mount of 'A' and 'B'. The table
below contains quick reference.
---------------------------------------------------------------------------
| BIND MOUNT OPERATION |
|**************************************************************************
|source(A)->| shared | private | slave | unbindable |
| dest(B) | | | | |
| | | | | | |
| v | | | | |
|**************************************************************************
| shared | shared | shared | shared & slave | invalid |
| | | | | |
|non-shared| shared | private | slave | invalid |
***************************************************************************
below contains quick reference::
--------------------------------------------------------------------------
| BIND MOUNT OPERATION |
|************************************************************************|
|source(A)->| shared | private | slave | unbindable |
| dest(B) | | | | |
| | | | | | |
| v | | | | |
|************************************************************************|
| shared | shared | shared | shared & slave | invalid |
| | | | | |
|non-shared| shared | private | slave | invalid |
**************************************************************************
Details:
1. 'A' is a shared mount and 'B' is a shared mount. A new mount 'C'
1. 'A' is a shared mount and 'B' is a shared mount. A new mount 'C'
which is clone of 'A', is created. Its root dentry is 'a' . 'C' is
mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
are created and mounted at the dentry 'b' on all mounts where 'B'
@ -371,7 +390,7 @@ replicas continue to be exactly same.
'B'. And finally the peer-group of 'C' is merged with the peer group
of 'A'.
2. 'A' is a private mount and 'B' is a shared mount. A new mount 'C'
2. 'A' is a private mount and 'B' is a shared mount. A new mount 'C'
which is clone of 'A', is created. Its root dentry is 'a'. 'C' is
mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
are created and mounted at the dentry 'b' on all mounts where 'B'
@ -379,7 +398,7 @@ replicas continue to be exactly same.
'C', 'C1', .., 'Cn' with exactly the same configuration as the
propagation tree for 'B'.
3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. A new
3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. A new
mount 'C' which is clone of 'A', is created. Its root dentry is 'a' .
'C' is mounted on mount 'B' at dentry 'b'. Also new mounts 'C1', 'C2',
'C3' ... are created and mounted at the dentry 'b' on all mounts where
@ -389,19 +408,19 @@ replicas continue to be exactly same.
is made the slave of mount 'Z'. In other words, mount 'C' is in the
state 'slave and shared'.
4. 'A' is a unbindable mount and 'B' is a shared mount. This is a
4. 'A' is a unbindable mount and 'B' is a shared mount. This is a
invalid operation.
5. 'A' is a private mount and 'B' is a non-shared(private or slave or
5. 'A' is a private mount and 'B' is a non-shared(private or slave or
unbindable) mount. A new mount 'C' which is clone of 'A', is created.
Its root dentry is 'a'. 'C' is mounted on mount 'B' at dentry 'b'.
6. 'A' is a shared mount and 'B' is a non-shared mount. A new mount 'C'
6. 'A' is a shared mount and 'B' is a non-shared mount. A new mount 'C'
which is a clone of 'A' is created. Its root dentry is 'a'. 'C' is
mounted on mount 'B' at dentry 'b'. 'C' is made a member of the
peer-group of 'A'.
7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount. A
7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount. A
new mount 'C' which is a clone of 'A' is created. Its root dentry is
'a'. 'C' is mounted on mount 'B' at dentry 'b'. Also 'C' is set as a
slave mount of 'Z'. In other words 'A' and 'C' are both slave mounts of
@ -409,7 +428,7 @@ replicas continue to be exactly same.
mount/unmount on 'A' do not propagate anywhere else. Similarly
mount/unmount on 'C' do not propagate anywhere else.
8. 'A' is a unbindable mount and 'B' is a non-shared mount. This is a
8. 'A' is a unbindable mount and 'B' is a non-shared mount. This is a
invalid operation. A unbindable mount cannot be bind mounted.
5c) Rbind semantics
@ -422,7 +441,9 @@ replicas continue to be exactly same.
then the subtree under the unbindable mount is pruned in the new
location.
eg: let's say we have the following mount tree.
eg:
let's say we have the following mount tree::
A
/ \
@ -430,12 +451,12 @@ replicas continue to be exactly same.
/ \ / \
D E F G
Let's say all the mount except the mount C in the tree are
of a type other than unbindable.
Let's say all the mount except the mount C in the tree are
of a type other than unbindable.
If this tree is rbound to say Z
If this tree is rbound to say Z
We will have the following tree at the new location.
We will have the following tree at the new location::
Z
|
@ -457,24 +478,26 @@ replicas continue to be exactly same.
the dentry in the destination mount.
The outcome depends on the type of the mount of 'A' and 'B'. The table
below is a quick reference.
---------------------------------------------------------------------------
| MOVE MOUNT OPERATION |
|**************************************************************************
| source(A)->| shared | private | slave | unbindable |
| dest(B) | | | | |
| | | | | | |
| v | | | | |
|**************************************************************************
| shared | shared | shared |shared and slave| invalid |
| | | | | |
|non-shared| shared | private | slave | unbindable |
***************************************************************************
NOTE: moving a mount residing under a shared mount is invalid.
below is a quick reference::
---------------------------------------------------------------------------
| MOVE MOUNT OPERATION |
|**************************************************************************
| source(A)->| shared | private | slave | unbindable |
| dest(B) | | | | |
| | | | | | |
| v | | | | |
|**************************************************************************
| shared | shared | shared |shared and slave| invalid |
| | | | | |
|non-shared| shared | private | slave | unbindable |
***************************************************************************
.. Note:: moving a mount residing under a shared mount is invalid.
Details follow:
1. 'A' is a shared mount and 'B' is a shared mount. The mount 'A' is
1. 'A' is a shared mount and 'B' is a shared mount. The mount 'A' is
mounted on mount 'B' at dentry 'b'. Also new mounts 'A1', 'A2'...'An'
are created and mounted at dentry 'b' on all mounts that receive
propagation from mount 'B'. A new propagation tree is created in the
@ -483,7 +506,7 @@ replicas continue to be exactly same.
propagation tree is appended to the already existing propagation tree
of 'A'.
2. 'A' is a private mount and 'B' is a shared mount. The mount 'A' is
2. 'A' is a private mount and 'B' is a shared mount. The mount 'A' is
mounted on mount 'B' at dentry 'b'. Also new mount 'A1', 'A2'... 'An'
are created and mounted at dentry 'b' on all mounts that receive
propagation from mount 'B'. The mount 'A' becomes a shared mount and a
@ -491,7 +514,7 @@ replicas continue to be exactly same.
'B'. This new propagation tree contains all the new mounts 'A1',
'A2'... 'An'.
3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. The
3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. The
mount 'A' is mounted on mount 'B' at dentry 'b'. Also new mounts 'A1',
'A2'... 'An' are created and mounted at dentry 'b' on all mounts that
receive propagation from mount 'B'. A new propagation tree is created
@ -501,32 +524,32 @@ replicas continue to be exactly same.
'A'. Mount 'A' continues to be the slave mount of 'Z' but it also
becomes 'shared'.
4. 'A' is a unbindable mount and 'B' is a shared mount. The operation
4. 'A' is a unbindable mount and 'B' is a shared mount. The operation
is invalid. Because mounting anything on the shared mount 'B' can
create new mounts that get mounted on the mounts that receive
propagation from 'B'. And since the mount 'A' is unbindable, cloning
it to mount at other mountpoints is not possible.
5. 'A' is a private mount and 'B' is a non-shared(private or slave or
5. 'A' is a private mount and 'B' is a non-shared(private or slave or
unbindable) mount. The mount 'A' is mounted on mount 'B' at dentry 'b'.
6. 'A' is a shared mount and 'B' is a non-shared mount. The mount 'A'
6. 'A' is a shared mount and 'B' is a non-shared mount. The mount 'A'
is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
shared mount.
7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount.
7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount.
The mount 'A' is mounted on mount 'B' at dentry 'b'. Mount 'A'
continues to be a slave mount of mount 'Z'.
8. 'A' is a unbindable mount and 'B' is a non-shared mount. The mount
8. 'A' is a unbindable mount and 'B' is a non-shared mount. The mount
'A' is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
unbindable mount.
5e) Mount semantics
Consider the following command
Consider the following command::
mount device B/b
mount device B/b
'B' is the destination mount and 'b' is the dentry in the destination
mount.
@ -537,9 +560,9 @@ replicas continue to be exactly same.
5f) Unmount semantics
Consider the following command
Consider the following command::
umount A
umount A
where 'A' is a mount mounted on mount 'B' at dentry 'b'.
@ -592,10 +615,12 @@ replicas continue to be exactly same.
A. What is the result of the following command sequence?
mount --bind /mnt /mnt
mount --make-shared /mnt
mount --bind /mnt /tmp
mount --move /tmp /mnt/1
::
mount --bind /mnt /mnt
mount --make-shared /mnt
mount --bind /mnt /tmp
mount --move /tmp /mnt/1
what should be the contents of /mnt /mnt/1 /mnt/1/1 should be?
Should they all be identical? or should /mnt and /mnt/1 be
@ -604,23 +629,27 @@ replicas continue to be exactly same.
B. What is the result of the following command sequence?
mount --make-rshared /
mkdir -p /v/1
mount --rbind / /v/1
::
mount --make-rshared /
mkdir -p /v/1
mount --rbind / /v/1
what should be the content of /v/1/v/1 be?
C. What is the result of the following command sequence?
mount --bind /mnt /mnt
mount --make-shared /mnt
mkdir -p /mnt/1/2/3 /mnt/1/test
mount --bind /mnt/1 /tmp
mount --make-slave /mnt
mount --make-shared /mnt
mount --bind /mnt/1/2 /tmp1
mount --make-slave /mnt
::
mount --bind /mnt /mnt
mount --make-shared /mnt
mkdir -p /mnt/1/2/3 /mnt/1/test
mount --bind /mnt/1 /tmp
mount --make-slave /mnt
mount --make-shared /mnt
mount --bind /mnt/1/2 /tmp1
mount --make-slave /mnt
At this point we have the first mount at /tmp and
its root dentry is 1. Let's call this mount 'A'
@ -668,7 +697,8 @@ replicas continue to be exactly same.
step 1:
let's say the root tree has just two directories with
one vfsmount.
one vfsmount::
root
/ \
tmp usr
@ -676,14 +706,17 @@ replicas continue to be exactly same.
And we want to replicate the tree at multiple
mountpoints under /root/tmp
step2:
mount --make-shared /root
step 2:
::
mkdir -p /tmp/m1
mount --rbind /root /tmp/m1
mount --make-shared /root
the new tree now looks like this:
mkdir -p /tmp/m1
mount --rbind /root /tmp/m1
the new tree now looks like this::
root
/ \
@ -697,11 +730,13 @@ replicas continue to be exactly same.
it has two vfsmounts
step3:
step 3:
::
mkdir -p /tmp/m2
mount --rbind /root /tmp/m2
the new tree now looks like this:
the new tree now looks like this::
root
/ \
@ -724,6 +759,7 @@ replicas continue to be exactly same.
it has 6 vfsmounts
step 4:
::
mkdir -p /tmp/m3
mount --rbind /root /tmp/m3
@ -740,7 +776,8 @@ replicas continue to be exactly same.
step 1:
let's say the root tree has just two directories with
one vfsmount.
one vfsmount::
root
/ \
tmp usr
@ -748,17 +785,20 @@ replicas continue to be exactly same.
How do we set up the same tree at multiple locations under
/root/tmp
step2:
mount --bind /root/tmp /root/tmp
step 2:
::
mount --make-rshared /root
mount --make-unbindable /root/tmp
mkdir -p /tmp/m1
mount --bind /root/tmp /root/tmp
mount --rbind /root /tmp/m1
mount --make-rshared /root
mount --make-unbindable /root/tmp
the new tree now looks like this:
mkdir -p /tmp/m1
mount --rbind /root /tmp/m1
the new tree now looks like this::
root
/ \
@ -768,11 +808,13 @@ replicas continue to be exactly same.
/ \
tmp usr
step3:
step 3:
::
mkdir -p /tmp/m2
mount --rbind /root /tmp/m2
the new tree now looks like this:
the new tree now looks like this::
root
/ \
@ -782,12 +824,13 @@ replicas continue to be exactly same.
/ \ / \
tmp usr tmp usr
step4:
step 4:
::
mkdir -p /tmp/m3
mount --rbind /root /tmp/m3
the new tree now looks like this:
the new tree now looks like this::
root
/ \
@ -801,25 +844,31 @@ replicas continue to be exactly same.
8A) Datastructure
4 new fields are introduced to struct vfsmount
->mnt_share
->mnt_slave_list
->mnt_slave
->mnt_master
4 new fields are introduced to struct vfsmount:
->mnt_share links together all the mount to/from which this vfsmount
* ->mnt_share
* ->mnt_slave_list
* ->mnt_slave
* ->mnt_master
->mnt_share
links together all the mount to/from which this vfsmount
send/receives propagation events.
->mnt_slave_list links all the mounts to which this vfsmount propagates
->mnt_slave_list
links all the mounts to which this vfsmount propagates
to.
->mnt_slave links together all the slaves that its master vfsmount
->mnt_slave
links together all the slaves that its master vfsmount
propagates to.
->mnt_master points to the master vfsmount from which this vfsmount
->mnt_master
points to the master vfsmount from which this vfsmount
receives propagation.
->mnt_flags takes two more flags to indicate the propagation status of
->mnt_flags
takes two more flags to indicate the propagation status of
the vfsmount. MNT_SHARE indicates that the vfsmount is a shared
vfsmount. MNT_UNCLONABLE indicates that the vfsmount cannot be
replicated.
@ -842,7 +891,7 @@ replicas continue to be exactly same.
A example propagation tree looks as shown in the figure below.
[ NOTE: Though it looks like a forest, if we consider all the shared
mounts as a conceptual entity called 'pnode', it becomes a tree]
mounts as a conceptual entity called 'pnode', it becomes a tree]::
A <--> B <--> C <---> D
@ -864,14 +913,19 @@ replicas continue to be exactly same.
A's ->mnt_slave_list links with ->mnt_slave of 'E', 'K', 'F' and 'G'
E's ->mnt_share links with ->mnt_share of K
'E', 'K', 'F', 'G' have their ->mnt_master point to struct
vfsmount of 'A'
'E', 'K', 'F', 'G' have their ->mnt_master point to struct vfsmount of 'A'
'M', 'L', 'N' have their ->mnt_master point to struct vfsmount of 'K'
K's ->mnt_slave_list links with ->mnt_slave of 'M', 'L' and 'N'
C's ->mnt_slave_list links with ->mnt_slave of 'J' and 'K'
J and K's ->mnt_master points to struct vfsmount of C
and finally D's ->mnt_slave_list links with ->mnt_slave of 'H' and 'I'
'H' and 'I' have their ->mnt_master pointing to struct vfsmount of 'D'.
@ -903,6 +957,7 @@ replicas continue to be exactly same.
Prepare phase:
for each mount in the source tree:
a) Create the necessary number of mount trees to
be attached to each of the mounts that receive
propagation from the destination mount.
@ -929,11 +984,12 @@ replicas continue to be exactly same.
Abort phase
delete all the newly created trees.
NOTE: all the propagation related functionality resides in the file
pnode.c
.. Note::
all the propagation related functionality resides in the file pnode.c
------------------------------------------------------------------------
version 0.1 (created the initial document, Ram Pai linuxram@us.ibm.com)
version 0.2 (Incorporated comments from Al Viro)