Commit cf06612c authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab Committed by Jonathan Corbet
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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: default avatarMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6692b8abc177130e9e53aace94117a2ad076cab5.1588021877.git.mchehab+huawei@kernel.org


Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 53a41d3e

Documentation/filesystems/index.rst

+1 −0
Original line number Diff line number Diff line
@@ -33,6 +33,7 @@ algorithms work. 
   mount_api

   quota

   seq_file

   sharedsubtree



   automount-support

Documentation/filesystems/proc.rst

+1 −1
Original line number Diff line number Diff line
@@ -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

Documentation/filesystems/sharedsubtree.txt→Documentation/filesystems/sharedsubtree.rst

+227 −171
Original line number Diff line number Diff line 
.. SPDX-License-Identifier: GPL-2.0



===============

Shared Subtrees

---------------

===============



Contents:

.. Contents:

	1) Overview

	2) Features

	3) Setting mount states
@@ -41,24 +44,31 @@ replicas continue to be exactly same. 


	Here is an example:



	Let's say /mnt has a mount that is shared.

	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



	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



	Now let's say we mount a device at /tmp/a

	Now let's say we mount a device at /tmp/a::



	    # mount /dev/sd0  /tmp/a



	    #ls /tmp/a
@@ -123,11 +133,12 @@ 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



	 Let's try to bind mount this mount somewhere else.

	 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
@@ -138,7 +149,7 @@ 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
@@ -154,7 +165,8 @@ replicas continue to be exactly same. 


	   Solution:



		The system administrator can make the mount at /cdrom shared

		The system administrator can make the mount at /cdrom shared::



		    mount --bind /cdrom /cdrom

		    mount --make-shared /cdrom
@@ -172,12 +184,12 @@ 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 /



		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
@@ -206,7 +218,7 @@ 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
@@ -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
@@ -307,9 +323,11 @@ replicas continue to be exactly same. 




   	State diagram:



   	The state diagram below explains the state transition of a mount,

	in response to various commands.

	------------------------------------------------------------------------

	in response to various commands::



	    -----------------------------------------------------------------------

	    |             |make-shared |  make-slave  | make-private |make-unbindab|

	    --------------|------------|--------------|--------------|-------------|

	    |shared	  |shared      |*slave/private|   private    | unbindable  |
@@ -338,7 +356,7 @@ replicas continue to be exactly same. 


5b) Bind semantics



	Consider the following command

	Consider the following command::



	    mount --bind A/a  B/b
@@ -346,19 +364,20 @@ replicas continue to be exactly same. 
	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.

   ---------------------------------------------------------------------------

	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:
@@ -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

	      /   \
@@ -435,7 +456,7 @@ replicas continue to be exactly same. 


	  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

		|
@@ -457,7 +478,8 @@ 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.

	below is a quick reference::



	    ---------------------------------------------------------------------------

	    |         		MOVE MOUNT OPERATION                                 |

	    |**************************************************************************
@@ -470,7 +492,8 @@ replicas continue to be exactly same. 
	    |          |               |                |                |            |

	    |non-shared| shared        |      private   |    slave       | unbindable |

	    ***************************************************************************

	NOTE: moving a mount residing under a shared mount is invalid.



	.. Note:: moving a mount residing under a shared mount is invalid.



      Details follow:
@@ -524,7 +547,7 @@ replicas continue to be exactly same. 


5e) Mount semantics



	Consider the following command

	Consider the following command::



	    mount device  B/b
@@ -537,7 +560,7 @@ replicas continue to be exactly same. 


5f) Unmount semantics



	Consider the following command

	Consider the following command::



	    umount A
@@ -592,6 +615,8 @@ 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
@@ -604,6 +629,8 @@ 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
@@ -613,6 +640,8 @@ replicas continue to be exactly same. 


	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
@@ -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
@@ -677,13 +707,16 @@ replicas continue to be exactly same. 
		    mountpoints under /root/tmp



		step 2:

		      ::





			mount --make-shared /root



			mkdir -p /tmp/m1



			mount --rbind /root /tmp/m1



		      the new tree now looks like this:

		      the new tree now looks like this::



				    root

				   /    \
@@ -698,10 +731,12 @@ replicas continue to be exactly same. 
			  it has two vfsmounts



		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
@@ -749,6 +786,9 @@ replicas continue to be exactly same. 
		    /root/tmp



		step 2:

		      ::





			mount --bind /root/tmp /root/tmp



			mount --make-rshared /root
@@ -758,7 +798,7 @@ replicas continue to be exactly same. 


			mount --rbind /root /tmp/m1



		      the new tree now looks like this:

		      the new tree now looks like this::



				    root

				   /    \
@@ -769,10 +809,12 @@ replicas continue to be exactly same. 
			     tmp  usr



		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

				   /    \
@@ -783,11 +825,12 @@ replicas continue to be exactly same. 
			     tmp  usr tmp usr



		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)