aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/filesystems/sharedsubtree.txt
blob: 736540045dc7b7c9e448fb90bd72fc63346e082d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
Shared Subtrees
---------------

Contents:
	1) Overview
	2) Features
	3) smount command
	4) Use-case
	5) Detailed semantics
	6) Quiz
	7) FAQ
	8) Implementation


1) Overview
-----------

Consider the following situation:

A process wants to clone its own namespace, but still wants to access the CD
that got mounted recently.  Shared subtree semantics provide the necessary
mechanism to accomplish the above.

It provides the necessary building blocks for features like per-user-namespace
and versioned filesystem.

2) Features
-----------

Shared subtree provides four different flavors of mounts; struct vfsmount to be
precise

	a. shared mount
	b. slave mount
	c. private mount
	d. unbindable mount


2a) A shared mount can be replicated to as many mountpoints and all the
replicas continue to be exactly same.

	Here is an example:

	Lets say /mnt has a mount that is shared.
	mount --make-shared /mnt

	note: mount command does not yet support the --make-shared flag.
	I have included a small C program which does the same by executing
	'smount /mnt shared'

	#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 lets say we mount a device at /tmp/a
	#mount /dev/sd0  /tmp/a

	#ls /tmp/a
	t1 t2 t2

	#ls /mnt/a
	t1 t2 t2

	Note that the mount has propagated to the mount at /mnt as well.

	And the same is true even when /dev/sd0 is mounted on /mnt/a. The
	contents will be visible under /tmp/a too.


2b) A slave mount is like a shared mount except that mount and umount events
	only propagate towards it.

	All slave mounts have a master mount which is a shared.

	Here is an example:

	Lets say /mnt has a mount which is shared.
	#mount --make-shared /mnt

	Lets bind mount /mnt to /tmp
	#mount --bind /mnt /tmp

	the new mount at /tmp becomes a shared mount and it is a replica of
	the mount at /mnt.

	Now lets make the mount at /tmp; a slave of /mnt
	#mount --make-slave /tmp
	[or smount /tmp slave]

	lets mount /dev/sd0 on /mnt/a
	#mount /dev/sd0 /mnt/a

	#ls /mnt/a
	t1 t2 t3

	#ls /tmp/a
	t1 t2 t3

	Note the mount event has propagated to the mount at /tmp

	However lets see what happens if we mount something on the mount at /tmp

	#mount /dev/sd1 /tmp/b

	#ls /tmp/b
	s1 s2 s3

	#ls /mnt/b

	Note how the mount event has not propagated to the mount at
	/mnt


2c) A private mount does not forward or receive propagation.

	This is the mount we are familiar with. Its the default type.


2d) A unbindable mount is a unbindable private mount

	lets say we have a mount at /mnt and we make is unbindable

	#mount --make-unbindable /mnt
	 [ smount /mnt  unbindable ]

	 Lets 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.


3) smount command

	Currently the mount command is not aware of shared subtree features.
	Work is in progress to add the support in mount ( util-linux package ).
	Till then use the following program.

	------------------------------------------------------------------------
	//
	//this code was developed my Miklos Szeredi <miklos@szeredi.hu>
	//and modified by Ram Pai <linuxram@us.ibm.com>
	// sample usage:
	//              smount /tmp shared
	//
	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <string.h>
	#include <sys/mount.h>
	#include <sys/fsuid.h>

	#ifndef MS_REC
	#define MS_REC		0x4000	/* 16384: Recursive loopback */
	#endif

	#ifndef MS_SHARED
	#define MS_SHARED		1<<20	/* Shared */
	#endif

	#ifndef MS_PRIVATE
	#define MS_PRIVATE		1<<18	/* Private */
	#endif

	#ifndef MS_SLAVE
	#define MS_SLAVE		1<<19	/* Slave */
	#endif

	#ifndef MS_UNBINDABLE
	#define MS_UNBINDABLE		1<<17	/* Unbindable */
	#endif

	int main(int argc, char *argv[])
	{
		int type;
		if(argc != 3) {
			fprintf(stderr, "usage: %s dir "
			"<rshared|rslave|rprivate|runbindable|shared|slave"
			"|private|unbindable>\n" , argv[0]);
			return 1;
		}

		fprintf(stdout, "%s %s %s\n", argv[0], argv[1], argv[2]);

		if (strcmp(argv[2],"rshared")==0)
			type=(MS_SHARED|MS_REC);
		else if (strcmp(argv[2],"rslave")==0)
			type=(MS_SLAVE|MS_REC);
		else if (strcmp(argv[2],"rprivate")==0)
			type=(MS_PRIVATE|MS_REC);
		else if (strcmp(argv[2],"runbindable")==0)
			type=(MS_UNBINDABLE|MS_REC);
		else if (strcmp(argv[2],"shared")==0)
			type=MS_SHARED;
		else if (strcmp(argv[2],"slave")==0)
			type=MS_SLAVE;
		else if (strcmp(argv[2],"private")==0)
			type=MS_PRIVATE;
		else if (strcmp(argv[2],"unbindable")==0)
			type=MS_UNBINDABLE;
		else {
			fprintf(stderr, "invalid operation: %s\n", argv[2]);
			return 1;
		}
		setfsuid(getuid());

		if(mount("", argv[1], "dontcare", type, "") == -1) {
			perror("mount");
			return 1;
		}
		return 0;
	}
	-----------------------------------------------------------------------

	Copy the above code snippet into smount.c
	gcc -o smount smount.c


	(i) To mark all the mounts under /mnt as shared execute the following
	command:

	 	smount /mnt rshared
		the corresponding syntax planned for mount command is
		mount --make-rshared /mnt

	    just to mark a mount /mnt as shared, execute the following
	    command:
	 	smount /mnt shared
		the corresponding syntax planned for mount command is
		mount --make-shared /mnt

	(ii) To mark all the shared mounts under /mnt as slave execute the
	following

	     command:
		smount /mnt rslave
		the corresponding syntax planned for mount command is
		mount --make-rslave /mnt

	    just to mark a mount /mnt as slave, execute the following
	    command:
	 	smount /mnt slave
		the corresponding syntax planned for mount command is
		mount --make-slave /mnt

	(iii) To mark all the mounts under /mnt as private execute the
	following command:

		smount /mnt rprivate
		the corresponding syntax planned for mount command is
		mount --make-rprivate /mnt

	    just to mark a mount /mnt as private, execute the following
	    command:
	 	smount /mnt private
		the corresponding syntax planned for mount command is
		mount --make-private /mnt

	      NOTE: by default all the mounts are created as private. But if
	      you want to change some shared/slave/unbindable  mount as
	      private at a later point in time, this command can help.

	(iv) To mark all the mounts under /mnt as unbindable execute the
	following

	     command:
		smount /mnt runbindable
		the corresponding syntax planned for mount command is
		mount --make-runbindable /mnt

	    just to mark a mount /mnt as unbindable, execute the following
	    command:
	 	smount /mnt unbindable
		the corresponding syntax planned for mount command is
		mount --make-unbindable /mnt


4) Use cases
------------

	A) A process wants to clone its own namespace, but still wants to
	   access the CD that got mounted recently.

	   Solution:

		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
		parent namespace.

		So when a CD is inserted and mounted at /cdrom that mount gets
		propagated to the other mount at /cdrom in all the other clone
		namespaces.

	B) A process wants its mounts invisible to any other process, but
	still be able to see the other system mounts.

	   Solution:

		To begin with, the administrator can mark the entire mount tree
		as shareable.

		mount --make-rshared /

		A new process can clone off a new namespace. And mark some part
		of its namespace as slave

		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
		done in the parent namespace under /myprivatetree still shows
		up in the process's namespace.


	Apart from the above semantics this feature provides the
	building blocks to solve the following problems:

	C)  Per-user namespace

		The above semantics allows a way to share mounts across
		namespaces.  But namespaces are associated with processes. If
		namespaces are made first class objects with user API to
		associate/disassociate a namespace with userid, then each user
		could have his/her own namespace and tailor it to his/her
		requirements. Offcourse its needs support from PAM.

	D)  Versioned files

		If the entire mount tree is visible at multiple locations, then
		a underlying versioning file system can return different
		version of the file depending on the path used to access that
		file.

		An example is:

		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, than that
		mount appears at /view/v1/usr, /view/v2/usr, /view/v3/usr and
		/view/v4/usr too

		A user can request v3 version of the file /usr/fs/namespace.c
		by accessing /view/v3/usr/fs/namespace.c . The underlying
		versioning filesystem can then decipher that v3 version of the
		filesystem is being requested and return the corresponding
		inode.

5) Detailed semantics:
-------------------
	The section below explains the detailed semantics of
	bind, rbind, move, mount, umount and clone-namespace operations.

	Note: the word 'vfsmount' and the noun 'mount' have been used
	to mean the same thing, throughout this document.

5a) Mount states

	A given mount can be in one of the following states
	1) shared
	2) slave
	3) shared and slave
	4) private
	5) unbindable

	A 'propagation event' is defined as event generated on a vfsmount
	that leads to mount or unmount actions in other vfsmounts.

	A 'peer group' is defined as a group of vfsmounts that propagate
	events to each other.

	(1) Shared mounts

		A 'shared mount' is defined as a vfsmount that belongs to a
		'peer group'.

		For example:
			mount --make-shared /mnt
			mount --bin /mnt /tmp

		The mount at /mnt and that at /tmp are both shared and belong
		to the same peer group. Anything mounted or unmounted under
		/mnt or /tmp reflect in all the other mounts of its peer
		group.


	(2) Slave mounts

		A 'slave mount' is defined as a vfsmount that receives
		propagation events and does not forward propagation events.

		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

			mount --make-slave mount

		A shared mount that is made as a slave is no more shared unless
		modified to become shared.

	(3) Shared and Slave

		A vfsmount can be both shared as well as slave.  This state
		indicates that the mount is a slave of some vfsmount, and
		has its own peer group too.  This vfsmount receives propagation
		events from its master vfsmount, and also forwards propagation
		events to its 'peer group' and to its slave vfsmounts.

		Strictly speaking, the vfsmount is shared having its own
		peer group, and this peer-group is a slave of some other
		peer group.

		Only a slave vfsmount can be made as 'shared and slave' by
		either executing the following command
			mount --make-shared mount
		or by moving the slave vfsmount under a shared vfsmount.

	(4) Private mount

		A 'private mount' is defined as vfsmount that does not
		receive or forward any propagation events.

	(5) Unbindable mount

		A 'unbindable mount' is defined as vfsmount that does not
		receive or forward any propagation events and cannot
		be bind mounted.


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

	* 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
	explained in section 5d.

5b) Bind semantics

	Consider the following command

	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   |
   ***************************************************************************

     	Details:

	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'
	propagates to. A new propagation tree containing 'C1',..,'Cn' is
	created. This propagation tree is identical to the propagation tree of
	'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'
	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'
	propagates to. A new propagation tree is set containing all new mounts
	'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
	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
	'B' propagates to. A new propagation tree containing the new mounts
	'C','C1',..  'Cn' is created. This propagation tree is identical to the
	propagation tree for 'B'. And finally the mount 'C' and its peer group
	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
	invalid operation.

	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'
	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
	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
	'Z'.  All mount/unmount events on 'Z' propagates to 'A' and 'C'. But
	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
	invalid operation. A unbindable mount cannot be bind mounted.

5c) Rbind semantics

	rbind is same as bind. Bind replicates the specified mount.  Rbind
	replicates all the mounts in the tree belonging to the specified mount.
	Rbind mount is bind mount applied to all the mounts in the tree.

	If the source tree that is rbind has some unbindable mounts,
	then the subtree under the unbindable mount is pruned in the new
	location.

	eg: lets say we have the following mount tree.

		A
	      /   \
	      B   C
	     / \ / \
	     D E F G

	     Lets 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

	     We will have the following tree at the new location.

		Z
		|
		A'
	       /
	      B'		Note how the tree under C is pruned
	     / \ 		in the new location.
	    D' E'



5d) Move semantics

	Consider the following command

	mount --move A  B/b

	where 'A' is the source mount, 'B' is the destination mount and 'b' is
	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.

      Details follow:

	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
	exact same configuration as that of 'B'. This new propagation tree
	contains all the new mounts 'A1', 'A2'...  'An'.  And this new
	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
	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
	propagation tree is created which is identical to that of
	'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
	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 exact same configuration as that of 'B'. This new propagation
	tree contains all the new mounts 'A1', 'A2'...  'An'.  And this new
	propagation tree is appended to the already existing propagation tree of
	'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
	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
	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'
	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.
	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
	'A' is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
	unbindable mount.

5e) Mount semantics

	Consider the following command

	mount device  B/b

	'B' is the destination mount and 'b' is the dentry in the destination
	mount.

	The above operation is the same as bind operation with the exception
	that the source mount is always a private mount.


5f) Unmount semantics

	Consider the following command

	umount A

	where 'A' is a mount mounted on mount 'B' at dentry 'b'.

	If mount 'B' is shared, then all most-recently-mounted mounts at dentry
	'b' on mounts that receive propagation from mount 'B' and does not have
	sub-mounts within them are unmounted.

	Example: Lets say 'B1', 'B2', 'B3' are shared mounts that propagate to
	each other.

	lets say 'A1', 'A2', 'A3' are first mounted at dentry 'b' on mount
	'B1', 'B2' and 'B3' respectively.

	lets say 'C1', 'C2', 'C3' are next mounted at the same dentry 'b' on
	mount 'B1', 'B2' and 'B3' respectively.

	if 'C1' is unmounted, all the mounts that are most-recently-mounted on
	'B1' and on the mounts that 'B1' propagates-to are unmounted.

	'B1' propagates to 'B2' and 'B3'. And the most recently mounted mount
	on 'B2' at dentry 'b' is 'C2', and that of mount 'B3' is 'C3'.

	So all 'C1', 'C2' and 'C3' should be unmounted.

	If any of 'C2' or 'C3' has some child mounts, then that mount is not
	unmounted, but all other mounts are unmounted. However if 'C1' is told
	to be unmounted and 'C1' has some sub-mounts, the umount operation is
	failed entirely.

5g) Clone Namespace

	A cloned namespace contains all the mounts as that of the parent
	namespace.

	Lets say 'A' and 'B' are the corresponding mounts in the parent and the
	child namespace.

	If 'A' is shared, then 'B' is also shared and 'A' and 'B' propagate to
	each other.

	If 'A' is a slave mount of 'Z', then 'B' is also the slave mount of
	'Z'.

	If 'A' is a private mount, then 'B' is a private mount too.

	If 'A' is unbindable mount, then 'B' is a unbindable mount too.


6) Quiz

	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

		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
		identical only?


	B. What is the result of the following command sequence?

		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

		At this point we have the first mount at /tmp and
		its root dentry is 1. Lets call this mount 'A'
		And then we have a second mount at /tmp1 with root
		dentry 2. Lets call this mount 'B'
		Next we have a third mount at /mnt with root dentry
		mnt. Lets call this mount 'C'

		'B' is the slave of 'A' and 'C' is a slave of 'B'
		A -> B -> C

		at this point if we execute the following command

		mount --bind /bin /tmp/test

		The mount is attempted on 'A'

		will the mount propagate to 'B' and 'C' ?

		what would be the contents of
		/mnt/1/test be?

7) FAQ

	Q1. Why is bind mount needed? How is it different from symbolic links?
		symbolic links can get stale if the destination mount gets
		unmounted or moved. Bind mounts continue to exist even if the
		other mount is unmounted or moved.

	Q2. Why can't the shared subtree be implemented using exportfs?

		exportfs is a heavyweight way of accomplishing part of what
		shared subtree can do. I cannot imagine a way to implement the
		semantics of slave mount using exportfs?

	Q3 Why is unbindable mount needed?

		Lets say we want to replicate the mount tree at multiple
		locations within the same subtree.

		if one rbind mounts a tree within the same subtree 'n' times
		the number of mounts created is an exponential function of 'n'.
		Having unbindable mount can help prune the unneeded bind
		mounts. Here is a example.

		step 1:
		   lets say the root tree has just two directories with
		   one vfsmount.
				    root
				   /    \
				  tmp    usr

		    And we want to replicate the tree at multiple
		    mountpoints under /root/tmp

		step2:
		      mount --make-shared /root

		      mkdir -p /tmp/m1

		      mount --rbind /root /tmp/m1

		      the new tree now looks like this:

				    root
				   /    \
				 tmp    usr
				/
			       m1
			      /  \
			     tmp  usr
			     /
			    m1

			  it has two vfsmounts

		step3:
			    mkdir -p /tmp/m2
			    mount --rbind /root /tmp/m2

			the new tree now looks like this:

				      root
				     /    \
				   tmp     usr
				  /    \
				m1       m2
			       / \       /  \
			     tmp  usr   tmp  usr
			     / \          /
			    m1  m2      m1
				/ \     /  \
			      tmp usr  tmp   usr
			      /        / \
			     m1       m1  m2
			    /  \
			  tmp   usr
			  /  \
			 m1   m2

		       it has 6 vfsmounts

		step 4:
			  mkdir -p /tmp/m3
			  mount --rbind /root /tmp/m3

			  I wont' draw the tree..but it has 24 vfsmounts


		at step i the number of vfsmounts is V[i] = i*V[i-1].
		This is an exponential function. And this tree has way more
		mounts than what we really needed in the first place.

		One could use a series of umount at each step to prune
		out the unneeded mounts. But there is a better solution.
		Unclonable mounts come in handy here.

		step 1:
		   lets say the root tree has just two directories with
		   one vfsmount.
				    root
				   /    \
				  tmp    usr

		    How do we set up the same tree at multiple locations under
		    /root/tmp

		step2:
		      mount --bind /root/tmp /root/tmp

		      mount --make-rshared /root
		      mount --make-unbindable /root/tmp

		      mkdir -p /tmp/m1

		      mount --rbind /root /tmp/m1

		      the new tree now looks like this:

				    root
				   /    \
				 tmp    usr
				/
			       m1
			      /  \
			     tmp  usr

		step3:
			    mkdir -p /tmp/m2
			    mount --rbind /root /tmp/m2

		      the new tree now looks like this:

				    root
				   /    \
				 tmp    usr
				/   \
			       m1     m2
			      /  \     / \
			     tmp  usr tmp usr

		step4:

			    mkdir -p /tmp/m3
			    mount --rbind /root /tmp/m3

		      the new tree now looks like this:

				    	  root
				      /    	  \
				     tmp    	   usr
			         /    \    \
			       m1     m2     m3
			      /  \     / \    /  \
			     tmp  usr tmp usr tmp usr

8) Implementation

8A) Datastructure

	4 new fields are introduced to struct 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
		to.

	->mnt_slave links together all the slaves that its master vfsmount
		propagates to.

	->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
		the vfsmount.  MNT_SHARE indicates that the vfsmount is a shared
		vfsmount.  MNT_UNCLONABLE indicates that the vfsmount cannot be
		replicated.

	All the shared vfsmounts in a peer group form a cyclic list through
	->mnt_share.

	All vfsmounts with the same ->mnt_master form on a cyclic list anchored
	in ->mnt_master->mnt_slave_list and going through ->mnt_slave.

	 ->mnt_master can point to arbitrary (and possibly different) members
	 of master peer group.  To find all immediate slaves of a peer group
	 you need to go through _all_ ->mnt_slave_list of its members.
	 Conceptually it's just a single set - distribution among the
	 individual lists does not affect propagation or the way propagation
	 tree is modified by operations.

	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]


		        A <--> B <--> C <---> D
		       /|\	      /|      |\
		      / F G	     J K      H I
		     /
		    E<-->K
			/|\
		       M L N

	In the above figure  A,B,C and D all are shared and propagate to each
	other.   'A' has got 3 slave mounts 'E' 'F' and 'G' 'C' has got 2 slave
	mounts 'J' and 'K'  and  'D' has got two slave mounts 'H' and 'I'.
	'E' is also shared with 'K' and they propagate to each other.  And
	'K' has 3 slaves 'M', 'L' and 'N'

	A's ->mnt_share links with the ->mnt_share of 'B' 'C' and 'D'

	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'
	'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'.


	NOTE: The propagation tree is orthogonal to the mount tree.


8B Algorithm:

	The crux of the implementation resides in rbind/move operation.

	The overall algorithm breaks the operation into 3 phases: (look at
	attach_recursive_mnt() and propagate_mnt())

	1. prepare phase.
	2. commit phases.
	3. abort phases.

	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.
		   b) Do not attach any of the trees to its destination.
		      However note down its ->mnt_parent and ->mnt_mountpoint
		   c) Link all the new mounts to form a propagation tree that
		      is identical to the propagation tree of the destination
		      mount.

		   If this phase is successful, there should be 'n' new
		   propagation trees; where 'n' is the number of mounts in the
		   source tree.  Go to the commit phase

		   Also there should be 'm' new mount trees, where 'm' is
		   the number of mounts to which the destination mount
		   propagates to.

		   if any memory allocations fail, go to the abort phase.

	Commit phase
		attach each of the mount trees to their corresponding
		destination mounts.

	Abort phase
		delete all the newly created trees.

	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)