| blob | ccf1cebe744f8b4a6901ff7f61bd23029562344a |
1 Shared Subtrees
2 ---------------
4 Contents:
5 1) Overview
6 2) Features
7 3) smount command
8 4) Use-case
9 5) Detailed semantics
10 6) Quiz
11 7) FAQ
12 8) Implementation
15 1) Overview
16 -----------
18 Consider the following situation:
20 A process wants to clone its own namespace, but still wants to access the CD
21 that got mounted recently. Shared subtree semantics provide the necessary
22 mechanism to accomplish the above.
24 It provides the necessary building blocks for features like per-user-namespace
25 and versioned filesystem.
27 2) Features
28 -----------
30 Shared subtree provides four different flavors of mounts; struct vfsmount to be
31 precise
33 a. shared mount
34 b. slave mount
35 c. private mount
36 d. unbindable mount
39 2a) A shared mount can be replicated to as many mountpoints and all the
40 replicas continue to be exactly same.
42 Here is an example:
44 Lets say /mnt has a mount that is shared.
45 mount --make-shared /mnt
47 note: mount command does not yet support the --make-shared flag.
48 I have included a small C program which does the same by executing
49 'smount /mnt shared'
51 #mount --bind /mnt /tmp
52 The above command replicates the mount at /mnt to the mountpoint /tmp
53 and the contents of both the mounts remain identical.
55 #ls /mnt
56 a b c
58 #ls /tmp
59 a b c
61 Now lets say we mount a device at /tmp/a
62 #mount /dev/sd0 /tmp/a
64 #ls /tmp/a
65 t1 t2 t2
67 #ls /mnt/a
68 t1 t2 t2
70 Note that the mount has propagated to the mount at /mnt as well.
72 And the same is true even when /dev/sd0 is mounted on /mnt/a. The
73 contents will be visible under /tmp/a too.
76 2b) A slave mount is like a shared mount except that mount and umount events
77 only propagate towards it.
79 All slave mounts have a master mount which is a shared.
81 Here is an example:
83 Lets say /mnt has a mount which is shared.
84 #mount --make-shared /mnt
86 Lets bind mount /mnt to /tmp
87 #mount --bind /mnt /tmp
89 the new mount at /tmp becomes a shared mount and it is a replica of
90 the mount at /mnt.
92 Now lets make the mount at /tmp; a slave of /mnt
93 #mount --make-slave /tmp
94 [or smount /tmp slave]
96 lets mount /dev/sd0 on /mnt/a
97 #mount /dev/sd0 /mnt/a
99 #ls /mnt/a
100 t1 t2 t3
102 #ls /tmp/a
103 t1 t2 t3
105 Note the mount event has propagated to the mount at /tmp
107 However lets see what happens if we mount something on the mount at /tmp
109 #mount /dev/sd1 /tmp/b
111 #ls /tmp/b
112 s1 s2 s3
114 #ls /mnt/b
116 Note how the mount event has not propagated to the mount at
117 /mnt
120 2c) A private mount does not forward or receive propagation.
122 This is the mount we are familiar with. Its the default type.
125 2d) A unbindable mount is a unbindable private mount
127 lets say we have a mount at /mnt and we make is unbindable
129 #mount --make-unbindable /mnt
130 [ smount /mnt unbindable ]
132 Lets try to bind mount this mount somewhere else.
133 # mount --bind /mnt /tmp
134 mount: wrong fs type, bad option, bad superblock on /mnt,
135 or too many mounted file systems
137 Binding a unbindable mount is a invalid operation.
140 3) smount command
142 Currently the mount command is not aware of shared subtree features.
143 Work is in progress to add the support in mount ( util-linux package ).
144 Till then use the following program.
146 ------------------------------------------------------------------------
147 //
148 //this code was developed my Miklos Szeredi <miklos@szeredi.hu>
149 //and modified by Ram Pai <linuxram@us.ibm.com>
150 // sample usage:
151 // smount /tmp shared
152 //
153 #include <stdio.h>
154 #include <stdlib.h>
155 #include <unistd.h>
156 #include <sys/mount.h>
157 #include <sys/fsuid.h>
159 #ifndef MS_REC
160 #define MS_REC 0x4000 /* 16384: Recursive loopback */
161 #endif
163 #ifndef MS_SHARED
164 #define MS_SHARED 1<<20 /* Shared */
165 #endif
167 #ifndef MS_PRIVATE
168 #define MS_PRIVATE 1<<18 /* Private */
169 #endif
171 #ifndef MS_SLAVE
172 #define MS_SLAVE 1<<19 /* Slave */
173 #endif
175 #ifndef MS_UNBINDABLE
176 #define MS_UNBINDABLE 1<<17 /* Unbindable */
177 #endif
179 int main(int argc, char *argv[])
180 {
181 int type;
182 if(argc != 3) {
183 fprintf(stderr, "usage: %s dir "
184 "<rshared|rslave|rprivate|runbindable|shared|slave"
185 "|private|unbindable>\n" , argv[0]);
186 return 1;
187 }
189 fprintf(stdout, "%s %s %s\n", argv[0], argv[1], argv[2]);
191 if (strcmp(argv[2],"rshared")==0)
192 type=(MS_SHARED|MS_REC);
193 else if (strcmp(argv[2],"rslave")==0)
194 type=(MS_SLAVE|MS_REC);
195 else if (strcmp(argv[2],"rprivate")==0)
196 type=(MS_PRIVATE|MS_REC);
197 else if (strcmp(argv[2],"runbindable")==0)
198 type=(MS_UNBINDABLE|MS_REC);
199 else if (strcmp(argv[2],"shared")==0)
200 type=MS_SHARED;
201 else if (strcmp(argv[2],"slave")==0)
202 type=MS_SLAVE;
203 else if (strcmp(argv[2],"private")==0)
204 type=MS_PRIVATE;
205 else if (strcmp(argv[2],"unbindable")==0)
206 type=MS_UNBINDABLE;
207 else {
208 fprintf(stderr, "invalid operation: %s\n", argv[2]);
209 return 1;
210 }
211 setfsuid(getuid());
213 if(mount("", argv[1], "dontcare", type, "") == -1) {
214 perror("mount");
215 return 1;
216 }
217 return 0;
218 }
219 -----------------------------------------------------------------------
221 Copy the above code snippet into smount.c
222 gcc -o smount smount.c
225 (i) To mark all the mounts under /mnt as shared execute the following
226 command:
228 smount /mnt rshared
229 the corresponding syntax planned for mount command is
230 mount --make-rshared /mnt
232 just to mark a mount /mnt as shared, execute the following
233 command:
234 smount /mnt shared
235 the corresponding syntax planned for mount command is
236 mount --make-shared /mnt
238 (ii) To mark all the shared mounts under /mnt as slave execute the
239 following
241 command:
242 smount /mnt rslave
243 the corresponding syntax planned for mount command is
244 mount --make-rslave /mnt
246 just to mark a mount /mnt as slave, execute the following
247 command:
248 smount /mnt slave
249 the corresponding syntax planned for mount command is
250 mount --make-slave /mnt
252 (iii) To mark all the mounts under /mnt as private execute the
253 following command:
255 smount /mnt rprivate
256 the corresponding syntax planned for mount command is
257 mount --make-rprivate /mnt
259 just to mark a mount /mnt as private, execute the following
260 command:
261 smount /mnt private
262 the corresponding syntax planned for mount command is
263 mount --make-private /mnt
265 NOTE: by default all the mounts are created as private. But if
266 you want to change some shared/slave/unbindable mount as
267 private at a later point in time, this command can help.
269 (iv) To mark all the mounts under /mnt as unbindable execute the
270 following
272 command:
273 smount /mnt runbindable
274 the corresponding syntax planned for mount command is
275 mount --make-runbindable /mnt
277 just to mark a mount /mnt as unbindable, execute the following
278 command:
279 smount /mnt unbindable
280 the corresponding syntax planned for mount command is
281 mount --make-unbindable /mnt
284 4) Use cases
285 ------------
287 A) A process wants to clone its own namespace, but still wants to
288 access the CD that got mounted recently.
290 Solution:
292 The system administrator can make the mount at /cdrom shared
293 mount --bind /cdrom /cdrom
294 mount --make-shared /cdrom
296 Now any process that clones off a new namespace will have a
297 mount at /cdrom which is a replica of the same mount in the
298 parent namespace.
300 So when a CD is inserted and mounted at /cdrom that mount gets
301 propagated to the other mount at /cdrom in all the other clone
302 namespaces.
304 B) A process wants its mounts invisible to any other process, but
305 still be able to see the other system mounts.
307 Solution:
309 To begin with, the administrator can mark the entire mount tree
310 as shareable.
312 mount --make-rshared /
314 A new process can clone off a new namespace. And mark some part
315 of its namespace as slave
317 mount --make-rslave /myprivatetree
319 Hence forth any mounts within the /myprivatetree done by the
320 process will not show up in any other namespace. However mounts
321 done in the parent namespace under /myprivatetree still shows
322 up in the process's namespace.
325 Apart from the above semantics this feature provides the
326 building blocks to solve the following problems:
328 C) Per-user namespace
330 The above semantics allows a way to share mounts across
331 namespaces. But namespaces are associated with processes. If
332 namespaces are made first class objects with user API to
333 associate/disassociate a namespace with userid, then each user
334 could have his/her own namespace and tailor it to his/her
335 requirements. Offcourse its needs support from PAM.
337 D) Versioned files
339 If the entire mount tree is visible at multiple locations, then
340 a underlying versioning file system can return different
341 version of the file depending on the path used to access that
342 file.
344 An example is:
346 mount --make-shared /
347 mount --rbind / /view/v1
348 mount --rbind / /view/v2
349 mount --rbind / /view/v3
350 mount --rbind / /view/v4
352 and if /usr has a versioning filesystem mounted, than that
353 mount appears at /view/v1/usr, /view/v2/usr, /view/v3/usr and
354 /view/v4/usr too
356 A user can request v3 version of the file /usr/fs/namespace.c
357 by accessing /view/v3/usr/fs/namespace.c . The underlying
358 versioning filesystem can then decipher that v3 version of the
359 filesystem is being requested and return the corresponding
360 inode.
362 5) Detailed semantics:
363 -------------------
364 The section below explains the detailed semantics of
365 bind, rbind, move, mount, umount and clone-namespace operations.
367 Note: the word 'vfsmount' and the noun 'mount' have been used
368 to mean the same thing, throughout this document.
370 5a) Mount states
372 A given mount can be in one of the following states
373 1) shared
374 2) slave
375 3) shared and slave
376 4) private
377 5) unbindable
379 A 'propagation event' is defined as event generated on a vfsmount
380 that leads to mount or unmount actions in other vfsmounts.
382 A 'peer group' is defined as a group of vfsmounts that propagate
383 events to each other.
385 (1) Shared mounts
387 A 'shared mount' is defined as a vfsmount that belongs to a
388 'peer group'.
390 For example:
391 mount --make-shared /mnt
392 mount --bin /mnt /tmp
394 The mount at /mnt and that at /tmp are both shared and belong
395 to the same peer group. Anything mounted or unmounted under
396 /mnt or /tmp reflect in all the other mounts of its peer
397 group.
400 (2) Slave mounts
402 A 'slave mount' is defined as a vfsmount that receives
403 propagation events and does not forward propagation events.
405 A slave mount as the name implies has a master mount from which
406 mount/unmount events are received. Events do not propagate from
407 the slave mount to the master. Only a shared mount can be made
408 a slave by executing the following command
410 mount --make-slave mount
412 A shared mount that is made as a slave is no more shared unless
413 modified to become shared.
415 (3) Shared and Slave
417 A vfsmount can be both shared as well as slave. This state
418 indicates that the mount is a slave of some vfsmount, and
419 has its own peer group too. This vfsmount receives propagation
420 events from its master vfsmount, and also forwards propagation
421 events to its 'peer group' and to its slave vfsmounts.
423 Strictly speaking, the vfsmount is shared having its own
424 peer group, and this peer-group is a slave of some other
425 peer group.
427 Only a slave vfsmount can be made as 'shared and slave' by
428 either executing the following command
429 mount --make-shared mount
430 or by moving the slave vfsmount under a shared vfsmount.
432 (4) Private mount
434 A 'private mount' is defined as vfsmount that does not
435 receive or forward any propagation events.
437 (5) Unbindable mount
439 A 'unbindable mount' is defined as vfsmount that does not
440 receive or forward any propagation events and cannot
441 be bind mounted.
444 State diagram:
445 The state diagram below explains the state transition of a mount,
446 in response to various commands.
447 ------------------------------------------------------------------------
448 | |make-shared | make-slave | make-private |make-unbindab|
449 --------------|------------|--------------|--------------|-------------|
450 |shared |shared |*slave/private| private | unbindable |
451 | | | | | |
452 |-------------|------------|--------------|--------------|-------------|
453 |slave |shared | **slave | private | unbindable |
454 | |and slave | | | |
455 |-------------|------------|--------------|--------------|-------------|
456 |shared |shared | slave | private | unbindable |
457 |and slave |and slave | | | |
458 |-------------|------------|--------------|--------------|-------------|
459 |private |shared | **private | private | unbindable |
460 |-------------|------------|--------------|--------------|-------------|
461 |unbindable |shared |**unbindable | private | unbindable |
462 ------------------------------------------------------------------------
464 * if the shared mount is the only mount in its peer group, making it
465 slave, makes it private automatically. Note that there is no master to
466 which it can be slaved to.
468 ** slaving a non-shared mount has no effect on the mount.
470 Apart from the commands listed below, the 'move' operation also changes
471 the state of a mount depending on type of the destination mount. Its
472 explained in section 5d.
474 5b) Bind semantics
476 Consider the following command
478 mount --bind A/a B/b
480 where 'A' is the source mount, 'a' is the dentry in the mount 'A', 'B'
481 is the destination mount and 'b' is the dentry in the destination mount.
483 The outcome depends on the type of mount of 'A' and 'B'. The table
484 below contains quick reference.
485 ---------------------------------------------------------------------------
486 | BIND MOUNT OPERATION |
487 |**************************************************************************
488 |source(A)->| shared | private | slave | unbindable |
489 | dest(B) | | | | |
490 | | | | | | |
491 | v | | | | |
492 |**************************************************************************
493 | shared | shared | shared | shared & slave | invalid |
494 | | | | | |
495 |non-shared| shared | private | slave | invalid |
496 ***************************************************************************
498 Details:
500 1. 'A' is a shared mount and 'B' is a shared mount. A new mount 'C'
501 which is clone of 'A', is created. Its root dentry is 'a' . 'C' is
502 mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
503 are created and mounted at the dentry 'b' on all mounts where 'B'
504 propagates to. A new propagation tree containing 'C1',..,'Cn' is
505 created. This propagation tree is identical to the propagation tree of
506 'B'. And finally the peer-group of 'C' is merged with the peer group
507 of 'A'.
509 2. 'A' is a private mount and 'B' is a shared mount. A new mount 'C'
510 which is clone of 'A', is created. Its root dentry is 'a'. 'C' is
511 mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
512 are created and mounted at the dentry 'b' on all mounts where 'B'
513 propagates to. A new propagation tree is set containing all new mounts
514 'C', 'C1', .., 'Cn' with exactly the same configuration as the
515 propagation tree for 'B'.
517 3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. A new
518 mount 'C' which is clone of 'A', is created. Its root dentry is 'a' .
519 'C' is mounted on mount 'B' at dentry 'b'. Also new mounts 'C1', 'C2',
520 'C3' ... are created and mounted at the dentry 'b' on all mounts where
521 'B' propagates to. A new propagation tree containing the new mounts
522 'C','C1',.. 'Cn' is created. This propagation tree is identical to the
523 propagation tree for 'B'. And finally the mount 'C' and its peer group
524 is made the slave of mount 'Z'. In other words, mount 'C' is in the
525 state 'slave and shared'.
527 4. 'A' is a unbindable mount and 'B' is a shared mount. This is a
528 invalid operation.
530 5. 'A' is a private mount and 'B' is a non-shared(private or slave or
531 unbindable) mount. A new mount 'C' which is clone of 'A', is created.
532 Its root dentry is 'a'. 'C' is mounted on mount 'B' at dentry 'b'.
534 6. 'A' is a shared mount and 'B' is a non-shared mount. A new mount 'C'
535 which is a clone of 'A' is created. Its root dentry is 'a'. 'C' is
536 mounted on mount 'B' at dentry 'b'. 'C' is made a member of the
537 peer-group of 'A'.
539 7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount. A
540 new mount 'C' which is a clone of 'A' is created. Its root dentry is
541 'a'. 'C' is mounted on mount 'B' at dentry 'b'. Also 'C' is set as a
542 slave mount of 'Z'. In other words 'A' and 'C' are both slave mounts of
543 'Z'. All mount/unmount events on 'Z' propagates to 'A' and 'C'. But
544 mount/unmount on 'A' do not propagate anywhere else. Similarly
545 mount/unmount on 'C' do not propagate anywhere else.
547 8. 'A' is a unbindable mount and 'B' is a non-shared mount. This is a
548 invalid operation. A unbindable mount cannot be bind mounted.
550 5c) Rbind semantics
552 rbind is same as bind. Bind replicates the specified mount. Rbind
553 replicates all the mounts in the tree belonging to the specified mount.
554 Rbind mount is bind mount applied to all the mounts in the tree.
556 If the source tree that is rbind has some unbindable mounts,
557 then the subtree under the unbindable mount is pruned in the new
558 location.
560 eg: lets say we have the following mount tree.
562 A
563 / \
564 B C
565 / \ / \
566 D E F G
568 Lets say all the mount except the mount C in the tree are
569 of a type other than unbindable.
571 If this tree is rbound to say Z
573 We will have the following tree at the new location.
575 Z
576 |
577 A'
578 /
579 B' Note how the tree under C is pruned
580 / \ in the new location.
581 D' E'
585 5d) Move semantics
587 Consider the following command
589 mount --move A B/b
591 where 'A' is the source mount, 'B' is the destination mount and 'b' is
592 the dentry in the destination mount.
594 The outcome depends on the type of the mount of 'A' and 'B'. The table
595 below is a quick reference.
596 ---------------------------------------------------------------------------
597 | MOVE MOUNT OPERATION |
598 |**************************************************************************
599 | source(A)->| shared | private | slave | unbindable |
600 | dest(B) | | | | |
601 | | | | | | |
602 | v | | | | |
603 |**************************************************************************
604 | shared | shared | shared |shared and slave| invalid |
605 | | | | | |
606 |non-shared| shared | private | slave | unbindable |
607 ***************************************************************************
608 NOTE: moving a mount residing under a shared mount is invalid.
610 Details follow:
612 1. 'A' is a shared mount and 'B' is a shared mount. The mount 'A' is
613 mounted on mount 'B' at dentry 'b'. Also new mounts 'A1', 'A2'...'An'
614 are created and mounted at dentry 'b' on all mounts that receive
615 propagation from mount 'B'. A new propagation tree is created in the
616 exact same configuration as that of 'B'. This new propagation tree
617 contains all the new mounts 'A1', 'A2'... 'An'. And this new
618 propagation tree is appended to the already existing propagation tree
619 of 'A'.
621 2. 'A' is a private mount and 'B' is a shared mount. The mount 'A' is
622 mounted on mount 'B' at dentry 'b'. Also new mount 'A1', 'A2'... 'An'
623 are created and mounted at dentry 'b' on all mounts that receive
624 propagation from mount 'B'. The mount 'A' becomes a shared mount and a
625 propagation tree is created which is identical to that of
626 'B'. This new propagation tree contains all the new mounts 'A1',
627 'A2'... 'An'.
629 3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. The
630 mount 'A' is mounted on mount 'B' at dentry 'b'. Also new mounts 'A1',
631 'A2'... 'An' are created and mounted at dentry 'b' on all mounts that
632 receive propagation from mount 'B'. A new propagation tree is created
633 in the exact same configuration as that of 'B'. This new propagation
634 tree contains all the new mounts 'A1', 'A2'... 'An'. And this new
635 propagation tree is appended to the already existing propagation tree of
636 'A'. Mount 'A' continues to be the slave mount of 'Z' but it also
637 becomes 'shared'.
639 4. 'A' is a unbindable mount and 'B' is a shared mount. The operation
640 is invalid. Because mounting anything on the shared mount 'B' can
641 create new mounts that get mounted on the mounts that receive
642 propagation from 'B'. And since the mount 'A' is unbindable, cloning
643 it to mount at other mountpoints is not possible.
645 5. 'A' is a private mount and 'B' is a non-shared(private or slave or
646 unbindable) mount. The mount 'A' is mounted on mount 'B' at dentry 'b'.
648 6. 'A' is a shared mount and 'B' is a non-shared mount. The mount 'A'
649 is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
650 shared mount.
652 7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount.
653 The mount 'A' is mounted on mount 'B' at dentry 'b'. Mount 'A'
654 continues to be a slave mount of mount 'Z'.
656 8. 'A' is a unbindable mount and 'B' is a non-shared mount. The mount
657 'A' is mounted on mount 'B' at dentry 'b'. Mount 'A' continues to be a
658 unbindable mount.
660 5e) Mount semantics
662 Consider the following command
664 mount device B/b
666 'B' is the destination mount and 'b' is the dentry in the destination
667 mount.
669 The above operation is the same as bind operation with the exception
670 that the source mount is always a private mount.
673 5f) Unmount semantics
675 Consider the following command
677 umount A
679 where 'A' is a mount mounted on mount 'B' at dentry 'b'.
681 If mount 'B' is shared, then all most-recently-mounted mounts at dentry
682 'b' on mounts that receive propagation from mount 'B' and does not have
683 sub-mounts within them are unmounted.
685 Example: Lets say 'B1', 'B2', 'B3' are shared mounts that propagate to
686 each other.
688 lets say 'A1', 'A2', 'A3' are first mounted at dentry 'b' on mount
689 'B1', 'B2' and 'B3' respectively.
691 lets say 'C1', 'C2', 'C3' are next mounted at the same dentry 'b' on
692 mount 'B1', 'B2' and 'B3' respectively.
694 if 'C1' is unmounted, all the mounts that are most-recently-mounted on
695 'B1' and on the mounts that 'B1' propagates-to are unmounted.
697 'B1' propagates to 'B2' and 'B3'. And the most recently mounted mount
698 on 'B2' at dentry 'b' is 'C2', and that of mount 'B3' is 'C3'.
700 So all 'C1', 'C2' and 'C3' should be unmounted.
702 If any of 'C2' or 'C3' has some child mounts, then that mount is not
703 unmounted, but all other mounts are unmounted. However if 'C1' is told
704 to be unmounted and 'C1' has some sub-mounts, the umount operation is
705 failed entirely.
707 5g) Clone Namespace
709 A cloned namespace contains all the mounts as that of the parent
710 namespace.
712 Lets say 'A' and 'B' are the corresponding mounts in the parent and the
713 child namespace.
715 If 'A' is shared, then 'B' is also shared and 'A' and 'B' propagate to
716 each other.
718 If 'A' is a slave mount of 'Z', then 'B' is also the slave mount of
719 'Z'.
721 If 'A' is a private mount, then 'B' is a private mount too.
723 If 'A' is unbindable mount, then 'B' is a unbindable mount too.
726 6) Quiz
728 A. What is the result of the following command sequence?
730 mount --bind /mnt /mnt
731 mount --make-shared /mnt
732 mount --bind /mnt /tmp
733 mount --move /tmp /mnt/1
735 what should be the contents of /mnt /mnt/1 /mnt/1/1 should be?
736 Should they all be identical? or should /mnt and /mnt/1 be
737 identical only?
740 B. What is the result of the following command sequence?
742 mount --make-rshared /
743 mkdir -p /v/1
744 mount --rbind / /v/1
746 what should be the content of /v/1/v/1 be?
749 C. What is the result of the following command sequence?
751 mount --bind /mnt /mnt
752 mount --make-shared /mnt
753 mkdir -p /mnt/1/2/3 /mnt/1/test
754 mount --bind /mnt/1 /tmp
755 mount --make-slave /mnt
756 mount --make-shared /mnt
757 mount --bind /mnt/1/2 /tmp1
758 mount --make-slave /mnt
760 At this point we have the first mount at /tmp and
761 its root dentry is 1. Lets call this mount 'A'
762 And then we have a second mount at /tmp1 with root
763 dentry 2. Lets call this mount 'B'
764 Next we have a third mount at /mnt with root dentry
765 mnt. Lets call this mount 'C'
767 'B' is the slave of 'A' and 'C' is a slave of 'B'
768 A -> B -> C
770 at this point if we execute the following command
772 mount --bind /bin /tmp/test
774 The mount is attempted on 'A'
776 will the mount propagate to 'B' and 'C' ?
778 what would be the contents of
779 /mnt/1/test be?
781 7) FAQ
783 Q1. Why is bind mount needed? How is it different from symbolic links?
784 symbolic links can get stale if the destination mount gets
785 unmounted or moved. Bind mounts continue to exist even if the
786 other mount is unmounted or moved.
788 Q2. Why can't the shared subtree be implemented using exportfs?
790 exportfs is a heavyweight way of accomplishing part of what
791 shared subtree can do. I cannot imagine a way to implement the
792 semantics of slave mount using exportfs?
794 Q3 Why is unbindable mount needed?
796 Lets say we want to replicate the mount tree at multiple
797 locations within the same subtree.
799 if one rbind mounts a tree within the same subtree 'n' times
800 the number of mounts created is an exponential function of 'n'.
801 Having unbindable mount can help prune the unneeded bind
802 mounts. Here is a example.
804 step 1:
805 lets say the root tree has just two directories with
806 one vfsmount.
807 root
808 / \
809 tmp usr
811 And we want to replicate the tree at multiple
812 mountpoints under /root/tmp
814 step2:
815 mount --make-shared /root
817 mkdir -p /tmp/m1
819 mount --rbind /root /tmp/m1
821 the new tree now looks like this:
823 root
824 / \
825 tmp usr
826 /
827 m1
828 / \
829 tmp usr
830 /
831 m1
833 it has two vfsmounts
835 step3:
836 mkdir -p /tmp/m2
837 mount --rbind /root /tmp/m2
839 the new tree now looks like this:
841 root
842 / \
843 tmp usr
844 / \
845 m1 m2
846 / \ / \
847 tmp usr tmp usr
848 / \ /
849 m1 m2 m1
850 / \ / \
851 tmp usr tmp usr
852 / / \
853 m1 m1 m2
854 / \
855 tmp usr
856 / \
857 m1 m2
859 it has 6 vfsmounts
861 step 4:
862 mkdir -p /tmp/m3
863 mount --rbind /root /tmp/m3
865 I wont' draw the tree..but it has 24 vfsmounts
868 at step i the number of vfsmounts is V[i] = i*V[i-1].
869 This is an exponential function. And this tree has way more
870 mounts than what we really needed in the first place.
872 One could use a series of umount at each step to prune
873 out the unneeded mounts. But there is a better solution.
874 Unclonable mounts come in handy here.
876 step 1:
877 lets say the root tree has just two directories with
878 one vfsmount.
879 root
880 / \
881 tmp usr
883 How do we set up the same tree at multiple locations under
884 /root/tmp
886 step2:
887 mount --bind /root/tmp /root/tmp
889 mount --make-rshared /root
890 mount --make-unbindable /root/tmp
892 mkdir -p /tmp/m1
894 mount --rbind /root /tmp/m1
896 the new tree now looks like this:
898 root
899 / \
900 tmp usr
901 /
902 m1
903 / \
904 tmp usr
906 step3:
907 mkdir -p /tmp/m2
908 mount --rbind /root /tmp/m2
910 the new tree now looks like this:
912 root
913 / \
914 tmp usr
915 / \
916 m1 m2
917 / \ / \
918 tmp usr tmp usr
920 step4:
922 mkdir -p /tmp/m3
923 mount --rbind /root /tmp/m3
925 the new tree now looks like this:
927 root
928 / \
929 tmp usr
930 / \ \
931 m1 m2 m3
932 / \ / \ / \
933 tmp usr tmp usr tmp usr
935 8) Implementation
937 8A) Datastructure
939 4 new fields are introduced to struct vfsmount
940 ->mnt_share
941 ->mnt_slave_list
942 ->mnt_slave
943 ->mnt_master
945 ->mnt_share links together all the mount to/from which this vfsmount
946 send/receives propagation events.
948 ->mnt_slave_list links all the mounts to which this vfsmount propagates
949 to.
951 ->mnt_slave links together all the slaves that its master vfsmount
952 propagates to.
954 ->mnt_master points to the master vfsmount from which this vfsmount
955 receives propagation.
957 ->mnt_flags takes two more flags to indicate the propagation status of
958 the vfsmount. MNT_SHARE indicates that the vfsmount is a shared
959 vfsmount. MNT_UNCLONABLE indicates that the vfsmount cannot be
960 replicated.
962 All the shared vfsmounts in a peer group form a cyclic list through
963 ->mnt_share.
965 All vfsmounts with the same ->mnt_master form on a cyclic list anchored
966 in ->mnt_master->mnt_slave_list and going through ->mnt_slave.
968 ->mnt_master can point to arbitrary (and possibly different) members
969 of master peer group. To find all immediate slaves of a peer group
970 you need to go through _all_ ->mnt_slave_list of its members.
971 Conceptually it's just a single set - distribution among the
972 individual lists does not affect propagation or the way propagation
973 tree is modified by operations.
975 A example propagation tree looks as shown in the figure below.
976 [ NOTE: Though it looks like a forest, if we consider all the shared
977 mounts as a conceptual entity called 'pnode', it becomes a tree]
980 A <--> B <--> C <---> D
981 /|\ /| |\
982 / F G J K H I
983 /
984 E<-->K
985 /|\
986 M L N
988 In the above figure A,B,C and D all are shared and propagate to each
989 other. 'A' has got 3 slave mounts 'E' 'F' and 'G' 'C' has got 2 slave
990 mounts 'J' and 'K' and 'D' has got two slave mounts 'H' and 'I'.
991 'E' is also shared with 'K' and they propagate to each other. And
992 'K' has 3 slaves 'M', 'L' and 'N'
994 A's ->mnt_share links with the ->mnt_share of 'B' 'C' and 'D'
996 A's ->mnt_slave_list links with ->mnt_slave of 'E', 'K', 'F' and 'G'
998 E's ->mnt_share links with ->mnt_share of K
999 'E', 'K', 'F', 'G' have their ->mnt_master point to struct
1000 vfsmount of 'A'
1001 'M', 'L', 'N' have their ->mnt_master point to struct vfsmount of 'K'
1002 K's ->mnt_slave_list links with ->mnt_slave of 'M', 'L' and 'N'
1004 C's ->mnt_slave_list links with ->mnt_slave of 'J' and 'K'
1005 J and K's ->mnt_master points to struct vfsmount of C
1006 and finally D's ->mnt_slave_list links with ->mnt_slave of 'H' and 'I'
1007 'H' and 'I' have their ->mnt_master pointing to struct vfsmount of 'D'.
1010 NOTE: The propagation tree is orthogonal to the mount tree.
1013 8B Algorithm:
1015 The crux of the implementation resides in rbind/move operation.
1017 The overall algorithm breaks the operation into 3 phases: (look at
1018 attach_recursive_mnt() and propagate_mnt())
1020 1. prepare phase.
1021 2. commit phases.
1022 3. abort phases.
1024 Prepare phase:
1026 for each mount in the source tree:
1027 a) Create the necessary number of mount trees to
1028 be attached to each of the mounts that receive
1029 propagation from the destination mount.
1030 b) Do not attach any of the trees to its destination.
1031 However note down its ->mnt_parent and ->mnt_mountpoint
1032 c) Link all the new mounts to form a propagation tree that
1033 is identical to the propagation tree of the destination
1034 mount.
1036 If this phase is successful, there should be 'n' new
1037 propagation trees; where 'n' is the number of mounts in the
1038 source tree. Go to the commit phase
1040 Also there should be 'm' new mount trees, where 'm' is
1041 the number of mounts to which the destination mount
1042 propagates to.
1044 if any memory allocations fail, go to the abort phase.
1046 Commit phase
1047 attach each of the mount trees to their corresponding
1048 destination mounts.
1050 Abort phase
1051 delete all the newly created trees.
1053 NOTE: all the propagation related functionality resides in the file
1054 pnode.c
1057 ------------------------------------------------------------------------
1059 version 0.1 (created the initial document, Ram Pai linuxram@us.ibm.com)
1060 version 0.2 (Incorporated comments from Al Viro)