1062 lines
32 KiB
Plaintext
1062 lines
32 KiB
Plaintext
Shared Subtrees
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---------------
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Contents:
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1) Overview
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2) Features
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3) smount command
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4) Use-case
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5) Detailed semantics
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6) Quiz
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7) FAQ
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8) Implementation
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1) Overview
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-----------
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Consider the following situation:
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A process wants to clone its own namespace, but still wants to access the CD
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that got mounted recently. Shared subtree semantics provide the necessary
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mechanism to accomplish the above.
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It provides the necessary building blocks for features like per-user-namespace
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and versioned filesystem.
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2) Features
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-----------
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Shared subtree provides four different flavors of mounts; struct vfsmount to be
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precise
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a. shared mount
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b. slave mount
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c. private mount
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d. unbindable mount
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2a) A shared mount can be replicated to as many mountpoints and all the
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replicas continue to be exactly same.
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Here is an example:
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Lets say /mnt has a mount that is shared.
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mount --make-shared /mnt
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note: mount command does not yet support the --make-shared flag.
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I have included a small C program which does the same by executing
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'smount /mnt shared'
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#mount --bind /mnt /tmp
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The above command replicates the mount at /mnt to the mountpoint /tmp
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and the contents of both the mounts remain identical.
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#ls /mnt
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a b c
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#ls /tmp
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a b c
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Now lets say we mount a device at /tmp/a
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#mount /dev/sd0 /tmp/a
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#ls /tmp/a
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t1 t2 t2
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#ls /mnt/a
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t1 t2 t2
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Note that the mount has propagated to the mount at /mnt as well.
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And the same is true even when /dev/sd0 is mounted on /mnt/a. The
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contents will be visible under /tmp/a too.
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2b) A slave mount is like a shared mount except that mount and umount events
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only propagate towards it.
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All slave mounts have a master mount which is a shared.
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Here is an example:
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Lets say /mnt has a mount which is shared.
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#mount --make-shared /mnt
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Lets bind mount /mnt to /tmp
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#mount --bind /mnt /tmp
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the new mount at /tmp becomes a shared mount and it is a replica of
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the mount at /mnt.
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Now lets make the mount at /tmp; a slave of /mnt
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#mount --make-slave /tmp
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[or smount /tmp slave]
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lets mount /dev/sd0 on /mnt/a
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#mount /dev/sd0 /mnt/a
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#ls /mnt/a
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t1 t2 t3
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#ls /tmp/a
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t1 t2 t3
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Note the mount event has propagated to the mount at /tmp
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However lets see what happens if we mount something on the mount at /tmp
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#mount /dev/sd1 /tmp/b
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#ls /tmp/b
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s1 s2 s3
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#ls /mnt/b
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Note how the mount event has not propagated to the mount at
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/mnt
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2c) A private mount does not forward or receive propagation.
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This is the mount we are familiar with. Its the default type.
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2d) A unbindable mount is a unbindable private mount
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lets say we have a mount at /mnt and we make is unbindable
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#mount --make-unbindable /mnt
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[ smount /mnt unbindable ]
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Lets try to bind mount this mount somewhere else.
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# mount --bind /mnt /tmp
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mount: wrong fs type, bad option, bad superblock on /mnt,
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or too many mounted file systems
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Binding a unbindable mount is a invalid operation.
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3) smount command
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Currently the mount command is not aware of shared subtree features.
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Work is in progress to add the support in mount ( util-linux package ).
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Till then use the following program.
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------------------------------------------------------------------------
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//
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//this code was developed my Miklos Szeredi <miklos@szeredi.hu>
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//and modified by Ram Pai <linuxram@us.ibm.com>
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// sample usage:
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// smount /tmp shared
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//
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <string.h>
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#include <sys/mount.h>
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#include <sys/fsuid.h>
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#ifndef MS_REC
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#define MS_REC 0x4000 /* 16384: Recursive loopback */
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#endif
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#ifndef MS_SHARED
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#define MS_SHARED 1<<20 /* Shared */
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#endif
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#ifndef MS_PRIVATE
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#define MS_PRIVATE 1<<18 /* Private */
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#endif
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#ifndef MS_SLAVE
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#define MS_SLAVE 1<<19 /* Slave */
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#endif
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#ifndef MS_UNBINDABLE
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#define MS_UNBINDABLE 1<<17 /* Unbindable */
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#endif
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int main(int argc, char *argv[])
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{
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int type;
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if(argc != 3) {
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fprintf(stderr, "usage: %s dir "
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"<rshared|rslave|rprivate|runbindable|shared|slave"
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"|private|unbindable>\n" , argv[0]);
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return 1;
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}
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fprintf(stdout, "%s %s %s\n", argv[0], argv[1], argv[2]);
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if (strcmp(argv[2],"rshared")==0)
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type=(MS_SHARED|MS_REC);
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else if (strcmp(argv[2],"rslave")==0)
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type=(MS_SLAVE|MS_REC);
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else if (strcmp(argv[2],"rprivate")==0)
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type=(MS_PRIVATE|MS_REC);
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else if (strcmp(argv[2],"runbindable")==0)
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type=(MS_UNBINDABLE|MS_REC);
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else if (strcmp(argv[2],"shared")==0)
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type=MS_SHARED;
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else if (strcmp(argv[2],"slave")==0)
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type=MS_SLAVE;
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else if (strcmp(argv[2],"private")==0)
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type=MS_PRIVATE;
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else if (strcmp(argv[2],"unbindable")==0)
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type=MS_UNBINDABLE;
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else {
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fprintf(stderr, "invalid operation: %s\n", argv[2]);
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return 1;
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}
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setfsuid(getuid());
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if(mount("", argv[1], "dontcare", type, "") == -1) {
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perror("mount");
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return 1;
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}
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return 0;
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}
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-----------------------------------------------------------------------
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Copy the above code snippet into smount.c
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gcc -o smount smount.c
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(i) To mark all the mounts under /mnt as shared execute the following
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command:
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smount /mnt rshared
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the corresponding syntax planned for mount command is
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mount --make-rshared /mnt
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just to mark a mount /mnt as shared, execute the following
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command:
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smount /mnt shared
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the corresponding syntax planned for mount command is
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mount --make-shared /mnt
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(ii) To mark all the shared mounts under /mnt as slave execute the
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following
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command:
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smount /mnt rslave
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the corresponding syntax planned for mount command is
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mount --make-rslave /mnt
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just to mark a mount /mnt as slave, execute the following
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command:
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smount /mnt slave
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the corresponding syntax planned for mount command is
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mount --make-slave /mnt
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(iii) To mark all the mounts under /mnt as private execute the
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following command:
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smount /mnt rprivate
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the corresponding syntax planned for mount command is
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mount --make-rprivate /mnt
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just to mark a mount /mnt as private, execute the following
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command:
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smount /mnt private
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the corresponding syntax planned for mount command is
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mount --make-private /mnt
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NOTE: by default all the mounts are created as private. But if
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you want to change some shared/slave/unbindable mount as
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private at a later point in time, this command can help.
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(iv) To mark all the mounts under /mnt as unbindable execute the
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following
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command:
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smount /mnt runbindable
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the corresponding syntax planned for mount command is
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mount --make-runbindable /mnt
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just to mark a mount /mnt as unbindable, execute the following
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command:
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smount /mnt unbindable
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the corresponding syntax planned for mount command is
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mount --make-unbindable /mnt
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4) Use cases
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------------
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A) A process wants to clone its own namespace, but still wants to
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access the CD that got mounted recently.
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Solution:
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The system administrator can make the mount at /cdrom shared
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mount --bind /cdrom /cdrom
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mount --make-shared /cdrom
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Now any process that clones off a new namespace will have a
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mount at /cdrom which is a replica of the same mount in the
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parent namespace.
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So when a CD is inserted and mounted at /cdrom that mount gets
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propagated to the other mount at /cdrom in all the other clone
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namespaces.
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B) A process wants its mounts invisible to any other process, but
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still be able to see the other system mounts.
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Solution:
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To begin with, the administrator can mark the entire mount tree
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as shareable.
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mount --make-rshared /
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A new process can clone off a new namespace. And mark some part
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of its namespace as slave
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mount --make-rslave /myprivatetree
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Hence forth any mounts within the /myprivatetree done by the
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process will not show up in any other namespace. However mounts
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done in the parent namespace under /myprivatetree still shows
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up in the process's namespace.
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Apart from the above semantics this feature provides the
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building blocks to solve the following problems:
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C) Per-user namespace
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The above semantics allows a way to share mounts across
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namespaces. But namespaces are associated with processes. If
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namespaces are made first class objects with user API to
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associate/disassociate a namespace with userid, then each user
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could have his/her own namespace and tailor it to his/her
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requirements. Offcourse its needs support from PAM.
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D) Versioned files
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If the entire mount tree is visible at multiple locations, then
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a underlying versioning file system can return different
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version of the file depending on the path used to access that
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file.
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An example is:
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mount --make-shared /
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mount --rbind / /view/v1
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mount --rbind / /view/v2
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mount --rbind / /view/v3
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mount --rbind / /view/v4
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and if /usr has a versioning filesystem mounted, than that
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mount appears at /view/v1/usr, /view/v2/usr, /view/v3/usr and
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/view/v4/usr too
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A user can request v3 version of the file /usr/fs/namespace.c
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by accessing /view/v3/usr/fs/namespace.c . The underlying
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versioning filesystem can then decipher that v3 version of the
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filesystem is being requested and return the corresponding
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inode.
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5) Detailed semantics:
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-------------------
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The section below explains the detailed semantics of
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bind, rbind, move, mount, umount and clone-namespace operations.
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Note: the word 'vfsmount' and the noun 'mount' have been used
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to mean the same thing, throughout this document.
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5a) Mount states
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A given mount can be in one of the following states
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1) shared
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2) slave
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3) shared and slave
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4) private
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5) unbindable
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A 'propagation event' is defined as event generated on a vfsmount
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that leads to mount or unmount actions in other vfsmounts.
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A 'peer group' is defined as a group of vfsmounts that propagate
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events to each other.
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(1) Shared mounts
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A 'shared mount' is defined as a vfsmount that belongs to a
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'peer group'.
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For example:
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mount --make-shared /mnt
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mount --bin /mnt /tmp
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The mount at /mnt and that at /tmp are both shared and belong
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to the same peer group. Anything mounted or unmounted under
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/mnt or /tmp reflect in all the other mounts of its peer
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group.
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(2) Slave mounts
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A 'slave mount' is defined as a vfsmount that receives
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propagation events and does not forward propagation events.
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A slave mount as the name implies has a master mount from which
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mount/unmount events are received. Events do not propagate from
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the slave mount to the master. Only a shared mount can be made
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a slave by executing the following command
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mount --make-slave mount
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A shared mount that is made as a slave is no more shared unless
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modified to become shared.
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(3) Shared and Slave
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A vfsmount can be both shared as well as slave. This state
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indicates that the mount is a slave of some vfsmount, and
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has its own peer group too. This vfsmount receives propagation
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events from its master vfsmount, and also forwards propagation
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events to its 'peer group' and to its slave vfsmounts.
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Strictly speaking, the vfsmount is shared having its own
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peer group, and this peer-group is a slave of some other
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peer group.
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Only a slave vfsmount can be made as 'shared and slave' by
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either executing the following command
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mount --make-shared mount
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or by moving the slave vfsmount under a shared vfsmount.
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(4) Private mount
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A 'private mount' is defined as vfsmount that does not
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receive or forward any propagation events.
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(5) Unbindable mount
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A 'unbindable mount' is defined as vfsmount that does not
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receive or forward any propagation events and cannot
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be bind mounted.
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State diagram:
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The state diagram below explains the state transition of a mount,
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in response to various commands.
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------------------------------------------------------------------------
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| |make-shared | make-slave | make-private |make-unbindab|
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--------------|------------|--------------|--------------|-------------|
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|shared |shared |*slave/private| private | unbindable |
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| | | | | |
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|-------------|------------|--------------|--------------|-------------|
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|slave |shared | **slave | private | unbindable |
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| |and slave | | | |
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|-------------|------------|--------------|--------------|-------------|
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|shared |shared | slave | private | unbindable |
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|and slave |and slave | | | |
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|-------------|------------|--------------|--------------|-------------|
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|private |shared | **private | private | unbindable |
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|-------------|------------|--------------|--------------|-------------|
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|unbindable |shared |**unbindable | private | unbindable |
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------------------------------------------------------------------------
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* if the shared mount is the only mount in its peer group, making it
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slave, makes it private automatically. Note that there is no master to
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which it can be slaved to.
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** slaving a non-shared mount has no effect on the mount.
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Apart from the commands listed below, the 'move' operation also changes
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the state of a mount depending on type of the destination mount. Its
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explained in section 5d.
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5b) Bind semantics
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Consider the following command
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mount --bind A/a B/b
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where 'A' is the source mount, 'a' is the dentry in the mount 'A', 'B'
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is the destination mount and 'b' is the dentry in the destination mount.
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The outcome depends on the type of mount of 'A' and 'B'. The table
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below contains quick reference.
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---------------------------------------------------------------------------
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| BIND MOUNT OPERATION |
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|**************************************************************************
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|source(A)->| shared | private | slave | unbindable |
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| dest(B) | | | | |
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| | | | | | |
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| v | | | | |
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|**************************************************************************
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| shared | shared | shared | shared & slave | invalid |
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| | | | | |
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|non-shared| shared | private | slave | invalid |
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***************************************************************************
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Details:
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1. 'A' is a shared mount and 'B' is a shared mount. A new mount 'C'
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which is clone of 'A', is created. Its root dentry is 'a' . 'C' is
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mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
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are created and mounted at the dentry 'b' on all mounts where 'B'
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propagates to. A new propagation tree containing 'C1',..,'Cn' is
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created. This propagation tree is identical to the propagation tree of
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'B'. And finally the peer-group of 'C' is merged with the peer group
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of 'A'.
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2. 'A' is a private mount and 'B' is a shared mount. A new mount 'C'
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which is clone of 'A', is created. Its root dentry is 'a'. 'C' is
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mounted on mount 'B' at dentry 'b'. Also new mount 'C1', 'C2', 'C3' ...
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are created and mounted at the dentry 'b' on all mounts where 'B'
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propagates to. A new propagation tree is set containing all new mounts
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'C', 'C1', .., 'Cn' with exactly the same configuration as the
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propagation tree for 'B'.
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3. 'A' is a slave mount of mount 'Z' and 'B' is a shared mount. A new
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mount 'C' which is clone of 'A', is created. Its root dentry is 'a' .
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'C' is mounted on mount 'B' at dentry 'b'. Also new mounts 'C1', 'C2',
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'C3' ... are created and mounted at the dentry 'b' on all mounts where
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'B' propagates to. A new propagation tree containing the new mounts
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'C','C1',.. 'Cn' is created. This propagation tree is identical to the
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propagation tree for 'B'. And finally the mount 'C' and its peer group
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is made the slave of mount 'Z'. In other words, mount 'C' is in the
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state 'slave and shared'.
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4. 'A' is a unbindable mount and 'B' is a shared mount. This is a
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invalid operation.
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5. 'A' is a private mount and 'B' is a non-shared(private or slave or
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unbindable) mount. A new mount 'C' which is clone of 'A', is created.
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Its root dentry is 'a'. 'C' is mounted on mount 'B' at dentry 'b'.
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6. 'A' is a shared mount and 'B' is a non-shared mount. A new mount 'C'
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which is a clone of 'A' is created. Its root dentry is 'a'. 'C' is
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mounted on mount 'B' at dentry 'b'. 'C' is made a member of the
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peer-group of 'A'.
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7. 'A' is a slave mount of mount 'Z' and 'B' is a non-shared mount. A
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new mount 'C' which is a clone of 'A' is created. Its root dentry is
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'a'. 'C' is mounted on mount 'B' at dentry 'b'. Also 'C' is set as a
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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)
|