mirror of https://github.com/mkerrisk/man-pages
mount.2: Restructure discussion of 'mountflags' into functional groups
The existing text makes no differentiation between different "classes" of mount flags. However, certain flags such as MS_REMOUNT, MS_BIND, MS_MOVE, etc. determine the general type of operation that mount() performs. Furthermore, the choice of which class of operation to perform is performed in a certain order, and that order is significant if multiple flags are specified. Restructure and extend the text to reflect these details. Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
parent
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327
man2/mount.2
327
man2/mount.2
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@ -80,68 +80,59 @@ See
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.BR mount (8)
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for details of the options available for each filesystem type.
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The following flags may be specified in
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.IR mountflags :
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.\" FIXME 2.6.15 added flags for "shared subtree" functionality:
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A call to
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.BR mount ()
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performs one of a number of general types of operation.
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depending on the bits specified in
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.IR mountflags .
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The choice of operation is determined by testing the bits set in
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.IR mountflags ,
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with the tests being conducted in the order listed here:
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.IP * 3
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Remount an existing mount:
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.IR mountflags
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includes
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.BR MS_REMOUNT .
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.IP *
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Create a bind mount:
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.IR mountflags
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includes
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.BR MS_BIND .
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.IP *
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Change the propagation type of an existing mount:
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.IR mountflags
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includes one of
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.BR MS_SHARED ,
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.BR MS_PRIVATE ,
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.BR MS_SLAVE ,
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or
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.BR MS_UNBINDABLE .
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.IP *
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Move an existing mount to a new location:
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.IR mountflags
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includes
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.BR MS_MOVE .
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.IP *
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Create a new mount:
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.IR mountflags
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includes none of the above flags.
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.PP
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Each of these operations is detailed later in this page.
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Further flags may be specified in
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.IR mountflags
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to modify the behavior of
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.BR mount (),
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as described below.
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.\"
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.\" MS_PRIVATE
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.\" All mounts are private by default. Previously shared mounts
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.\" can be re-marked PRIVATE.
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.\" MS_SHARED
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.\" Mount points that are marked SHARED propagate mount events
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.\" to one another after being cloned.
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.\" mount --make-rshared ==> MS_SHARED | MS_REC
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.\" MS_SLAVE
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.\" A previously shared mount point can be marked SLAVE, meaning
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.\" it receives propagated events, but does not propagate events.
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.\" MS_UNBINDABLE
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.\" mounts cannot be bound into other places, and will not be
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.\" propagated into new subtrees
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.\"
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.\" These settings are visible in /proc/$$/mountinfo
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.\"
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.\" These need to be documented on this page.
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.\" See:
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.\"
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.\" * Documentation/filesystems/sharedsubtree.txt
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.\"
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.\" * http://lwn.net/Articles/159077/
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.\"
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.\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
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.\" Shared-Subtree Concept, Implementation, and Applications in Linux
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.\" Al Viro viro@ftp.linux.org.uk
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.\" Ram Pai linuxram@us.ibm.com
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.\"
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.\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
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.\" Applying mount namespaces
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.\"
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.\" Uncover practical applications for advanced Linux mounts features
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.\" Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
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.\" Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
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.\" Date: 17 Sep 2007
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.SS Additional mount flags
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The list below describes the additional flags that can be specified in
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.IR mountflags .
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Note that some operation types ignore some or all of these flags,
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as described later in this page.
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.\"
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.\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
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.\"
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.TP
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.BR MS_BIND " (Linux 2.4 onward)"
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.\" since 2.4.0-test9
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Perform a bind mount, making a file or a directory subtree visible at
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another point within a filesystem.
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Bind mounts may cross filesystem boundaries and span
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.BR chroot (2)
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jails.
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The
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.IR filesystemtype
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and
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.IR data
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arguments are ignored.
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The remaining bits in the
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.I mountflags
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argument are also ignored, with the exception of
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.B MS_REC
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(the bind mount has the same mount options as
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the underlying mount point).
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.TP
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.BR MS_DIRSYNC " (since Linux 2.5.19)"
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Make directory changes on this filesystem synchronous.
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(This property can be obtained for individual directories
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@ -201,21 +192,6 @@ this mount option requires the
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capability.
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.\" FIXME Describe the MS_MOVE flag in more detail
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.TP
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.B MS_MOVE
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Move a subtree.
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.I source
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specifies an existing mount point and
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.I target
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specifies the new location.
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The move is atomic: at no point is the subtree unmounted.
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The remaining bits in the
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.IR mountflags
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argument are ignored, as are the
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.IR filesystemtype
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and
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.IR data
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arguments.
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.TP
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.B MS_NOATIME
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Do not update access times for (all types of) files on this filesystem.
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.TP
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@ -271,8 +247,49 @@ is more than 1 day old.
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.\" that this lets utilities such as tmpreaper (which deletes
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.\" files based on last access time) work correctly.
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.TP
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.BR MS_SILENT " (since Linux 2.6.17)"
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Suppress the display of certain
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.RI ( printk ())
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warning messages in the kernel log.
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This flag supersedes the misnamed and obsolete
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.BR MS_VERBOSE
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flag (available since Linux 2.4.12), which has the same meaning.
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.TP
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.BR MS_STRICTATIME " (since Linux 2.6.30)"
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Always update the last access time (atime) when files on this
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filesystem are accessed.
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(This was the default behavior before Linux 2.6.30.)
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Specifying this flag overrides the effect of setting the
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.BR MS_NOATIME
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and
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.BR MS_RELATIME
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flags.
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.TP
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.B MS_SYNCHRONOUS
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Make writes on this filesystem synchronous (as though
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the
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.B O_SYNC
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flag to
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.BR open (2)
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was specified for all file opens to this filesystem).
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.PP
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From Linux 2.4 onward, the
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.BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
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flags are settable on a per-mount-point basis.
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From kernel 2.6.16 onward,
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.B MS_NOATIME
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and
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.B MS_NODIRATIME
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are also settable on a per-mount-point basis.
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The
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.B MS_RELATIME
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flag is also settable on a per-mount-point basis.
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.\"
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.SS Remounting an existing mount
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An existing mount may be remounted by specifying
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.B MS_REMOUNT
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Remount an existing mount.
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in
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.IR mountflags .
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This allows you to change the
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.I mountflags
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and
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@ -286,6 +303,7 @@ call;
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and
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.I filesystemtype
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are ignored.
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The
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.I mountflags
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and
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@ -336,44 +354,124 @@ as:
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Note that only the
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.BR MS_RDONLY
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setting of the bind mount can be changed in this manner.
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.TP
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.BR MS_SILENT " (since Linux 2.6.17)"
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Suppress the display of certain
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.RI ( printk ())
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warning messages in the kernel log.
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This flag supersedes the misnamed and obsolete
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.BR MS_VERBOSE
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flag (available since Linux 2.4.12), which has the same meaning.
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.TP
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.BR MS_STRICTATIME " (since Linux 2.6.30)"
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Always update the last access time (atime) when files on this
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filesystem are accessed.
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(This was the default behavior before Linux 2.6.30.)
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Specifying this flag overrides the effect of setting the
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.BR MS_NOATIME
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and
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.BR MS_RELATIME
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flags.
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.TP
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.B MS_SYNCHRONOUS
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Make writes on this filesystem synchronous (as though
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the
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.B O_SYNC
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flag to
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.BR open (2)
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was specified for all file opens to this filesystem).
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.PP
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From Linux 2.4 onward, the
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.BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
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flags are settable on a per-mount-point basis.
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From kernel 2.6.16 onward,
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.B MS_NOATIME
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and
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.B MS_NODIRATIME
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are also settable on a per-mount-point basis.
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.\"
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.SS Creating a bind mount
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If
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.I mountflags
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includes
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.BR MS_BIND
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(available since Linux 2.4),
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.\" since 2.4.0-test9
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then perform a bind mount.
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A bind mount makes a file or a directory subtree visible at
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another point within the single directory hierarchy.
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Bind mounts may cross filesystem boundaries and span
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.BR chroot (2)
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jails.
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The
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.B MS_RELATIME
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flag is also settable on a per-mount-point basis.
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.IR filesystemtype
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and
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.IR data
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arguments are ignored.
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The remaining bits in the
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.I mountflags
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argument are also ignored, with the exception of
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.BR MS_REC .
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(The bind mount has the same mount options as
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the underlying mount point.)
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However, see the discussion of remounting above,
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for a method of making an existing bind mount read-only.
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.\"
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.SS Changing the propagation type of an existing mount
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[TBC]
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.\"
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.\" FIXME 2.6.15 added flags for "shared subtree" functionality:
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.\"
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.\" MS_PRIVATE
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.\" All mounts are private by default. Previously shared mounts
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.\" can be re-marked PRIVATE.
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.\" MS_SHARED
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.\" Mount points that are marked SHARED propagate mount events
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.\" to one another after being cloned.
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.\" mount --make-rshared ==> MS_SHARED | MS_REC
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.\" MS_SLAVE
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.\" A previously shared mount point can be marked SLAVE, meaning
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.\" it receives propagated events, but does not propagate events.
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.\" MS_UNBINDABLE
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.\" mounts cannot be bound into other places, and will not be
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.\" propagated into new subtrees
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.\"
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.\" These settings are visible in /proc/$$/mountinfo
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.\"
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.\" These need to be documented on this page.
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.\" See:
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.\"
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.\" * Documentation/filesystems/sharedsubtree.txt
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.\"
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.\" * http://lwn.net/Articles/159077/
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.\"
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.\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
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.\" Shared-Subtree Concept, Implementation, and Applications in Linux
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.\" Al Viro viro@ftp.linux.org.uk
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.\" Ram Pai linuxram@us.ibm.com
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.\"
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.\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
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.\" Applying mount namespaces
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.\"
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.\" Uncover practical applications for advanced Linux mounts features
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.\" Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
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.\" Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
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.\" Date: 17 Sep 2007
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.\"
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.SS Moving a mount
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If
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.I mountflags
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contains the flag
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.BR MS_MOVE ,
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then move a subtree:
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.I source
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specifies an existing mount point and
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.I target
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specifies the new location to which that mount point is to be relocated.
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The move is atomic: at no point is the subtree unmounted.
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The remaining bits in the
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.IR mountflags
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argument are ignored, as are the
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.IR filesystemtype
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and
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.IR data
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arguments.
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.\"
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.SS Creating a new mount point
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If none of
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.BR MS_REMOUNT ,
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.BR MS_BIND ,
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.BR MS_MOVE ,
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.BR MS_SHARED ,
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.BR MS_PRIVATE ,
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.BR MS_SLAVE ,
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or
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.BR MS_UNBINDABLE
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is specified in
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.IR mountflags ,
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then
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.BR mount ()
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performs its default action: creating a new mount point.
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.IR source
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specifies the source for the new mount point, and
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.IR target
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specifies the directory at which to create the mount point.
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The
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.I filesystemtype
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and
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.I data
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arguments are employed, and further bits may be specified in
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.IR mountflags
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to modify the behavior of the call.
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.\"
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.SH RETURN VALUE
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On success, zero is returned.
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On error, \-1 is returned, and
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@ -477,13 +575,16 @@ The caller does not have the required privileges.
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The definitions of
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.BR MS_DIRSYNC ,
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.BR MS_MOVE ,
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.BR MS_PRIVATE ,
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.BR MS_REC ,
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.BR MS_RELATIME ,
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and
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.BR MS_SHARED ,
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.BR MS_SLAVED ,
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.BR MS_STRICTATIME
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and
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.BR MS_UNBINDABLE
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were added to glibc headers in version 2.12.
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.\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
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.\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
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.\"
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.SH CONFORMING TO
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This function is Linux-specific and should not be used in
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programs intended to be portable.
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