-.\" Hey Emacs! This file is -*- nroff -*- source.
-.\"
.\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
.\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
.\" and Copyright (C) 2002, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" Modified 1996-11-04 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2001-10-13 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
+.\" 2008-10-06, mtk: move umount*() material into separate umount.2 page.
+.\" 2008-10-06, mtk: Add discussion of namespaces.
.\"
-.TH MOUNT 2 2004-05-18 "Linux" "Linux Programmer's Manual"
+.TH MOUNT 2 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
-mount, umount, umount2 \- mount and unmount filesystems
+mount \- mount filesystem
.SH SYNOPSIS
.nf
.B "#include <sys/mount.h>"
.BI "int mount(const char *" source ", const char *" target ,
.BI " const char *" filesystemtype ", unsigned long " mountflags ,
.BI " const void *" data );
-.sp
-.BI "int umount(const char *" target );
-.sp
-.BI "int umount2(const char *" target ", int " flags );
.fi
.SH DESCRIPTION
.BR mount ()
or a dummy) to the directory specified by
.IR target .
-.BR umount ()
-and
-.BR umount2 ()
-remove the attachment of the (topmost) filesystem mounted on
-.IR target .
-
Appropriate privilege (Linux: the
.B CAP_SYS_ADMIN
-capability) is required to mount and unmount filesystems.
+capability) is required to mount filesystems.
Since Linux 2.4 a single filesystem can be visible at
multiple mount points, and multiple mounts can be stacked
.\" Multiple mounts on same mount point: since 2.3.99pre7.
Values for the
-.IR filesystemtype
+.I filesystemtype
argument supported by the kernel are listed in
.I /proc/filesystems
-(like "minix", "ext2", "msdos", "proc", "nfs", "iso9660" etc.).
+(e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs",
+"msdos", "proc", "nfs", "iso9660").
Further types may become available when the appropriate modules
are loaded.
The
-.IR mountflags
+.I mountflags
argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
in the top 16 bits (this was required in kernel versions prior to 2.4, but
is no longer required and ignored if specified),
-and various mount flags (as defined in \fI<linux/fs.h>\fP for libc4 and libc5
-and in \fI<sys/mount.h>\fP for glibc2) in the low order 16 bits:
-.\" FIXME 2.6.15 added flags for "shared sub-tree" functionality:
-.\" MS_UNBINDABLE, MS_PRIVATE, MS_SHARED, MS_SLAVE
+and various mount flags
+.\" (as defined in \fI<linux/fs.h>\fP for libc4 and libc5
+.\" and in \fI<sys/mount.h>\fP for glibc2)
+in the low order 16 bits:
+.\" FIXME 2.6.15 added flags for "shared subtree" functionality:
+.\"
+.\" MS_PRIVATE
+.\" All mounts are private by default. Previously shared mounts
+.\" can be re-marked PRIVATE.
+.\" MS_SHARED
+.\" Mount points that are marked SHARED propagate mount events
+.\" to one another after being cloned.
+.\" mount --make-rshared ==> MS_SHARED | MS_REC
+.\" MS_SLAVE
+.\" A previously shared mount point can be marked SLAVE, meaning
+.\" it receives propagated events, but does not propagate events.
+.\" MS_UNBINDABLE
+.\" mounts cannot be bound into other places, and will not be
+.\" propagated into new subtrees
+.\"
+.\" These settings are visible in /proc/$$/mountinfo
+.\"
.\" These need to be documented on this page.
-.\" See Documentation/sharedsubtree.txt
+.\" See:
+.\"
+.\" * Documentation/filesystems/sharedsubtree.txt
+.\"
+.\" * http://lwn.net/Articles/159077/
+.\"
+.\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
+.\" Shared-Subtree Concept, Implementation, and Applications in Linux
+.\" Al Viro viro@ftp.linux.org.uk
+.\" Ram Pai linuxram@us.ibm.com
+.\"
+.\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
+.\" Applying mount namespaces
+.\"
+.\" Uncover practical applications for advanced Linux mounts features
+.\" Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
+.\" Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
+.\" Date: 17 Sep 2007
+.\"
+.\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
+.\"
.TP
-.BR MS_BIND " (Linux 2.4 onwards)"
+.BR MS_BIND " (Linux 2.4 onward)"
.\" since 2.4.0-test9
Perform a bind mount, making a file or a directory subtree visible at
-another point within a file system.
-Bind mounts may cross file system boundaries and span
+another point within a filesystem.
+Bind mounts may cross filesystem boundaries and span
.BR chroot (2)
jails.
The
-.IR filesystemtype ", " mountflags ", and " data
+.IR filesystemtype
+and
+.IR data
arguments are ignored.
+Up until Linux 2.6.26,
+.I mountflags
+was also ignored
.\" with the exception of the "hidden" MS_REC mountflags bit
+(the bind mount has the same mount options as
+the underlying mount point).
.TP
.BR MS_DIRSYNC " (since Linux 2.5.19)"
-Make directory changes on this file system synchronous.
+Make directory changes on this filesystem synchronous.
(This property can be obtained for individual directories
or subtrees using
-.BR chattr (8).)
+.BR chattr (1).)
+.TP
+.BR MS_LAZYTIME " (since Linux 4.0)"
+.\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
+.\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
+.\" commit a26f49926da938f47561f386be56a83dd37a496d
+Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
+by maintaining these changes only in memory.
+The on-disk timestamps are updated only when:
+.RS
+.IP (a) 5
+the inode needs to be updated for some change unrelated to file timestamps;
+.IP (b)
+the application employs
+.BR fsync (2),
+.BR syncfs (2),
+or
+.BR sync (2);
+.IP (c)
+an undeleted inode is evicted from memory; or
+.IP (d)
+more than 24 hours have passed since the inode was written to disk.
+.RE
+.IP
+This mount option significantly reduces writes
+needed to update the inode's timestamps, especially mtime and atime.
+However, in the event of a system crash, the atime and mtime fields
+on disk might be out of date by up to 24 hours.
+
+Examples of workloads where this option could be of significant benefit
+include frequent random writes to preallocated files,
+as well as cases where the
+.B MS_STRICTATIME
+mount option is also enabled.
+(The advantage of combining
+.BR MS_STRICTATIME
+and
+.BR MS_LAZYTIME
+is that
+.BR stat (2)
+will return the correctly updated atime, but the atime updates
+will be flushed to disk only in the cases listed above.)
.TP
.B MS_MANDLOCK
-Permit mandatory locking on files in this file system.
+Permit mandatory locking on files in this filesystem.
(Mandatory locking must still be enabled on a per-file basis,
as described in
.BR fcntl (2).)
-.\" FIXME Say more about MS_MOVE
+.\" FIXME Describe the MS_MOVE flag in more detail
.TP
.B MS_MOVE
Move a subtree.
arguments are ignored.
.TP
.B MS_NOATIME
-Do not update access times for (all types of) files on this file system.
+Do not update access times for (all types of) files on this filesystem.
.TP
.B MS_NODEV
-Do not allow access to devices (special files) on this file system.
+Do not allow access to devices (special files) on this filesystem.
.TP
.B MS_NODIRATIME
-Do not update access times for directories on this file system.
+Do not update access times for directories on this filesystem.
+This flag provides a subset of the functionality provided by
+.BR MS_NOATIME ;
+that is,
+.BR MS_NOATIME
+implies
+.BR MS_NODIRATIME .
.TP
.B MS_NOEXEC
-Do not allow programs to be executed from this file system.
-.\" (Possibly useful for a file system that contains non-Linux executables.
+Do not allow programs to be executed from this filesystem.
+.\" (Possibly useful for a filesystem that contains non-Linux executables.
.\" Often used as a security feature, e.g., to make sure that restricted
.\" users cannot execute files uploaded using ftp or so.)
.TP
.B MS_NOSUID
Do not honor set-user-ID and set-group-ID bits when executing
-programs from this file system.
+programs from this filesystem.
.\" (This is a security feature to prevent users executing set-user-ID and
.\" set-group-ID programs from removable disk devices.)
.TP
.B MS_RDONLY
-Mount file system read-only.
+Mount filesystem read-only.
.\"
.\" FIXME Document MS_REC, available since 2.4.11.
.\" This flag has meaning in conjunction with MS_BIND and
-.\" also with the shared sub-tree flags.
+.\" also with the shared subtree flags.
.TP
-.BR MS_RELATIME " (Since Linux 2.6.20)"
-When a file on this file system is accessed,
-only update the file's last accessed time (atime) if the current value
-of atime is less than or equal to the file's last modified (mtime)
+.BR MS_RELATIME " (since Linux 2.6.20)"
+When a file on this filesystem is accessed,
+update the file's last access time (atime) only if the current value
+of atime is less than or equal to the file's last modification time (mtime)
or last status change time (ctime).
This option is useful for programs, such as
.BR mutt (1),
that need to know when a file has been read since it was last modified.
+Since Linux 2.6.30, the kernel defaults to the behavior provided
+by this flag (unless
+.BR MS_NOATIME
+was specified), and the
+.B MS_STRICTATIME
+flag is required to obtain traditional semantics.
+In addition, since Linux 2.6.30,
+the file's last access time is always updated if it
+is more than 1 day old.
+.\" Matthew Garrett notes in the patch that added this behavior
+.\" that this lets utilities such as tmpreaper (which deletes
+.\" files based on last access time) work correctly.
.TP
.B MS_REMOUNT
Remount an existing mount.
.I mountflags
and
.I data
-of an existing mount without having to unmount and remount the file system.
-.I source
-and
+of an existing mount without having to unmount and remount the filesystem.
.I target
-should be the same values specified in the initial
+should be the same value specified in the initial
.BR mount ()
call;
+.I source
+and
.I filesystemtype
-is ignored.
+are ignored.
+The
+.I mountflags
+and
+.I data
+arguments should match the values used in the original
+.BR mount ()
+call, except for those parameters that are being deliberately changed.
The following
.I mountflags
.BR MS_SYNCHRONOUS ,
.BR MS_MANDLOCK ;
before kernel 2.6.16, the following could also be changed:
-.BR MS_NOATIME
+.B MS_NOATIME
and
.BR MS_NODIRATIME ;
-and, additionally, before kernel 2.4, the following could also be changed:
+and, additionally, before kernel 2.4.10, the following could also be changed:
.BR MS_NOSUID ,
.BR MS_NODEV ,
.BR MS_NOEXEC .
.TP
+.BR MS_SILENT " (since Linux 2.6.17)"
+Suppress the display of certain
+.RI ( printk ())
+warning messages in the kernel log.
+This flag supersedes the misnamed and obsolete
+.BR MS_VERBOSE
+flag (available since Linux 2.4.12), which has the same meaning.
+.TP
+.BR MS_STRICTATIME " (since Linux 2.6.30)"
+Always update the last access time (atime) when files on this
+filesystem are accessed.
+(This was the default behavior before Linux 2.6.30.)
+Specifying this flag overrides the effect of setting the
+.BR MS_NOATIME
+and
+.BR MS_RELATIME
+flags.
+.TP
.B MS_SYNCHRONOUS
-Make writes on this file system synchronous (as though
+Make writes on this filesystem synchronous (as though
the
.B O_SYNC
flag to
.BR open (2)
-was specified for all file opens to this file system).
+was specified for all file opens to this filesystem).
.PP
-From Linux 2.4 onwards, the
+From Linux 2.4 onward, the
.BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
flags are settable on a per-mount-point basis.
-From kernel 2.6.16 onwards,
+From kernel 2.6.16 onward,
.B MS_NOATIME
and
.B MS_NODIRATIME
flag is also settable on a per-mount-point basis.
.PP
The
-.IR data
-argument is interpreted by the different file systems.
+.I data
+argument is interpreted by the different filesystems.
Typically it is a string of comma-separated options
-understood by this file system.
+understood by this filesystem.
See
.BR mount (8)
for details of the options available for each filesystem type.
-.PP
-.\" Note: the kernel naming differs from the glibc naming
-.\" umount2 is the glibc name for what the kernel now calls umount
-.\" and umount is the glibc name for oldumount
-Linux 2.1.116 added the
-.BR umount2 ()
-system call, which, like
-.BR umount (),
-unmounts a target, but allows additional
-.I flags
-controlling the behavior of the operation:
-.TP
-.BR MNT_FORCE " (since Linux 2.1.116)"
-Force unmount even if busy.
-This can cause data loss.
-(Only for NFS mounts.)
-.\" FIXME Can MNT_FORCE result in data loss? According to
-.\" the Solaris manual page it can cause data loss on Solaris.
-.\" If the same holds on Linux, then this should be documented.
-.TP
-.BR MNT_DETACH " (since Linux 2.4.11)"
-Perform a lazy unmount: make the mount point unavailable for
-new accesses, and actually perform the unmount when the mount point
-ceases to be busy.
-.TP
-.BR MNT_EXPIRE " (since Linux 2.6.8)"
-Mark the mount point as expired.
-If a mount point is not currently in use, then an initial call to
-.BR umount2 ()
-with this flag fails with the error
-.BR EAGAIN ,
-but marks the mount point as expired.
-The mount point remains expired as long as it isn't accessed
-by any process.
-A second
-.BR umount2 ()
-call specifying
-.B MNT_EXPIRE
-unmounts an expired mount point.
-This flag cannot be specified with either
-.B MNT_FORCE
-or
-.BR MNT_DETACH .
-.SH "RETURN VALUE"
+.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
errors.
Each filesystem type may have its own special errors and its
own special behavior.
-See the kernel source code for details.
+See the Linux kernel source code for details.
.TP
.B EACCES
A component of a path was not searchable.
.\" mtk: Probably: write permission is required for MS_BIND, with
.\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
.TP
-.B EAGAIN
-A call to
-.BR umount2 ()
-specifying
-.B MNT_EXPIRE
-successfully marked an unbusy file system as expired.
-.TP
.B EBUSY
.I source
is already mounted.
.I target
because
.I target
-is still busy (it is the working directory of some task,
+is still busy (it is the working directory of some thread,
the mount point of another device, has open files, etc.).
-Or, it could not be unmounted because it is busy.
.TP
.B EFAULT
One of the pointer arguments points outside the user address space.
.RB ( MS_MOVE )
was attempted, but
.I source
-was not a mount point, or was '/'.
-Or, an unmount was attempted, but
-.I target
-was not a mount point.
-Or,
-.BR umount2 ()
-was called with
-.B MNT_EXPIRE
-and either
-.B MNT_DETACH
-or
-.BR MNT_FORCE .
+was not a mount point, or was \(aq/\(aq.
.TP
.B ELOOP
-Too many link encountered during pathname resolution.
+Too many links encountered during pathname resolution.
Or, a move was attempted, while
.I target
is a descendant of
is not a block device (and a device was required).
.TP
.B ENOTDIR
-The second argument, or a prefix of the first argument, is not
-a directory.
+.IR target ,
+or a prefix of
+.IR source ,
+is not a directory.
.TP
.B ENXIO
The major number of the block device
.TP
.B EPERM
The caller does not have the required privileges.
-.SH "CONFORMING TO"
-These functions are Linux specific and should not be used in
+.SH VERSIONS
+The definitions of
+.BR MS_DIRSYNC ,
+.BR MS_MOVE ,
+.BR MS_REC ,
+.BR MS_RELATIME ,
+and
+.BR MS_STRICTATIME
+were added to glibc headers in version 2.12.
+.\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
+.\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
+.SH CONFORMING TO
+This function is Linux-specific and should not be used in
programs intended to be portable.
.SH NOTES
-.SS Linux Notes
The original
-.BR umount ()
-function was called as \fIumount(device)\fP and would return
-.B ENOTBLK
-when called with something other than a block device.
-In Linux 0.98p4 a call \fIumount(dir)\fP was added, in order to
-support anonymous devices.
-In Linux 2.3.99-pre7 the call \fIumount(device)\fP was removed,
-leaving only \fIumount(dir)\fP (since now devices can be mounted
-in more than one place, so specifying the device does not suffice).
-.LP
-The original
-.BR MS_SYNC
+.B MS_SYNC
flag was renamed
-.BR MS_SYNCHRONOUS
+.B MS_SYNCHRONOUS
in 1.1.69
when a different
-.BR MS_SYNC
+.B MS_SYNC
was added to \fI<mman.h>\fP.
.LP
Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
Since Linux 2.4 the set-user-ID and set-group-ID bits are
just silently ignored in this case.
.\" The change is in patch-2.4.0-prerelease.
-.SH "SEE ALSO"
+.SS Per-process namespaces
+Starting with kernel 2.4.19, Linux provides
+per-process mount namespaces.
+A mount namespace is the set of filesystem mounts that
+are visible to a process.
+Mount-point namespaces can be (and usually are)
+shared between multiple processes,
+and changes to the namespace (i.e., mounts and unmounts) by one process
+are visible to all other processes sharing the same namespace.
+(The pre-2.4.19 Linux situation can be considered as one in which
+a single namespace was shared by every process on the system.)
+
+A child process created by
+.BR fork (2)
+shares its parent's mount namespace;
+the mount namespace is preserved across an
+.BR execve (2).
+
+A process can obtain a private mount namespace if:
+it was created using the
+.BR clone (2)
+.BR CLONE_NEWNS
+flag,
+in which case its new namespace is initialized to be a
+.I copy
+of the namespace of the process that called
+.BR clone (2);
+or it calls
+.BR unshare (2)
+with the
+.BR CLONE_NEWNS
+flag,
+which causes the caller's mount namespace to obtain a private copy
+of the namespace that it was previously sharing with other processes,
+so that future mounts and unmounts by the caller are invisible
+to other processes (except child processes that the caller
+subsequently creates) and vice versa.
+
+The Linux-specific
+.I /proc/PID/mounts
+file exposes the list of mount points in the mount
+namespace of the process with the specified ID; see
+.BR proc (5)
+for details.
+.SH SEE ALSO
+.BR umount (2),
+.BR namespaces (7),
.BR path_resolution (7),
+.BR lsblk (8),
+.BR findmnt (8),
.BR mount (8),
.BR umount (8)
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