mount() attaches the filesystem specified by source (which is
often a pathname referring to a device, but can also be the
pathname of a directory or file, or a dummy string) to the
location (a directory or file) specified by the pathname in
target.
Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is
required to mount filesystems.
Values for the filesystemtype argument supported by the kernel
are listed in /proc/filesystems (e.g., "btrfs", "ext4", "jfs",
"xfs", "vfat", "fuse", "tmpfs", "cgroup", "proc", "mqueue",
"nfs", "cifs", "iso9660"). Further types may become available
when the appropriate modules are loaded.
The data argument is interpreted by the different filesystems.
Typically it is a string of comma-separated options understood by
this filesystem. See mount(8) for details of the options
available for each filesystem type. This argument may be
specified as NULL, if there are no options.
A call to mount() performs one of a number of general types of
operation, depending on the bits specified in mountflags. The
choice of which operation to perform is determined by testing the
bits set in mountflags, with the tests being conducted in the
order listed here:
* Remount an existing mount: mountflags includes MS_REMOUNT.
* Create a bind mount: mountflags includes MS_BIND.
* Change the propagation type of an existing mount: mountflags
includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or
MS_UNBINDABLE.
* Move an existing mount to a new location: mountflags includes
MS_MOVE.
* Create a new mount: mountflags includes none of the above
flags.
Each of these operations is detailed later in this page. Further
flags may be specified in mountflags to modify the behavior of
mount(), as described below.
Additional mount flags
The list below describes the additional flags that can be
specified in mountflags. Note that some operation types ignore
some or all of these flags, as described later in this page.
MS_DIRSYNC (since Linux 2.5.19)
Make directory changes on this filesystem synchronous.
(This property can be obtained for individual directories
or subtrees using chattr(1).)
MS_LAZYTIME (since Linux 4.0)
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:
(a) the inode needs to be updated for some change
unrelated to file timestamps;
(b) the application employs fsync(2), syncfs(2), or
sync(2);
(c) an undeleted inode is evicted from memory; or
(d) more than 24 hours have passed since the inode was
written to disk.
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
MS_STRICTATIME mount option is also enabled. (The
advantage of combining MS_STRICTATIME and MS_LAZYTIME is
that stat(2) will return the correctly updated atime, but
the atime updates will be flushed to disk only in the
cases listed above.)
MS_MANDLOCK
Permit mandatory locking on files in this filesystem.
(Mandatory locking must still be enabled on a per-file
basis, as described in fcntl(2).) Since Linux 4.5, this
mount option requires the CAP_SYS_ADMIN capability and a
kernel configured with the CONFIG_MANDATORY_FILE_LOCKING
option.
MS_NOATIME
Do not update access times for (all types of) files on
this filesystem.
MS_NODEV
Do not allow access to devices (special files) on this
filesystem.
MS_NODIRATIME
Do not update access times for directories on this
filesystem. This flag provides a subset of the
functionality provided by MS_NOATIME; that is, MS_NOATIME
implies MS_NODIRATIME.
MS_NOEXEC
Do not allow programs to be executed from this filesystem.
MS_NOSUID
Do not honor set-user-ID and set-group-ID bits or file
capabilities when executing programs from this filesystem.
In addition, SELinux domain transitions require the
permission nosuid_transition, which in turn needs also the
policy capability nnp_nosuid_transition.
MS_RDONLY
Mount filesystem read-only.
MS_REC (since Linux 2.4.11)
Used in conjunction with MS_BIND to create a recursive
bind mount, and in conjunction with the propagation type
flags to recursively change the propagation type of all of
the mounts in a subtree. See below for further details.
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
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
MS_NOATIME was specified), and the 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.
MS_SILENT (since Linux 2.6.17)
Suppress the display of certain (printk()) warning
messages in the kernel log. This flag supersedes the
misnamed and obsolete MS_VERBOSE flag (available since
Linux 2.4.12), which has the same meaning.
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 MS_NOATIME and
MS_RELATIME flags.
MS_SYNCHRONOUS
Make writes on this filesystem synchronous (as though the
O_SYNC flag to open(2) was specified for all file opens to
this filesystem).
MS_NOSYMFOLLOW (since Linux 5.10)
Do not follow symbolic links when resolving paths.
Symbolic links can still be created, and readlink(1),
readlink(2), realpath(1), and realpath(3) all still work
properly.
From Linux 2.4 onward, some of the above flags are settable on a
per-mount basis, while others apply to the superblock of the
mounted filesystem, meaning that all mounts of the same
filesystem share those flags. (Previously, all of the flags were
per-superblock.)
The per-mount-point flags are as follows:
* Since Linux 2.4: MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are
settable on a per-mount-point basis.
* Additionally, since Linux 2.6.16: MS_NOATIME and
MS_NODIRATIME.
* Additionally, since Linux 2.6.20: MS_RELATIME.
The following flags are per-superblock: MS_DIRSYNC, MS_LAZYTIME,
MS_MANDLOCK, MS_SILENT, and MS_SYNCHRONOUS. The initial settings
of these flags are determined on the first mount of the
filesystem, and will be shared by all subsequent mounts of the
same filesystem. Subsequently, the settings of the flags can be
changed via a remount operation (see below). Such changes will
be visible via all mounts associated with the filesystem.
Since Linux 2.6.16, MS_RDONLY can be set or cleared on a per-
mount-point basis as well as on the underlying filesystem
superblock. The mounted filesystem will be writable only if
neither the filesystem nor the mountpoint are flagged as read-
only.
Remounting an existing mount
An existing mount may be remounted by specifying MS_REMOUNT in
mountflags. This allows you to change the mountflags and data of
an existing mount without having to unmount and remount the
filesystem. target should be the same value specified in the
initial mount() call.
The source and filesystemtype arguments are ignored.
The mountflags and data arguments should match the values used in
the original mount() call, except for those parameters that are
being deliberately changed.
The following mountflags can be changed: MS_LAZYTIME,
MS_MANDLOCK, MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC,
MS_NOSUID, MS_RELATIME, MS_RDONLY, MS_STRICTATIME (whose effect
is to clear the MS_NOATIME and MS_RELATIME flags), and
MS_SYNCHRONOUS. Attempts to change the setting of the MS_DIRSYNC
and MS_SILENT flags during a remount are silently ignored. Note
that changes to per-superblock flags are visible via all mounts
of the associated filesystem (because the per-superblock flags
are shared by all mounts).
Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME,
MS_RELATIME, or MS_STRICTATIME is specified in mountflags, then
the remount operation preserves the existing values of these
flags (rather than defaulting to MS_RELATIME).
Since Linux 2.6.26, the MS_REMOUNT flag can be used with MS_BIND
to modify only the per-mount-point flags. This is particularly
useful for setting or clearing the "read-only" flag on a mount
without changing the underlying filesystem. Specifying
mountflags as:
MS_REMOUNT | MS_BIND | MS_RDONLY
will make access through this mountpoint read-only, without
affecting other mounts.
Creating a bind mount
If mountflags includes MS_BIND (available since Linux 2.4), then
perform a bind mount. A bind mount makes a file or a directory
subtree visible at another point within the single directory
hierarchy. Bind mounts may cross filesystem boundaries and span
chroot(2) jails.
The filesystemtype and data arguments are ignored.
The remaining bits (other than MS_REC, described below) in the
mountflags argument are also ignored. (The bind mount has the
same mount options as the underlying mount.) However, see the
discussion of remounting above, for a method of making an
existing bind mount read-only.
By default, when a directory is bind mounted, only that directory
is mounted; if there are any submounts under the directory tree,
they are not bind mounted. If the MS_REC flag is also specified,
then a recursive bind mount operation is performed: all submounts
under the source subtree (other than unbindable mounts) are also
bind mounted at the corresponding location in the target subtree.
Changing the propagation type of an existing mount
If mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or
MS_UNBINDABLE (all available since Linux 2.6.15), then the
propagation type of an existing mount is changed. If more than
one of these flags is specified, an error results.
The only other flags that can be specified while changing the
propagation type are MS_REC (described below) and MS_SILENT
(which is ignored).
The source, filesystemtype, and data arguments are ignored.
The meanings of the propagation type flags are as follows:
MS_SHARED
Make this mount shared. Mount and unmount events
immediately under this mount will propagate to the other
mounts that are members of this mount's peer group.
Propagation here means that the same mount or unmount will
automatically occur under all of the other mounts in the
peer group. Conversely, mount and unmount events that
take place under peer mounts will propagate to this mount.
MS_PRIVATE
Make this mount private. Mount and unmount events do not
propagate into or out of this mount.
MS_SLAVE
If this is a shared mount that is a member of a peer group
that contains other members, convert it to a slave mount.
If this is a shared mount that is a member of a peer group
that contains no other members, convert it to a private
mount. Otherwise, the propagation type of the mount is
left unchanged.
When a mount is a slave, mount and unmount events
propagate into this mount from the (master) shared peer
group of which it was formerly a member. Mount and
unmount events under this mount do not propagate to any
peer.
A mount can be the slave of another peer group while at
the same time sharing mount and unmount events with a peer
group of which it is a member.
MS_UNBINDABLE
Make this mount unbindable. This is like a private mount,
and in addition this mount can't be bind mounted. When a
recursive bind mount (mount() with the MS_BIND and MS_REC
flags) is performed on a directory subtree, any unbindable
mounts within the subtree are automatically pruned (i.e.,
not replicated) when replicating that subtree to produce
the target subtree.
By default, changing the propagation type affects only the target
mount. If the MS_REC flag is also specified in mountflags, then
the propagation type of all mounts under target is also changed.
For further details regarding mount propagation types (including
the default propagation type assigned to new mounts), see
mount_namespaces(7).
Moving a mount
If mountflags contains the flag MS_MOVE (available since Linux
2.4.18), then move a subtree: source specifies an existing mount
and target specifies the new location to which that mount is to
be relocated. The move is atomic: at no point is the subtree
unmounted.
The remaining bits in the mountflags argument are ignored, as are
the filesystemtype and data arguments.
Creating a new mount
If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE,
MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then
mount() performs its default action: creating a new mount.
source specifies the source for the new mount, and target
specifies the directory at which to create the mount point.
The filesystemtype and data arguments are employed, and further
bits may be specified in mountflags to modify the behavior of the
call.
