.\" 2008-10-06, mtk: move umount*() material into separate umount.2 page.
.\" 2008-10-06, mtk: Add discussion of namespaces.
.\"
-.TH MOUNT 2 2016-07-17 "Linux" "Linux Programmer's Manual"
+.TH MOUNT 2 2017-07-13 "Linux" "Linux Programmer's Manual"
.SH NAME
mount \- mount filesystem
.SH SYNOPSIS
or a dummy string) to the location (a directory or file)
specified by the pathname in
.IR target .
-
+.PP
Appropriate privilege (Linux: the
.B CAP_SYS_ADMIN
capability) is required to mount filesystems.
-
+.PP
Values for the
.I filesystemtype
argument supported by the kernel are listed in
"tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs", "iso9660").
Further types may become available when the appropriate modules
are loaded.
-
+.PP
The
.I data
argument is interpreted by the different filesystems.
See
.BR mount (8)
for details of the options available for each filesystem type.
-
+.PP
A call to
.BR mount ()
performs one of a number of general types of operation,
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on disk might be out of date by up to 24 hours.
-
+.PP
Examples of workloads where this option could be of significant benefit
include frequent random writes to preallocated files,
as well as cases where the
this mount option requires the
.B CAP_SYS_ADMIN
capability.
-.\" FIXME Describe the MS_MOVE flag in more detail
.TP
.B MS_NOATIME
Do not update access times for (all types of) files on this filesystem.
The
.B MS_RELATIME
flag is also settable on a per-mount-point basis.
+Since Linux 2.6.16,
+.B MS_RDONLY
+can be set or cleared on a per-mount-point basis as well as on
+the underlying filesystem.
+The mounted filesystem will be writable only if neither the filesystem
+nor the mountpoint are flagged as read-only.
.\"
.SS Remounting an existing mount
An existing mount may be remounted by specifying
should be the same value specified in the initial
.BR mount ()
call.
-
+.PP
The
.I source
and
.I filesystemtype
arguments are ignored.
-
+.PP
The
.I mountflags
and
arguments should match the values used in the original
.BR mount ()
call, except for those parameters that are being deliberately changed.
-
+Another exception is that
+.B MS_BIND
+has a different meaning for remount, and it should be included only if
+explicitly desired.
+.PP
The following
.I mountflags
can be changed:
.\" See the definition of MS_RMT_MASK in include/uapi/linux/fs.h
.BR MS_DIRSYNC
flag during a remount are silently ignored.
-
+.PP
Since Linux 3.17,
.\" commit ffbc6f0ead47fa5a1dc9642b0331cb75c20a640e
if none of
then the remount operation preserves the existing values of these flags
(rather than defaulting to
.BR MS_RELATIME ).
-
-Since Linux 2.6.26, this flag can also be used to make
+.PP
+Since Linux 2.6.26, this flag can be used with
+.B MS_BIND
+to modify only the per-mount-point flags.
.\" See https://lwn.net/Articles/281157/
-an existing bind mount read-only by specifying
+This is particularly useful for setting or clearing the "read-only"
+flag on a mount point without changing the underlying filesystem.
+Specifying
.IR mountflags
as:
-
+.PP
MS_REMOUNT | MS_BIND | MS_RDONLY
-
-Note that only the
-.BR MS_RDONLY
-setting of the bind mount can be changed in this manner.
+.PP
+will make access through this mountpoint read-only, without affecting
+other mount points.
.\"
.SS Creating a bind mount
If
Bind mounts may cross filesystem boundaries and span
.BR chroot (2)
jails.
-
+.PP
The
.IR filesystemtype
and
.IR data
arguments are ignored.
-
+.PP
The remaining bits in the
.I mountflags
argument are also ignored, with the exception of
the underlying mount point.)
However, see the discussion of remounting above,
for a method of making an existing bind mount read-only.
-
+.PP
By default, when a directory is bind mounted,
only that directory is mounted;
if there are any submounts under the directory tree,
(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.
-
+.PP
The only flags that can be used with changing the propagation type are
.BR MS_REC
and
.BR MS_SILENT .
-
+.PP
The
.IR source ,
.IR filesystemtype ,
and
.IR data
arguments are ignored.
-
+.PP
The meanings of the propagation type flags are as follows:
.TP
.BR MS_SHARED
.BR MS_PRIVATE
Make this mount point private.
Mount and unmount events do not propagate into or out of this mount point.
-This is the default propagation type for newly created mount points.
.TP
.BR MS_SLAVE
If this is a shared mount point that is a member of a peer group
If this is a shared mount point 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 point is left unchanged.
-
+.PP
When a mount point is a slave,
mount and unmount events propagate into this mount point from
the (master) shared peer group of which it was formerly a member.
Mount and unmount events under this mount point do not propagate to any peer.
-
+.PP
A mount point 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.
then the propagation type of all mount points under
.IR target
is also changed.
-
-For further details regarding mount propagation types, see
+.PP
+For further details regarding mount propagation types
+(including the default propagation type assigned to new mounts), see
.BR mount_namespaces (7).
.\"
.SS Moving a mount
.I target
specifies the new location to which that mount point is to be relocated.
The move is atomic: at no point is the subtree unmounted.
-
+.PP
The remaining bits in the
.IR mountflags
argument are ignored, as are the
specifies the source for the new mount point, and
.IR target
specifies the directory at which to create the mount point.
-
+.PP
The
.I filesystemtype
and
.BR MS_REC ,
.BR MS_RELATIME ,
.BR MS_SHARED ,
-.BR MS_SLAVED ,
+.BR MS_SLAVE ,
.BR MS_STRICTATIME
and
.BR MS_UNBINDABLE
multiple mount points, and multiple mounts can be stacked
on the same mount point.
.\" Multiple mounts on same mount point: since 2.3.99pre7.
-
+.PP
The
.I mountflags
argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
.BR MS_MGC_VAL
was required in kernel versions prior to 2.4,
but since Linux 2.4 is no longer required and is ignored if specified.
-
+.PP
The original
.B MS_SYNC
flag was renamed
just silently ignored in this case.
.\" The change is in patch-2.4.0-prerelease.
.\"
-
.SS Per-process namespaces
Starting with kernel 2.4.19, Linux provides
per-process mount namespaces.
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.)
-
+.PP
A child process created by
.BR fork (2)
shares its parent's mount namespace;
the mount namespace is preserved across an
.BR execve (2).
-
+.PP
A process can obtain a private mount namespace if:
it was created using the
.BR clone (2)
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.
-
+.PP
The Linux-specific
-.I /proc/PID/mounts
+.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 mountpoint (1),
.BR umount (2),
.BR mount_namespaces (7),
.BR path_resolution (7),
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