.\" 2008-10-06, mtk: move umount*() material into separate umount.2 page.
.\" 2008-10-06, mtk: Add discussion of namespaces.
.\"
-.TH MOUNT 2 2016-03-15 "Linux" "Linux Programmer's Manual"
+.TH MOUNT 2 2019-08-02 "Linux" "Linux Programmer's Manual"
.SH NAME
mount \- mount filesystem
.SH SYNOPSIS
.nf
.B "#include <sys/mount.h>"
-.sp
+.PP
.BI "int mount(const char *" source ", const char *" target ,
.BI " const char *" filesystemtype ", unsigned long " mountflags ,
.BI " const void *" data );
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.
+performs one of a number of general types of operation,
depending on the bits specified in
.IR mountflags .
-The choice of operation is determined by testing the bits set in
+The choice of which operation to perform is determined by
+testing the bits set in
.IR mountflags ,
with the tests being conducted in the order listed here:
.IP * 3
as described later in this page.
.\"
.\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
+.\" commit 7a224228ed79d587ece2304869000aad1b8e97dd
+.\" (This is a per-superblock flag)
.\"
.TP
.BR MS_DIRSYNC " (since Linux 2.5.19)"
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on disk might be out of date by up to 24 hours.
-
+.IP
Examples of workloads where this option could be of significant benefit
include frequent random writes to preallocated files,
as well as cases where the
.\" commit 95ace75414f312f9a7b93d873f386987b92a5301
this mount option requires the
.B CAP_SYS_ADMIN
-capability.
-.\" FIXME Describe the MS_MOVE flag in more detail
+capability and a kernel configured with the
+.B CONFIG_MANDATORY_FILE_LOCKING
+option.
.TP
.B MS_NOATIME
Do not update access times for (all types of) files on this filesystem.
.BR open (2)
was specified for all file opens to this filesystem).
.PP
-From Linux 2.4 onward, the
+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.)
+.PP
+The per-mount-point flags are as follows:
+.IP * 3
+Since Linux 2.4:
.BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
flags are settable on a per-mount-point basis.
-From kernel 2.6.16 onward,
+.IP *
+Since Linux 2.6.16:
.B MS_NOATIME
and
-.B MS_NODIRATIME
-are also settable on a per-mount-point basis.
-The
-.B MS_RELATIME
-flag is also settable on a per-mount-point basis.
+.BR MS_NODIRATIME .
+.IP *
+Since Linux 2.6.20:
+.BR MS_RELATIME .
+.PP
+The following flags are per-superblock:
+.BR MS_DIRSYNC ,
+.BR MS_LAZYTIME ,
+.BR MS_MANDLOCK ,
+.BR MS_MS_SILENT ,
+and
+.BR MS_SYNCHRONOUS .
+.\" And MS_I_VERSION?
+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 mount points associated
+with the filesystem.
+.PP
+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 superblock.
+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.
-
+.PP
The following
.I mountflags
can be changed:
.BR MS_LAZYTIME ,
+.\" FIXME
+.\" MS_LAZYTIME seems to be available only on a few filesystems,
+.\" and on ext4, it seems (from experiment that this flag
+.\" can only be enabled (but not disabled) on a remount.
+.\" The following code in ext4_remount() (kernel 4.17) seems to
+.\" confirm this:
+.\"
+.\" if (*flags & SB_LAZYTIME)
+.\" sb->s_flags |= SB_LAZYTIME;
.BR MS_MANDLOCK ,
.BR MS_NOATIME ,
.BR MS_NODEV ,
.BR MS_NOSUID ,
.BR MS_RELATIME ,
.BR MS_RDONLY ,
+.BR MS_STRICTATIME
+(whose effect is to clear the
+.BR MS_NOATIME
+and
+.BR MS_RELATIME
+flags),
and
.BR MS_SYNCHRONOUS .
Attempts to change the setting of the
-.\" See the definition of MS_RMT_MASK in include/uapi/linux/fs.h
+.\" See the definition of MS_RMT_MASK in include/uapi/linux/fs.h,
+.\" which excludes MS_DIRSYNC and MS_SILENT, although SB_DIRSYNC
+.\" and SB_SILENT are split out as per-superblock flags in do_mount()
+.\" (Linux 4.17 source code)
.BR MS_DIRSYNC
-flag during a remount are silently ignored.
-
+and
+.BR MS_SILENT
+flags during a remount are silently ignored.
+Note that changes to per-superblock flags are visible via
+all mount points of the associated filesystem
+(because the per-superblock flags are shared by all mount points).
+.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, the
+.B MS_REMOUNT
+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:
-
- 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
+.in +4n
+.EX
+MS_REMOUNT | MS_BIND | MS_RDONLY
+.EE
+.in
+.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.
-
-The remaining bits in the
+.PP
+The remaining bits (other than
+.BR MS_REC ,
+described below) in the
.I mountflags
-argument are also ignored, with the exception of
-.BR MS_REC .
+argument are also ignored.
(The bind mount has the same mount options as
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.
-
-The only flags that can be used with changing the propagation type are
+.PP
+The only other flags that can be specified while changing
+the propagation type are
.BR MS_REC
-and
-.BR MS_SILENT .
-
+(described below) and
+.BR MS_SILENT
+(which is ignored).
+.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.
-
+.IP
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.
-
+.IP
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.
This is like a private mount,
and in addition this mount can't be bind mounted.
When a recursive bind mount
-.RB ( mount (2)
+.RB ( mount ()
with the
.BR MS_BIND
and
.BR MS_REC
flags) is performed on a directory subtree,
-any bind mounts within the subtree are automatically pruned
+any unbindable mounts within the subtree are automatically pruned
(i.e., not replicated)
when replicating that subtree to produce the target subtree.
.PP
then the propagation type of all mount points under
.IR target
is also changed.
+.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
If
.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
Mounting a read-only filesystem was attempted without giving the
.B MS_RDONLY
flag.
+.IP
+The file system may be read-only for various reasons, including:
+it resides on a read-only optical disk;
+it is resides on a device with a physical switch that has been set to
+mark the device read-only;
+the filesystem implementation was compiled with read-only support;
+or errors were detected when initially mounting the filesystem,
+so that it was marked read-only
+and can't be remounted as read-write (until the errors are fixed).
+.IP
+Some filesystems instead return the error
+.BR EROFS
+on an attempt to mount a read-only filesystem.
.TP
.B EACCES
The block device
.\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
.TP
.B EBUSY
+An attempt was made to stack a new mount directly on
+top of an existing mount point that was created in this
+mount namespace with the same
.I source
-is already mounted.
+and
+.IR target .
.TP
.B EBUSY
.I source
cannot be remounted read-only,
because it still holds files open for writing.
.TP
-.B EBUSY
-.I source
-cannot be mounted on
-.I target
-because
-.I target
-is still busy (it is the working directory of some thread,
-the mount point of another device, has open files, etc.).
-.TP
.B EFAULT
One of the pointer arguments points outside the user address space.
.TP
.B EINVAL
A move operation
.RB ( MS_MOVE )
+was attempted, but the mount tree under
+.I source
+includes unbindable mounts and
+.I target
+is a mount point that has propagation type
+.BR MS_SHARED .
+.TP
+.B EINVAL
+A move operation
+.RB ( MS_MOVE )
+was attempted, but the parent mount of
+.I source
+mount has propagation type
+.BR MS_SHARED .
+.TP
+.B EINVAL
+A move operation
+.RB ( MS_MOVE )
was attempted, but
.I source
was not a mount point, or was \(aq/\(aq.
.BR EINVAL
An attempt was made to bind mount an unbindable mount.
.TP
+.BR EINVAL
+In an unprivileged mount namespace
+(i.e., a mount namespace owned by a user namespace
+that was created by an unprivileged user),
+a bind mount operation
+.RB ( MS_BIND )
+was attempted without specifying
+.RB ( MS_REC ),
+which would have revealed the filesystem tree underneath one of
+the submounts of the directory being bound.
+.TP
.B ELOOP
Too many links encountered during pathname resolution.
.TP
.TP
.B EPERM
The caller does not have the required privileges.
+.TP
+.B EROFS
+Mounting a read-only filesystem was attempted without giving the
+.B MS_RDONLY
+flag.
+See
+.BR EACCES ,
+above.
.SH VERSIONS
The definitions of
.BR MS_DIRSYNC ,
.BR MS_REC ,
.BR MS_RELATIME ,
.BR MS_SHARED ,
-.BR MS_SLAVED ,
-.BR MS_STRICTATIME
+.BR MS_SLAVE ,
+.BR MS_STRICTATIME ,
and
.BR MS_UNBINDABLE
were added to glibc headers in version 2.12.
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
when a different
.B MS_SYNC
was added to \fI<mman.h>\fP.
-.LP
+.PP
Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
on a filesystem mounted with
.B MS_NOSUID
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
+namespace of the process with the specified ID.
+The
+.I /proc/[pid]/mountinfo
+file exposes even more information about mount points,
+including the propagation type and mount ID information that makes it
+possible to discover the parental relationship between mount points.
+See
.BR proc (5)
-for details.
+and
+.BR mount_namespaces (7)
+for details of these files.
.SH SEE ALSO
+.BR mountpoint (1),
+.BR chroot (2),
+.BR ioctl_iflags (2),
+.BR pivot_root (2),
.BR umount (2),
-.BR namespaces (7),
+.BR mount_namespaces (7),
.BR path_resolution (7),
-.BR lsblk (8),
.BR findmnt (8),
+.BR lsblk (8),
.BR mount (8),
.BR umount (8)
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