Manpages - mount.2

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:

1. the inode needs to be updated for some change unrelated to file timestamps;
2. the application employs *fsync*(2), *syncfs*(2), or *sync*(2);
3. an undeleted inode is evicted from memory; or
4. 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 However, in the event of a system crash, the atime and mtime fields on disk might be out of date by up to 24 hours.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.

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.

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.

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).

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.

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.

Starting with kernel 2.4.19, Linux provides mount namespaces. A mount namespace is the set of filesystem mounts that are visible to a process. Mount 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 *fork*(2) shares its parent’s mount namespace; the mount namespace is preserved across an *execve*(2).

A process can obtain a private mount namespace if: it was created using the clone*(2) *CLONE_NEWNS flag, in which case its new namespace is initialized to be a copy of the namespace of the process that called clone*(2); or it calls *unshare*(2) with the *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.

For further details on mount namespaces, see *mount_namespaces*(7).

Each mount has a parent mount. The overall parental relationship of all mounts defines the single directory hierarchy seen by the processes within a mount namespace.

The parent of a new mount is defined when the mount is created. In the usual case, the parent of a new mount is the mount of the filesystem containing the directory or file at which the new mount is attached. In the case where a new mount is stacked on top of an existing mount, the parent of the new mount is the previous mount that was stacked at that location.

The parental relationship between mounts can be discovered via the /proc[pid]/mountinfo/ file (see below).

The Linux-specific /proc[pid]/mounts/ file exposes the list of mounts in the mount namespace of the process with the specified ID. The /proc[pid]/mountinfo/ file exposes even more information about mounts, including the propagation type and mount ID information that makes it possible to discover the parental relationship between mounts. See *proc*(5) and *mount_namespaces*(7) for details of this file.