linux-zen-desktop/Documentation/filesystems/tmpfs.rst

.. SPDX-License-Identifier: GPL-2.0

=====
Tmpfs
=====

Tmpfs is a file system which keeps all of its files in virtual memory.


Everything in tmpfs is temporary in the sense that no files will be
created on your hard drive. If you unmount a tmpfs instance,
everything stored therein is lost.

tmpfs puts everything into the kernel internal caches and grows and
shrinks to accommodate the files it contains and is able to swap
unneeded pages out to swap space, if swap was enabled for the tmpfs
mount. tmpfs also supports THP.

tmpfs extends ramfs with a few userspace configurable options listed and
explained further below, some of which can be reconfigured dynamically on the
fly using a remount ('mount -o remount ...') of the filesystem. A tmpfs
filesystem can be resized but it cannot be resized to a size below its current
usage. tmpfs also supports POSIX ACLs, and extended attributes for the
trusted.* and security.* namespaces. ramfs does not use swap and you cannot
modify any parameter for a ramfs filesystem. The size limit of a ramfs
filesystem is how much memory you have available, and so care must be taken if
used so to not run out of memory.

An alternative to tmpfs and ramfs is to use brd to create RAM disks
(/dev/ram*), which allows you to simulate a block device disk in physical RAM.
To write data you would just then need to create an regular filesystem on top
this ramdisk. As with ramfs, brd ramdisks cannot swap. brd ramdisks are also
configured in size at initialization and you cannot dynamically resize them.
Contrary to brd ramdisks, tmpfs has its own filesystem, it does not rely on the
block layer at all.

Since tmpfs lives completely in the page cache and optionally on swap,
all tmpfs pages will be shown as "Shmem" in /proc/meminfo and "Shared" in
free(1). Notice that these counters also include shared memory
(shmem, see ipcs(1)). The most reliable way to get the count is
using df(1) and du(1).

tmpfs has the following uses:

1) There is always a kernel internal mount which you will not see at
   all. This is used for shared anonymous mappings and SYSV shared
   memory.

   This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
   set, the user visible part of tmpfs is not built. But the internal
   mechanisms are always present.

2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
   POSIX shared memory (shm_open, shm_unlink). Adding the following
   line to /etc/fstab should take care of this::

	tmpfs	/dev/shm	tmpfs	defaults	0 0

   Remember to create the directory that you intend to mount tmpfs on
   if necessary.

   This mount is _not_ needed for SYSV shared memory. The internal
   mount is used for that. (In the 2.3 kernel versions it was
   necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
   shared memory.)

3) Some people (including me) find it very convenient to mount it
   e.g. on /tmp and /var/tmp and have a big swap partition. And now
   loop mounts of tmpfs files do work, so mkinitrd shipped by most
   distributions should succeed with a tmpfs /tmp.

4) And probably a lot more I do not know about :-)


tmpfs has three mount options for sizing:

=========  ============================================================
size       The limit of allocated bytes for this tmpfs instance. The
           default is half of your physical RAM without swap. If you
           oversize your tmpfs instances the machine will deadlock
           since the OOM handler will not be able to free that memory.
nr_blocks  The same as size, but in blocks of PAGE_SIZE.
nr_inodes  The maximum number of inodes for this instance. The default
           is half of the number of your physical RAM pages, or (on a
           machine with highmem) the number of lowmem RAM pages,
           whichever is the lower.
=========  ============================================================

These parameters accept a suffix k, m or g for kilo, mega and giga and
can be changed on remount.  The size parameter also accepts a suffix %
to limit this tmpfs instance to that percentage of your physical RAM:
the default, when neither size nor nr_blocks is specified, is size=50%

If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
if nr_inodes=0, inodes will not be limited.  It is generally unwise to
mount with such options, since it allows any user with write access to
use up all the memory on the machine; but enhances the scalability of
that instance in a system with many CPUs making intensive use of it.

tmpfs blocks may be swapped out, when there is a shortage of memory.
tmpfs has a mount option to disable its use of swap:

======  ===========================================================
noswap  Disables swap. Remounts must respect the original settings.
        By default swap is enabled.
======  ===========================================================

tmpfs also supports Transparent Huge Pages which requires a kernel
configured with CONFIG_TRANSPARENT_HUGEPAGE and with huge supported for
your system (has_transparent_hugepage(), which is architecture specific).
The mount options for this are:

================ ==============================================================
huge=never       Do not allocate huge pages.  This is the default.
huge=always      Attempt to allocate huge page every time a new page is needed.
huge=within_size Only allocate huge page if it will be fully within i_size.
                 Also respect madvise(2) hints.
huge=advise      Only allocate huge page if requested with madvise(2).
================ ==============================================================

See also Documentation/admin-guide/mm/transhuge.rst, which describes the
sysfs file /sys/kernel/mm/transparent_hugepage/shmem_enabled: which can
be used to deny huge pages on all tmpfs mounts in an emergency, or to
force huge pages on all tmpfs mounts for testing.

tmpfs has a mount option to set the NUMA memory allocation policy for
all files in that instance (if CONFIG_NUMA is enabled) - which can be
adjusted on the fly via 'mount -o remount ...'

======================== ==============================================
mpol=default             use the process allocation policy
                         (see set_mempolicy(2))
mpol=prefer:Node         prefers to allocate memory from the given Node
mpol=bind:NodeList       allocates memory only from nodes in NodeList
mpol=interleave          prefers to allocate from each node in turn
mpol=interleave:NodeList allocates from each node of NodeList in turn
mpol=local		 prefers to allocate memory from the local node
======================== ==============================================

NodeList format is a comma-separated list of decimal numbers and ranges,
a range being two hyphen-separated decimal numbers, the smallest and
largest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15

A memory policy with a valid NodeList will be saved, as specified, for
use at file creation time.  When a task allocates a file in the file
system, the mount option memory policy will be applied with a NodeList,
if any, modified by the calling task's cpuset constraints
[See Documentation/admin-guide/cgroup-v1/cpusets.rst] and any optional flags,
listed below.  If the resulting NodeLists is the empty set, the effective
memory policy for the file will revert to "default" policy.

NUMA memory allocation policies have optional flags that can be used in
conjunction with their modes.  These optional flags can be specified
when tmpfs is mounted by appending them to the mode before the NodeList.
See Documentation/admin-guide/mm/numa_memory_policy.rst for a list of
all available memory allocation policy mode flags and their effect on
memory policy.

::

	=static		is equivalent to	MPOL_F_STATIC_NODES
	=relative	is equivalent to	MPOL_F_RELATIVE_NODES

For example, mpol=bind=static:NodeList, is the equivalent of an
allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.

Note that trying to mount a tmpfs with an mpol option will fail if the
running kernel does not support NUMA; and will fail if its nodelist
specifies a node which is not online.  If your system relies on that
tmpfs being mounted, but from time to time runs a kernel built without
NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
online, then it is advisable to omit the mpol option from automatic
mount options.  It can be added later, when the tmpfs is already mounted
on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.


To specify the initial root directory you can use the following mount
options:

====	==================================
mode	The permissions as an octal number
uid	The user id
gid	The group id
====	==================================

These options do not have any effect on remount. You can change these
parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.


tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode
numbers:

=======   ========================
inode64   Use 64-bit inode numbers
inode32   Use 32-bit inode numbers
=======   ========================

On a 32-bit kernel, inode32 is implicit, and inode64 is refused at mount time.
On a 64-bit kernel, CONFIG_TMPFS_INODE64 sets the default.  inode64 avoids the
possibility of multiple files with the same inode number on a single device;
but risks glibc failing with EOVERFLOW once 33-bit inode numbers are reached -
if a long-lived tmpfs is accessed by 32-bit applications so ancient that
opening a file larger than 2GiB fails with EINVAL.


So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
will give you tmpfs instance on /mytmpfs which can allocate 10GB
RAM/SWAP in 10240 inodes and it is only accessible by root.


:Author:
   Christoph Rohland <cr@sap.com>, 1.12.01
:Updated:
   Hugh Dickins, 4 June 2007
:Updated:
   KOSAKI Motohiro, 16 Mar 2010
:Updated:
   Chris Down, 13 July 2020