linux/Documentation/filesystems/tmpfs.txt

Tmpfs is a file system which keeps all 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. It has maximum size limits which can
be adjusted on the fly via 'mount -o remount ...'

If you compare it to ramfs (which was the template to create tmpfs)
you gain swapping and limit checking. Another similar thing is the RAM
disk (/dev/ram*), which simulates a fixed size hard disk in physical
RAM, where you have to create an ordinary filesystem on top. Ramdisks
cannot swap and you do not have the possibility to resize them. 

Since tmpfs lives completely in the page cache and 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 build. 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 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/cgroup-v1/cpusets.txt] 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.


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