linux/Documentation/filesystems/tmpfs.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3=====
   4Tmpfs
   5=====
   6
   7Tmpfs is a file system which keeps all of its files in virtual memory.
   8
   9
  10Everything in tmpfs is temporary in the sense that no files will be
  11created on your hard drive. If you unmount a tmpfs instance,
  12everything stored therein is lost.
  13
  14tmpfs puts everything into the kernel internal caches and grows and
  15shrinks to accommodate the files it contains and is able to swap
  16unneeded pages out to swap space. It has maximum size limits which can
  17be adjusted on the fly via 'mount -o remount ...'
  18
  19If you compare it to ramfs (which was the template to create tmpfs)
  20you gain swapping and limit checking. Another similar thing is the RAM
  21disk (/dev/ram*), which simulates a fixed size hard disk in physical
  22RAM, where you have to create an ordinary filesystem on top. Ramdisks
  23cannot swap and you do not have the possibility to resize them.
  24
  25Since tmpfs lives completely in the page cache and on swap, all tmpfs
  26pages will be shown as "Shmem" in /proc/meminfo and "Shared" in
  27free(1). Notice that these counters also include shared memory
  28(shmem, see ipcs(1)). The most reliable way to get the count is
  29using df(1) and du(1).
  30
  31tmpfs has the following uses:
  32
  331) There is always a kernel internal mount which you will not see at
  34   all. This is used for shared anonymous mappings and SYSV shared
  35   memory.
  36
  37   This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
  38   set, the user visible part of tmpfs is not built. But the internal
  39   mechanisms are always present.
  40
  412) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
  42   POSIX shared memory (shm_open, shm_unlink). Adding the following
  43   line to /etc/fstab should take care of this::
  44
  45        tmpfs   /dev/shm        tmpfs   defaults        0 0
  46
  47   Remember to create the directory that you intend to mount tmpfs on
  48   if necessary.
  49
  50   This mount is _not_ needed for SYSV shared memory. The internal
  51   mount is used for that. (In the 2.3 kernel versions it was
  52   necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
  53   shared memory.)
  54
  553) Some people (including me) find it very convenient to mount it
  56   e.g. on /tmp and /var/tmp and have a big swap partition. And now
  57   loop mounts of tmpfs files do work, so mkinitrd shipped by most
  58   distributions should succeed with a tmpfs /tmp.
  59
  604) And probably a lot more I do not know about :-)
  61
  62
  63tmpfs has three mount options for sizing:
  64
  65=========  ============================================================
  66size       The limit of allocated bytes for this tmpfs instance. The
  67           default is half of your physical RAM without swap. If you
  68           oversize your tmpfs instances the machine will deadlock
  69           since the OOM handler will not be able to free that memory.
  70nr_blocks  The same as size, but in blocks of PAGE_SIZE.
  71nr_inodes  The maximum number of inodes for this instance. The default
  72           is half of the number of your physical RAM pages, or (on a
  73           machine with highmem) the number of lowmem RAM pages,
  74           whichever is the lower.
  75=========  ============================================================
  76
  77These parameters accept a suffix k, m or g for kilo, mega and giga and
  78can be changed on remount.  The size parameter also accepts a suffix %
  79to limit this tmpfs instance to that percentage of your physical RAM:
  80the default, when neither size nor nr_blocks is specified, is size=50%
  81
  82If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
  83if nr_inodes=0, inodes will not be limited.  It is generally unwise to
  84mount with such options, since it allows any user with write access to
  85use up all the memory on the machine; but enhances the scalability of
  86that instance in a system with many CPUs making intensive use of it.
  87
  88
  89tmpfs has a mount option to set the NUMA memory allocation policy for
  90all files in that instance (if CONFIG_NUMA is enabled) - which can be
  91adjusted on the fly via 'mount -o remount ...'
  92
  93======================== ==============================================
  94mpol=default             use the process allocation policy
  95                         (see set_mempolicy(2))
  96mpol=prefer:Node         prefers to allocate memory from the given Node
  97mpol=bind:NodeList       allocates memory only from nodes in NodeList
  98mpol=interleave          prefers to allocate from each node in turn
  99mpol=interleave:NodeList allocates from each node of NodeList in turn
 100mpol=local               prefers to allocate memory from the local node
 101======================== ==============================================
 102
 103NodeList format is a comma-separated list of decimal numbers and ranges,
 104a range being two hyphen-separated decimal numbers, the smallest and
 105largest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15
 106
 107A memory policy with a valid NodeList will be saved, as specified, for
 108use at file creation time.  When a task allocates a file in the file
 109system, the mount option memory policy will be applied with a NodeList,
 110if any, modified by the calling task's cpuset constraints
 111[See Documentation/admin-guide/cgroup-v1/cpusets.rst] and any optional flags,
 112listed below.  If the resulting NodeLists is the empty set, the effective
 113memory policy for the file will revert to "default" policy.
 114
 115NUMA memory allocation policies have optional flags that can be used in
 116conjunction with their modes.  These optional flags can be specified
 117when tmpfs is mounted by appending them to the mode before the NodeList.
 118See Documentation/admin-guide/mm/numa_memory_policy.rst for a list of
 119all available memory allocation policy mode flags and their effect on
 120memory policy.
 121
 122::
 123
 124        =static         is equivalent to        MPOL_F_STATIC_NODES
 125        =relative       is equivalent to        MPOL_F_RELATIVE_NODES
 126
 127For example, mpol=bind=static:NodeList, is the equivalent of an
 128allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
 129
 130Note that trying to mount a tmpfs with an mpol option will fail if the
 131running kernel does not support NUMA; and will fail if its nodelist
 132specifies a node which is not online.  If your system relies on that
 133tmpfs being mounted, but from time to time runs a kernel built without
 134NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
 135online, then it is advisable to omit the mpol option from automatic
 136mount options.  It can be added later, when the tmpfs is already mounted
 137on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
 138
 139
 140To specify the initial root directory you can use the following mount
 141options:
 142
 143====    ==================================
 144mode    The permissions as an octal number
 145uid     The user id
 146gid     The group id
 147====    ==================================
 148
 149These options do not have any effect on remount. You can change these
 150parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
 151
 152
 153tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode
 154numbers:
 155
 156=======   ========================
 157inode64   Use 64-bit inode numbers
 158inode32   Use 32-bit inode numbers
 159=======   ========================
 160
 161On a 32-bit kernel, inode32 is implicit, and inode64 is refused at mount time.
 162On a 64-bit kernel, CONFIG_TMPFS_INODE64 sets the default.  inode64 avoids the
 163possibility of multiple files with the same inode number on a single device;
 164but risks glibc failing with EOVERFLOW once 33-bit inode numbers are reached -
 165if a long-lived tmpfs is accessed by 32-bit applications so ancient that
 166opening a file larger than 2GiB fails with EINVAL.
 167
 168
 169So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
 170will give you tmpfs instance on /mytmpfs which can allocate 10GB
 171RAM/SWAP in 10240 inodes and it is only accessible by root.
 172
 173
 174:Author:
 175   Christoph Rohland <cr@sap.com>, 1.12.01
 176:Updated:
 177   Hugh Dickins, 4 June 2007
 178:Updated:
 179   KOSAKI Motohiro, 16 Mar 2010
 180:Updated:
 181   Chris Down, 13 July 2020
 182