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, if swap was enabled for the tmpfs
17mount. tmpfs also supports THP.
18
19tmpfs extends ramfs with a few userspace configurable options listed and
20explained further below, some of which can be reconfigured dynamically on the
21fly using a remount ('mount -o remount ...') of the filesystem. A tmpfs
22filesystem can be resized but it cannot be resized to a size below its current
23usage. tmpfs also supports POSIX ACLs, and extended attributes for the
24trusted.* and security.* namespaces. ramfs does not use swap and you cannot
25modify any parameter for a ramfs filesystem. The size limit of a ramfs
26filesystem is how much memory you have available, and so care must be taken if
27used so to not run out of memory.
28
29An alternative to tmpfs and ramfs is to use brd to create RAM disks
30(/dev/ram*), which allows you to simulate a block device disk in physical RAM.
31To write data you would just then need to create an regular filesystem on top
32this ramdisk. As with ramfs, brd ramdisks cannot swap. brd ramdisks are also
33configured in size at initialization and you cannot dynamically resize them.
34Contrary to brd ramdisks, tmpfs has its own filesystem, it does not rely on the
35block layer at all.
36
37Since tmpfs lives completely in the page cache and optionally on swap,
38all tmpfs pages will be shown as "Shmem" in /proc/meminfo and "Shared" in
39free(1). Notice that these counters also include shared memory
40(shmem, see ipcs(1)). The most reliable way to get the count is
41using df(1) and du(1).
42
43tmpfs has the following uses:
44
451) There is always a kernel internal mount which you will not see at
46   all. This is used for shared anonymous mappings and SYSV shared
47   memory.
48
49   This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
50   set, the user visible part of tmpfs is not built. But the internal
51   mechanisms are always present.
52
532) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
54   POSIX shared memory (shm_open, shm_unlink). Adding the following
55   line to /etc/fstab should take care of this::
56
57	tmpfs	/dev/shm	tmpfs	defaults	0 0
58
59   Remember to create the directory that you intend to mount tmpfs on
60   if necessary.
61
62   This mount is _not_ needed for SYSV shared memory. The internal
63   mount is used for that. (In the 2.3 kernel versions it was
64   necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
65   shared memory.)
66
673) Some people (including me) find it very convenient to mount it
68   e.g. on /tmp and /var/tmp and have a big swap partition. And now
69   loop mounts of tmpfs files do work, so mkinitrd shipped by most
70   distributions should succeed with a tmpfs /tmp.
71
724) And probably a lot more I do not know about :-)
73
74
75tmpfs has three mount options for sizing:
76
77=========  ============================================================
78size       The limit of allocated bytes for this tmpfs instance. The
79           default is half of your physical RAM without swap. If you
80           oversize your tmpfs instances the machine will deadlock
81           since the OOM handler will not be able to free that memory.
82nr_blocks  The same as size, but in blocks of PAGE_SIZE.
83nr_inodes  The maximum number of inodes for this instance. The default
84           is half of the number of your physical RAM pages, or (on a
85           machine with highmem) the number of lowmem RAM pages,
86           whichever is the lower.
87noswap     Disables swap. Remounts must respect the original settings.
88           By default swap is enabled.
89=========  ============================================================
90
91These parameters accept a suffix k, m or g for kilo, mega and giga and
92can be changed on remount.  The size parameter also accepts a suffix %
93to limit this tmpfs instance to that percentage of your physical RAM:
94the default, when neither size nor nr_blocks is specified, is size=50%
95
96If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
97if nr_inodes=0, inodes will not be limited.  It is generally unwise to
98mount with such options, since it allows any user with write access to
99use up all the memory on the machine; but enhances the scalability of
100that instance in a system with many CPUs making intensive use of it.
101
102tmpfs also supports Transparent Huge Pages which requires a kernel
103configured with CONFIG_TRANSPARENT_HUGEPAGE and with huge supported for
104your system (has_transparent_hugepage(), which is architecture specific).
105The mount options for this are:
106
107======  ============================================================
108huge=0  never: disables huge pages for the mount
109huge=1  always: enables huge pages for the mount
110huge=2  within_size: only allocate huge pages if the page will be
111        fully within i_size, also respect fadvise()/madvise() hints.
112huge=3  advise: only allocate huge pages if requested with
113        fadvise()/madvise()
114======  ============================================================
115
116There is a sysfs file which you can also use to control system wide THP
117configuration for all tmpfs mounts, the file is:
118
119/sys/kernel/mm/transparent_hugepage/shmem_enabled
120
121This sysfs file is placed on top of THP sysfs directory and so is registered
122by THP code. It is however only used to control all tmpfs mounts with one
123single knob. Since it controls all tmpfs mounts it should only be used either
124for emergency or testing purposes. The values you can set for shmem_enabled are:
125
126==  ============================================================
127-1  deny: disables huge on shm_mnt and all mounts, for
128    emergency use
129-2  force: enables huge on shm_mnt and all mounts, w/o needing
130    option, for testing
131==  ============================================================
132
133tmpfs has a mount option to set the NUMA memory allocation policy for
134all files in that instance (if CONFIG_NUMA is enabled) - which can be
135adjusted on the fly via 'mount -o remount ...'
136
137======================== ==============================================
138mpol=default             use the process allocation policy
139                         (see set_mempolicy(2))
140mpol=prefer:Node         prefers to allocate memory from the given Node
141mpol=bind:NodeList       allocates memory only from nodes in NodeList
142mpol=interleave          prefers to allocate from each node in turn
143mpol=interleave:NodeList allocates from each node of NodeList in turn
144mpol=local		 prefers to allocate memory from the local node
145======================== ==============================================
146
147NodeList format is a comma-separated list of decimal numbers and ranges,
148a range being two hyphen-separated decimal numbers, the smallest and
149largest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15
150
151A memory policy with a valid NodeList will be saved, as specified, for
152use at file creation time.  When a task allocates a file in the file
153system, the mount option memory policy will be applied with a NodeList,
154if any, modified by the calling task's cpuset constraints
155[See Documentation/admin-guide/cgroup-v1/cpusets.rst] and any optional flags,
156listed below.  If the resulting NodeLists is the empty set, the effective
157memory policy for the file will revert to "default" policy.
158
159NUMA memory allocation policies have optional flags that can be used in
160conjunction with their modes.  These optional flags can be specified
161when tmpfs is mounted by appending them to the mode before the NodeList.
162See Documentation/admin-guide/mm/numa_memory_policy.rst for a list of
163all available memory allocation policy mode flags and their effect on
164memory policy.
165
166::
167
168	=static		is equivalent to	MPOL_F_STATIC_NODES
169	=relative	is equivalent to	MPOL_F_RELATIVE_NODES
170
171For example, mpol=bind=static:NodeList, is the equivalent of an
172allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
173
174Note that trying to mount a tmpfs with an mpol option will fail if the
175running kernel does not support NUMA; and will fail if its nodelist
176specifies a node which is not online.  If your system relies on that
177tmpfs being mounted, but from time to time runs a kernel built without
178NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
179online, then it is advisable to omit the mpol option from automatic
180mount options.  It can be added later, when the tmpfs is already mounted
181on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
182
183
184To specify the initial root directory you can use the following mount
185options:
186
187====	==================================
188mode	The permissions as an octal number
189uid	The user id
190gid	The group id
191====	==================================
192
193These options do not have any effect on remount. You can change these
194parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
195
196
197tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode
198numbers:
199
200=======   ========================
201inode64   Use 64-bit inode numbers
202inode32   Use 32-bit inode numbers
203=======   ========================
204
205On a 32-bit kernel, inode32 is implicit, and inode64 is refused at mount time.
206On a 64-bit kernel, CONFIG_TMPFS_INODE64 sets the default.  inode64 avoids the
207possibility of multiple files with the same inode number on a single device;
208but risks glibc failing with EOVERFLOW once 33-bit inode numbers are reached -
209if a long-lived tmpfs is accessed by 32-bit applications so ancient that
210opening a file larger than 2GiB fails with EINVAL.
211
212
213So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
214will give you tmpfs instance on /mytmpfs which can allocate 10GB
215RAM/SWAP in 10240 inodes and it is only accessible by root.
216
217
218:Author:
219   Christoph Rohland <cr@sap.com>, 1.12.01
220:Updated:
221   Hugh Dickins, 4 June 2007
222:Updated:
223   KOSAKI Motohiro, 16 Mar 2010
224:Updated:
225   Chris Down, 13 July 2020
226