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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