1 .. SPDX-License-Identifier: GPL-2.0
2
3 =====
4 Tmpfs
5 =====
6
7 Tmpfs is a file system which keeps all of its files in virtual memory.
8
9
10 Everything in tmpfs is temporary in the sense that no files will be
11 created on your hard drive. If you unmount a tmpfs instance,
12 everything stored therein is lost.
13
14 tmpfs puts everything into the kernel internal caches and grows and
15 shrinks to accommodate the files it contains and is able to swap
16 unneeded pages out to swap space, if swap was enabled for the tmpfs
17 mount. tmpfs also supports THP.
18
19 tmpfs extends ramfs with a few userspace configurable options listed and
20 explained further below, some of which can be reconfigured dynamically on the
21 fly using a remount ('mount -o remount ...') of the filesystem. A tmpfs
22 filesystem can be resized but it cannot be resized to a size below its current
23 usage. tmpfs also supports POSIX ACLs, and extended attributes for the
24 trusted.*, security.* and user.* namespaces. ramfs does not use swap and you
25 cannot modify any parameter for a ramfs filesystem. The size limit of a ramfs
26 filesystem is how much memory you have available, and so care must be taken if
27 used so to not run out of memory.
28
29 An 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.
31 To write data you would just then need to create an regular filesystem on top
32 this ramdisk. As with ramfs, brd ramdisks cannot swap. brd ramdisks are also
33 configured in size at initialization and you cannot dynamically resize them.
34 Contrary to brd ramdisks, tmpfs has its own filesystem, it does not rely on the
35 block layer at all.
36
37 Since tmpfs lives completely in the page cache and optionally on swap,
38 all tmpfs pages will be shown as "Shmem" in /proc/meminfo and "Shared" in
39 free(1). Notice that these counters also include shared memory
40 (shmem, see ipcs(1)). The most reliable way to get the count is
41 using df(1) and du(1).
42
43 tmpfs has the following uses:
44
45 1) 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
53 2) 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
67 3) 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
72 4) And probably a lot more I do not know about :-)
73
74
75 tmpfs has three mount options for sizing:
76
77 ========= ============================================================
78 size 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.
82 nr_blocks The same as size, but in blocks of PAGE_SIZE.
83 nr_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.
87 ========= ============================================================
88
89 These parameters accept a suffix k, m or g for kilo, mega and giga and
90 can be changed on remount. The size parameter also accepts a suffix %
91 to limit this tmpfs instance to that percentage of your physical RAM:
92 the default, when neither size nor nr_blocks is specified, is size=50%
93
94 If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
95 if nr_inodes=0, inodes will not be limited. It is generally unwise to
96 mount with such options, since it allows any user with write access to
97 use up all the memory on the machine; but enhances the scalability of
98 that instance in a system with many CPUs making intensive use of it.
99
100 If nr_inodes is not 0, that limited space for inodes is also used up by
101 extended attributes: "df -i"'s IUsed and IUse% increase, IFree decreases.
102
103 tmpfs blocks may be swapped out, when there is a shortage of memory.
104 tmpfs has a mount option to disable its use of swap:
105
106 ====== ===========================================================
107 noswap Disables swap. Remounts must respect the original settings.
108 By default swap is enabled.
109 ====== ===========================================================
110
111 tmpfs also supports Transparent Huge Pages which requires a kernel
112 configured with CONFIG_TRANSPARENT_HUGEPAGE and with huge supported for
113 your system (has_transparent_hugepage(), which is architecture specific).
114 The mount options for this are:
115
116 ================ ==============================================================
117 huge=never Do not allocate huge pages. This is the default.
118 huge=always Attempt to allocate huge page every time a new page is needed.
119 huge=within_size Only allocate huge page if it will be fully within i_size.
120 Also respect madvise(2) hints.
121 huge=advise Only allocate huge page if requested with madvise(2).
122 ================ ==============================================================
123
124 See also Documentation/admin-guide/mm/transhuge.rst, which describes the
125 sysfs file /sys/kernel/mm/transparent_hugepage/shmem_enabled: which can
126 be used to deny huge pages on all tmpfs mounts in an emergency, or to
127 force huge pages on all tmpfs mounts for testing.
128
129 tmpfs also supports quota with the following mount options
130
131 ======================== =================================================
132 quota User and group quota accounting and enforcement
133 is enabled on the mount. Tmpfs is using hidden
134 system quota files that are initialized on mount.
135 usrquota User quota accounting and enforcement is enabled
136 on the mount.
137 grpquota Group quota accounting and enforcement is enabled
138 on the mount.
139 usrquota_block_hardlimit Set global user quota block hard limit.
140 usrquota_inode_hardlimit Set global user quota inode hard limit.
141 grpquota_block_hardlimit Set global group quota block hard limit.
142 grpquota_inode_hardlimit Set global group quota inode hard limit.
143 ======================== =================================================
144
145 None of the quota related mount options can be set or changed on remount.
146
147 Quota limit parameters accept a suffix k, m or g for kilo, mega and giga
148 and can't be changed on remount. Default global quota limits are taking
149 effect for any and all user/group/project except root the first time the
150 quota entry for user/group/project id is being accessed - typically the
151 first time an inode with a particular id ownership is being created after
152 the mount. In other words, instead of the limits being initialized to zero,
153 they are initialized with the particular value provided with these mount
154 options. The limits can be changed for any user/group id at any time as they
155 normally can be.
156
157 Note that tmpfs quotas do not support user namespaces so no uid/gid
158 translation is done if quotas are enabled inside user namespaces.
159
160 tmpfs has a mount option to set the NUMA memory allocation policy for
161 all files in that instance (if CONFIG_NUMA is enabled) - which can be
162 adjusted on the fly via 'mount -o remount ...'
163
164 ======================== ==============================================
165 mpol=default use the process allocation policy
166 (see set_mempolicy(2))
167 mpol=prefer:Node prefers to allocate memory from the given Node
168 mpol=bind:NodeList allocates memory only from nodes in NodeList
169 mpol=interleave prefers to allocate from each node in turn
170 mpol=interleave:NodeList allocates from each node of NodeList in turn
171 mpol=local prefers to allocate memory from the local node
172 ======================== ==============================================
173
174 NodeList format is a comma-separated list of decimal numbers and ranges,
175 a range being two hyphen-separated decimal numbers, the smallest and
176 largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15
177
178 A memory policy with a valid NodeList will be saved, as specified, for
179 use at file creation time. When a task allocates a file in the file
180 system, the mount option memory policy will be applied with a NodeList,
181 if any, modified by the calling task's cpuset constraints
182 [See Documentation/admin-guide/cgroup-v1/cpusets.rst] and any optional flags,
183 listed below. If the resulting NodeLists is the empty set, the effective
184 memory policy for the file will revert to "default" policy.
185
186 NUMA memory allocation policies have optional flags that can be used in
187 conjunction with their modes. These optional flags can be specified
188 when tmpfs is mounted by appending them to the mode before the NodeList.
189 See Documentation/admin-guide/mm/numa_memory_policy.rst for a list of
190 all available memory allocation policy mode flags and their effect on
191 memory policy.
192
193 ::
194
195 =static is equivalent to MPOL_F_STATIC_NODES
196 =relative is equivalent to MPOL_F_RELATIVE_NODES
197
198 For example, mpol=bind=static:NodeList, is the equivalent of an
199 allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
200
201 Note that trying to mount a tmpfs with an mpol option will fail if the
202 running kernel does not support NUMA; and will fail if its nodelist
203 specifies a node which is not online. If your system relies on that
204 tmpfs being mounted, but from time to time runs a kernel built without
205 NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
206 online, then it is advisable to omit the mpol option from automatic
207 mount options. It can be added later, when the tmpfs is already mounted
208 on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
209
210
211 To specify the initial root directory you can use the following mount
212 options:
213
214 ==== ==================================
215 mode The permissions as an octal number
216 uid The user id
217 gid The group id
218 ==== ==================================
219
220 These options do not have any effect on remount. You can change these
221 parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
222
223
224 tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode
225 numbers:
226
227 ======= ========================
228 inode64 Use 64-bit inode numbers
229 inode32 Use 32-bit inode numbers
230 ======= ========================
231
232 On a 32-bit kernel, inode32 is implicit, and inode64 is refused at mount time.
233 On a 64-bit kernel, CONFIG_TMPFS_INODE64 sets the default. inode64 avoids the
234 possibility of multiple files with the same inode number on a single device;
235 but risks glibc failing with EOVERFLOW once 33-bit inode numbers are reached -
236 if a long-lived tmpfs is accessed by 32-bit applications so ancient that
237 opening a file larger than 2GiB fails with EINVAL.
238
239
240 So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
241 will give you tmpfs instance on /mytmpfs which can allocate 10GB
242 RAM/SWAP in 10240 inodes and it is only accessible by root.
243
244
245 :Author:
246 Christoph Rohland <cr@sap.com>, 1.12.01
247 :Updated:
248 Hugh Dickins, 4 June 2007
249 :Updated:
250 KOSAKI Motohiro, 16 Mar 2010
251 :Updated:
252 Chris Down, 13 July 2020
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