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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, 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.*, security.* and user.* namespaces. ramfs does not use swap and you
25cannot modify 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.
87========= ============================================================
88
89These parameters accept a suffix k, m or g for kilo, mega and giga and
90can be changed on remount. The size parameter also accepts a suffix %
91to limit this tmpfs instance to that percentage of your physical RAM:
92the default, when neither size nor nr_blocks is specified, is size=50%
93
94If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
95if nr_inodes=0, inodes will not be limited. It is generally unwise to
96mount with such options, since it allows any user with write access to
97use up all the memory on the machine; but enhances the scalability of
98that instance in a system with many CPUs making intensive use of it.
99
100If nr_inodes is not 0, that limited space for inodes is also used up by
101extended attributes: "df -i"'s IUsed and IUse% increase, IFree decreases.
102
103tmpfs blocks may be swapped out, when there is a shortage of memory.
104tmpfs has a mount option to disable its use of swap:
105
106====== ===========================================================
107noswap Disables swap. Remounts must respect the original settings.
108 By default swap is enabled.
109====== ===========================================================
110
111tmpfs also supports Transparent Huge Pages which requires a kernel
112configured with CONFIG_TRANSPARENT_HUGEPAGE and with huge supported for
113your system (has_transparent_hugepage(), which is architecture specific).
114The mount options for this are:
115
116================ ==============================================================
117huge=never Do not allocate huge pages. This is the default.
118huge=always Attempt to allocate huge page every time a new page is needed.
119huge=within_size Only allocate huge page if it will be fully within i_size.
120 Also respect madvise(2) hints.
121huge=advise Only allocate huge page if requested with madvise(2).
122================ ==============================================================
123
124See also Documentation/admin-guide/mm/transhuge.rst, which describes the
125sysfs file /sys/kernel/mm/transparent_hugepage/shmem_enabled: which can
126be used to deny huge pages on all tmpfs mounts in an emergency, or to
127force huge pages on all tmpfs mounts for testing.
128
129tmpfs also supports quota with the following mount options
130
131======================== =================================================
132quota 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.
135usrquota User quota accounting and enforcement is enabled
136 on the mount.
137grpquota Group quota accounting and enforcement is enabled
138 on the mount.
139usrquota_block_hardlimit Set global user quota block hard limit.
140usrquota_inode_hardlimit Set global user quota inode hard limit.
141grpquota_block_hardlimit Set global group quota block hard limit.
142grpquota_inode_hardlimit Set global group quota inode hard limit.
143======================== =================================================
144
145None of the quota related mount options can be set or changed on remount.
146
147Quota limit parameters accept a suffix k, m or g for kilo, mega and giga
148and can't be changed on remount. Default global quota limits are taking
149effect for any and all user/group/project except root the first time the
150quota entry for user/group/project id is being accessed - typically the
151first time an inode with a particular id ownership is being created after
152the mount. In other words, instead of the limits being initialized to zero,
153they are initialized with the particular value provided with these mount
154options. The limits can be changed for any user/group id at any time as they
155normally can be.
156
157Note that tmpfs quotas do not support user namespaces so no uid/gid
158translation is done if quotas are enabled inside user namespaces.
159
160tmpfs has a mount option to set the NUMA memory allocation policy for
161all files in that instance (if CONFIG_NUMA is enabled) - which can be
162adjusted on the fly via 'mount -o remount ...'
163
164======================== ==============================================
165mpol=default use the process allocation policy
166 (see set_mempolicy(2))
167mpol=prefer:Node prefers to allocate memory from the given Node
168mpol=bind:NodeList allocates memory only from nodes in NodeList
169mpol=interleave prefers to allocate from each node in turn
170mpol=interleave:NodeList allocates from each node of NodeList in turn
171mpol=local prefers to allocate memory from the local node
172======================== ==============================================
173
174NodeList format is a comma-separated list of decimal numbers and ranges,
175a range being two hyphen-separated decimal numbers, the smallest and
176largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15
177
178A memory policy with a valid NodeList will be saved, as specified, for
179use at file creation time. When a task allocates a file in the file
180system, the mount option memory policy will be applied with a NodeList,
181if any, modified by the calling task's cpuset constraints
182[See Documentation/admin-guide/cgroup-v1/cpusets.rst] and any optional flags,
183listed below. If the resulting NodeLists is the empty set, the effective
184memory policy for the file will revert to "default" policy.
185
186NUMA memory allocation policies have optional flags that can be used in
187conjunction with their modes. These optional flags can be specified
188when tmpfs is mounted by appending them to the mode before the NodeList.
189See Documentation/admin-guide/mm/numa_memory_policy.rst for a list of
190all available memory allocation policy mode flags and their effect on
191memory policy.
192
193::
194
195 =static is equivalent to MPOL_F_STATIC_NODES
196 =relative is equivalent to MPOL_F_RELATIVE_NODES
197
198For example, mpol=bind=static:NodeList, is the equivalent of an
199allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
200
201Note that trying to mount a tmpfs with an mpol option will fail if the
202running kernel does not support NUMA; and will fail if its nodelist
203specifies a node which is not online. If your system relies on that
204tmpfs being mounted, but from time to time runs a kernel built without
205NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
206online, then it is advisable to omit the mpol option from automatic
207mount options. It can be added later, when the tmpfs is already mounted
208on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
209
210
211To specify the initial root directory you can use the following mount
212options:
213
214==== ==================================
215mode The permissions as an octal number
216uid The user id
217gid The group id
218==== ==================================
219
220These options do not have any effect on remount. You can change these
221parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
222
223
224tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode
225numbers:
226
227======= ========================
228inode64 Use 64-bit inode numbers
229inode32 Use 32-bit inode numbers
230======= ========================
231
232On a 32-bit kernel, inode32 is implicit, and inode64 is refused at mount time.
233On a 64-bit kernel, CONFIG_TMPFS_INODE64 sets the default. inode64 avoids the
234possibility of multiple files with the same inode number on a single device;
235but risks glibc failing with EOVERFLOW once 33-bit inode numbers are reached -
236if a long-lived tmpfs is accessed by 32-bit applications so ancient that
237opening a file larger than 2GiB fails with EINVAL.
238
239
240So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
241will give you tmpfs instance on /mytmpfs which can allocate 10GB
242RAM/SWAP in 10240 inodes and it is only accessible by root.
243
244tmpfs has the following mounting options for case-insensitive lookup support:
245
246================= ==============================================================
247casefold Enable casefold support at this mount point using the given
248 argument as the encoding standard. Currently only UTF-8
249 encodings are supported. If no argument is used, it will load
250 the latest UTF-8 encoding available.
251strict_encoding Enable strict encoding at this mount point (disabled by
252 default). In this mode, the filesystem refuses to create file
253 and directory with names containing invalid UTF-8 characters.
254================= ==============================================================
255
256This option doesn't render the entire filesystem case-insensitive. One needs to
257still set the casefold flag per directory, by flipping the +F attribute in an
258empty directory. Nevertheless, new directories will inherit the attribute. The
259mountpoint itself cannot be made case-insensitive.
260
261Example::
262
263 $ mount -t tmpfs -o casefold=utf8-12.1.0,strict_encoding fs_name /mytmpfs
264 $ mount -t tmpfs -o casefold fs_name /mytmpfs
265
266
267:Author:
268 Christoph Rohland <cr@sap.com>, 1.12.01
269:Updated:
270 Hugh Dickins, 4 June 2007
271:Updated:
272 KOSAKI Motohiro, 16 Mar 2010
273:Updated:
274 Chris Down, 13 July 2020
275:Updated:
276 André Almeida, 23 Aug 2024
1.. SPDX-License-Identifier: GPL-2.0
2
3=====
4Tmpfs
5=====
6
7Tmpfs is a file system which keeps all 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 build. 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