17e7cd458SMauro Carvalho Chehab.. SPDX-License-Identifier: GPL-2.0 27e7cd458SMauro Carvalho Chehab 37e7cd458SMauro Carvalho Chehab===== 47e7cd458SMauro Carvalho ChehabTmpfs 57e7cd458SMauro Carvalho Chehab===== 67e7cd458SMauro Carvalho Chehab 77e7cd458SMauro Carvalho ChehabTmpfs is a file system which keeps all files in virtual memory. 87e7cd458SMauro Carvalho Chehab 97e7cd458SMauro Carvalho Chehab 107e7cd458SMauro Carvalho ChehabEverything in tmpfs is temporary in the sense that no files will be 117e7cd458SMauro Carvalho Chehabcreated on your hard drive. If you unmount a tmpfs instance, 127e7cd458SMauro Carvalho Chehabeverything stored therein is lost. 137e7cd458SMauro Carvalho Chehab 147e7cd458SMauro Carvalho Chehabtmpfs puts everything into the kernel internal caches and grows and 157e7cd458SMauro Carvalho Chehabshrinks to accommodate the files it contains and is able to swap 167e7cd458SMauro Carvalho Chehabunneeded pages out to swap space. It has maximum size limits which can 177e7cd458SMauro Carvalho Chehabbe adjusted on the fly via 'mount -o remount ...' 187e7cd458SMauro Carvalho Chehab 197e7cd458SMauro Carvalho ChehabIf you compare it to ramfs (which was the template to create tmpfs) 207e7cd458SMauro Carvalho Chehabyou gain swapping and limit checking. Another similar thing is the RAM 217e7cd458SMauro Carvalho Chehabdisk (/dev/ram*), which simulates a fixed size hard disk in physical 227e7cd458SMauro Carvalho ChehabRAM, where you have to create an ordinary filesystem on top. Ramdisks 237e7cd458SMauro Carvalho Chehabcannot swap and you do not have the possibility to resize them. 247e7cd458SMauro Carvalho Chehab 257e7cd458SMauro Carvalho ChehabSince tmpfs lives completely in the page cache and on swap, all tmpfs 267e7cd458SMauro Carvalho Chehabpages will be shown as "Shmem" in /proc/meminfo and "Shared" in 277e7cd458SMauro Carvalho Chehabfree(1). Notice that these counters also include shared memory 287e7cd458SMauro Carvalho Chehab(shmem, see ipcs(1)). The most reliable way to get the count is 297e7cd458SMauro Carvalho Chehabusing df(1) and du(1). 307e7cd458SMauro Carvalho Chehab 317e7cd458SMauro Carvalho Chehabtmpfs has the following uses: 327e7cd458SMauro Carvalho Chehab 337e7cd458SMauro Carvalho Chehab1) There is always a kernel internal mount which you will not see at 347e7cd458SMauro Carvalho Chehab all. This is used for shared anonymous mappings and SYSV shared 357e7cd458SMauro Carvalho Chehab memory. 367e7cd458SMauro Carvalho Chehab 377e7cd458SMauro Carvalho Chehab This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not 387e7cd458SMauro Carvalho Chehab set, the user visible part of tmpfs is not build. But the internal 397e7cd458SMauro Carvalho Chehab mechanisms are always present. 407e7cd458SMauro Carvalho Chehab 417e7cd458SMauro Carvalho Chehab2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for 427e7cd458SMauro Carvalho Chehab POSIX shared memory (shm_open, shm_unlink). Adding the following 437e7cd458SMauro Carvalho Chehab line to /etc/fstab should take care of this:: 447e7cd458SMauro Carvalho Chehab 457e7cd458SMauro Carvalho Chehab tmpfs /dev/shm tmpfs defaults 0 0 467e7cd458SMauro Carvalho Chehab 477e7cd458SMauro Carvalho Chehab Remember to create the directory that you intend to mount tmpfs on 487e7cd458SMauro Carvalho Chehab if necessary. 497e7cd458SMauro Carvalho Chehab 507e7cd458SMauro Carvalho Chehab This mount is _not_ needed for SYSV shared memory. The internal 517e7cd458SMauro Carvalho Chehab mount is used for that. (In the 2.3 kernel versions it was 527e7cd458SMauro Carvalho Chehab necessary to mount the predecessor of tmpfs (shm fs) to use SYSV 537e7cd458SMauro Carvalho Chehab shared memory) 547e7cd458SMauro Carvalho Chehab 557e7cd458SMauro Carvalho Chehab3) Some people (including me) find it very convenient to mount it 567e7cd458SMauro Carvalho Chehab e.g. on /tmp and /var/tmp and have a big swap partition. And now 577e7cd458SMauro Carvalho Chehab loop mounts of tmpfs files do work, so mkinitrd shipped by most 587e7cd458SMauro Carvalho Chehab distributions should succeed with a tmpfs /tmp. 597e7cd458SMauro Carvalho Chehab 607e7cd458SMauro Carvalho Chehab4) And probably a lot more I do not know about :-) 617e7cd458SMauro Carvalho Chehab 627e7cd458SMauro Carvalho Chehab 637e7cd458SMauro Carvalho Chehabtmpfs has three mount options for sizing: 647e7cd458SMauro Carvalho Chehab 657e7cd458SMauro Carvalho Chehab========= ============================================================ 667e7cd458SMauro Carvalho Chehabsize The limit of allocated bytes for this tmpfs instance. The 677e7cd458SMauro Carvalho Chehab default is half of your physical RAM without swap. If you 687e7cd458SMauro Carvalho Chehab oversize your tmpfs instances the machine will deadlock 697e7cd458SMauro Carvalho Chehab since the OOM handler will not be able to free that memory. 707e7cd458SMauro Carvalho Chehabnr_blocks The same as size, but in blocks of PAGE_SIZE. 717e7cd458SMauro Carvalho Chehabnr_inodes The maximum number of inodes for this instance. The default 727e7cd458SMauro Carvalho Chehab is half of the number of your physical RAM pages, or (on a 737e7cd458SMauro Carvalho Chehab machine with highmem) the number of lowmem RAM pages, 747e7cd458SMauro Carvalho Chehab whichever is the lower. 757e7cd458SMauro Carvalho Chehab========= ============================================================ 767e7cd458SMauro Carvalho Chehab 777e7cd458SMauro Carvalho ChehabThese parameters accept a suffix k, m or g for kilo, mega and giga and 787e7cd458SMauro Carvalho Chehabcan be changed on remount. The size parameter also accepts a suffix % 797e7cd458SMauro Carvalho Chehabto limit this tmpfs instance to that percentage of your physical RAM: 807e7cd458SMauro Carvalho Chehabthe default, when neither size nor nr_blocks is specified, is size=50% 817e7cd458SMauro Carvalho Chehab 827e7cd458SMauro Carvalho ChehabIf nr_blocks=0 (or size=0), blocks will not be limited in that instance; 837e7cd458SMauro Carvalho Chehabif nr_inodes=0, inodes will not be limited. It is generally unwise to 847e7cd458SMauro Carvalho Chehabmount with such options, since it allows any user with write access to 857e7cd458SMauro Carvalho Chehabuse up all the memory on the machine; but enhances the scalability of 867e7cd458SMauro Carvalho Chehabthat instance in a system with many cpus making intensive use of it. 877e7cd458SMauro Carvalho Chehab 887e7cd458SMauro Carvalho Chehab 897e7cd458SMauro Carvalho Chehabtmpfs has a mount option to set the NUMA memory allocation policy for 907e7cd458SMauro Carvalho Chehaball files in that instance (if CONFIG_NUMA is enabled) - which can be 917e7cd458SMauro Carvalho Chehabadjusted on the fly via 'mount -o remount ...' 927e7cd458SMauro Carvalho Chehab 937e7cd458SMauro Carvalho Chehab======================== ============================================== 947e7cd458SMauro Carvalho Chehabmpol=default use the process allocation policy 957e7cd458SMauro Carvalho Chehab (see set_mempolicy(2)) 967e7cd458SMauro Carvalho Chehabmpol=prefer:Node prefers to allocate memory from the given Node 977e7cd458SMauro Carvalho Chehabmpol=bind:NodeList allocates memory only from nodes in NodeList 987e7cd458SMauro Carvalho Chehabmpol=interleave prefers to allocate from each node in turn 997e7cd458SMauro Carvalho Chehabmpol=interleave:NodeList allocates from each node of NodeList in turn 1007e7cd458SMauro Carvalho Chehabmpol=local prefers to allocate memory from the local node 1017e7cd458SMauro Carvalho Chehab======================== ============================================== 1027e7cd458SMauro Carvalho Chehab 1037e7cd458SMauro Carvalho ChehabNodeList format is a comma-separated list of decimal numbers and ranges, 1047e7cd458SMauro Carvalho Chehaba range being two hyphen-separated decimal numbers, the smallest and 1057e7cd458SMauro Carvalho Chehablargest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15 1067e7cd458SMauro Carvalho Chehab 1077e7cd458SMauro Carvalho ChehabA memory policy with a valid NodeList will be saved, as specified, for 1087e7cd458SMauro Carvalho Chehabuse at file creation time. When a task allocates a file in the file 1097e7cd458SMauro Carvalho Chehabsystem, the mount option memory policy will be applied with a NodeList, 1107e7cd458SMauro Carvalho Chehabif any, modified by the calling task's cpuset constraints 1117e7cd458SMauro Carvalho Chehab[See Documentation/admin-guide/cgroup-v1/cpusets.rst] and any optional flags, 1127e7cd458SMauro Carvalho Chehablisted below. If the resulting NodeLists is the empty set, the effective 1137e7cd458SMauro Carvalho Chehabmemory policy for the file will revert to "default" policy. 1147e7cd458SMauro Carvalho Chehab 1157e7cd458SMauro Carvalho ChehabNUMA memory allocation policies have optional flags that can be used in 1167e7cd458SMauro Carvalho Chehabconjunction with their modes. These optional flags can be specified 1177e7cd458SMauro Carvalho Chehabwhen tmpfs is mounted by appending them to the mode before the NodeList. 1187e7cd458SMauro Carvalho ChehabSee Documentation/admin-guide/mm/numa_memory_policy.rst for a list of 1197e7cd458SMauro Carvalho Chehaball available memory allocation policy mode flags and their effect on 1207e7cd458SMauro Carvalho Chehabmemory policy. 1217e7cd458SMauro Carvalho Chehab 1227e7cd458SMauro Carvalho Chehab:: 1237e7cd458SMauro Carvalho Chehab 1247e7cd458SMauro Carvalho Chehab =static is equivalent to MPOL_F_STATIC_NODES 1257e7cd458SMauro Carvalho Chehab =relative is equivalent to MPOL_F_RELATIVE_NODES 1267e7cd458SMauro Carvalho Chehab 1277e7cd458SMauro Carvalho ChehabFor example, mpol=bind=static:NodeList, is the equivalent of an 1287e7cd458SMauro Carvalho Chehaballocation policy of MPOL_BIND | MPOL_F_STATIC_NODES. 1297e7cd458SMauro Carvalho Chehab 1307e7cd458SMauro Carvalho ChehabNote that trying to mount a tmpfs with an mpol option will fail if the 1317e7cd458SMauro Carvalho Chehabrunning kernel does not support NUMA; and will fail if its nodelist 1327e7cd458SMauro Carvalho Chehabspecifies a node which is not online. If your system relies on that 1337e7cd458SMauro Carvalho Chehabtmpfs being mounted, but from time to time runs a kernel built without 1347e7cd458SMauro Carvalho ChehabNUMA capability (perhaps a safe recovery kernel), or with fewer nodes 1357e7cd458SMauro Carvalho Chehabonline, then it is advisable to omit the mpol option from automatic 1367e7cd458SMauro Carvalho Chehabmount options. It can be added later, when the tmpfs is already mounted 1377e7cd458SMauro Carvalho Chehabon MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'. 1387e7cd458SMauro Carvalho Chehab 1397e7cd458SMauro Carvalho Chehab 1407e7cd458SMauro Carvalho ChehabTo specify the initial root directory you can use the following mount 1417e7cd458SMauro Carvalho Chehaboptions: 1427e7cd458SMauro Carvalho Chehab 1437e7cd458SMauro Carvalho Chehab==== ================================== 1447e7cd458SMauro Carvalho Chehabmode The permissions as an octal number 1457e7cd458SMauro Carvalho Chehabuid The user id 1467e7cd458SMauro Carvalho Chehabgid The group id 1477e7cd458SMauro Carvalho Chehab==== ================================== 1487e7cd458SMauro Carvalho Chehab 1497e7cd458SMauro Carvalho ChehabThese options do not have any effect on remount. You can change these 1507e7cd458SMauro Carvalho Chehabparameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem. 1517e7cd458SMauro Carvalho Chehab 1527e7cd458SMauro Carvalho Chehab 1537e7cd458SMauro Carvalho ChehabSo 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs' 1547e7cd458SMauro Carvalho Chehabwill give you tmpfs instance on /mytmpfs which can allocate 10GB 1557e7cd458SMauro Carvalho ChehabRAM/SWAP in 10240 inodes and it is only accessible by root. 1567e7cd458SMauro Carvalho Chehab 1577e7cd458SMauro Carvalho Chehab 1587e7cd458SMauro Carvalho Chehab:Author: 1597e7cd458SMauro Carvalho Chehab Christoph Rohland <cr@sap.com>, 1.12.01 1607e7cd458SMauro Carvalho Chehab:Updated: 1617e7cd458SMauro Carvalho Chehab Hugh Dickins, 4 June 2007 1627e7cd458SMauro Carvalho Chehab:Updated: 1637e7cd458SMauro Carvalho Chehab KOSAKI Motohiro, 16 Mar 2010 164