1Tmpfs is a file system which keeps all files in virtual memory. 2 3 4Everything in tmpfs is temporary in the sense that no files will be 5created on your hard drive. If you unmount a tmpfs instance, 6everything stored therein is lost. 7 8tmpfs puts everything into the kernel internal caches and grows and 9shrinks to accommodate the files it contains and is able to swap 10unneeded pages out to swap space. It has maximum size limits which can 11be adjusted on the fly via 'mount -o remount ...' 12 13If you compare it to ramfs (which was the template to create tmpfs) 14you gain swapping and limit checking. Another similar thing is the RAM 15disk (/dev/ram*), which simulates a fixed size hard disk in physical 16RAM, where you have to create an ordinary filesystem on top. Ramdisks 17cannot swap and you do not have the possibility to resize them. 18 19Since tmpfs lives completely in the page cache and on swap, all tmpfs 20pages will be shown as "Shmem" in /proc/meminfo and "Shared" in 21free(1). Notice that these counters also include shared memory 22(shmem, see ipcs(1)). The most reliable way to get the count is 23using df(1) and du(1). 24 25tmpfs has the following uses: 26 271) There is always a kernel internal mount which you will not see at 28 all. This is used for shared anonymous mappings and SYSV shared 29 memory. 30 31 This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not 32 set, the user visible part of tmpfs is not build. But the internal 33 mechanisms are always present. 34 352) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for 36 POSIX shared memory (shm_open, shm_unlink). Adding the following 37 line to /etc/fstab should take care of this: 38 39 tmpfs /dev/shm tmpfs defaults 0 0 40 41 Remember to create the directory that you intend to mount tmpfs on 42 if necessary. 43 44 This mount is _not_ needed for SYSV shared memory. The internal 45 mount is used for that. (In the 2.3 kernel versions it was 46 necessary to mount the predecessor of tmpfs (shm fs) to use SYSV 47 shared memory) 48 493) Some people (including me) find it very convenient to mount it 50 e.g. on /tmp and /var/tmp and have a big swap partition. And now 51 loop mounts of tmpfs files do work, so mkinitrd shipped by most 52 distributions should succeed with a tmpfs /tmp. 53 544) And probably a lot more I do not know about :-) 55 56 57tmpfs has three mount options for sizing: 58 59size: The limit of allocated bytes for this tmpfs instance. The 60 default is half of your physical RAM without swap. If you 61 oversize your tmpfs instances the machine will deadlock 62 since the OOM handler will not be able to free that memory. 63nr_blocks: The same as size, but in blocks of PAGE_SIZE. 64nr_inodes: The maximum number of inodes for this instance. The default 65 is half of the number of your physical RAM pages, or (on a 66 machine with highmem) the number of lowmem RAM pages, 67 whichever is the lower. 68 69These parameters accept a suffix k, m or g for kilo, mega and giga and 70can be changed on remount. The size parameter also accepts a suffix % 71to limit this tmpfs instance to that percentage of your physical RAM: 72the default, when neither size nor nr_blocks is specified, is size=50% 73 74If nr_blocks=0 (or size=0), blocks will not be limited in that instance; 75if nr_inodes=0, inodes will not be limited. It is generally unwise to 76mount with such options, since it allows any user with write access to 77use up all the memory on the machine; but enhances the scalability of 78that instance in a system with many cpus making intensive use of it. 79 80 81tmpfs has a mount option to set the NUMA memory allocation policy for 82all files in that instance (if CONFIG_NUMA is enabled) - which can be 83adjusted on the fly via 'mount -o remount ...' 84 85mpol=default use the process allocation policy 86 (see set_mempolicy(2)) 87mpol=prefer:Node prefers to allocate memory from the given Node 88mpol=bind:NodeList allocates memory only from nodes in NodeList 89mpol=interleave prefers to allocate from each node in turn 90mpol=interleave:NodeList allocates from each node of NodeList in turn 91mpol=local prefers to allocate memory from the local node 92 93NodeList format is a comma-separated list of decimal numbers and ranges, 94a range being two hyphen-separated decimal numbers, the smallest and 95largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15 96 97A memory policy with a valid NodeList will be saved, as specified, for 98use at file creation time. When a task allocates a file in the file 99system, the mount option memory policy will be applied with a NodeList, 100if any, modified by the calling task's cpuset constraints 101[See Documentation/admin-guide/cgroup-v1/cpusets.rst] and any optional flags, listed 102below. If the resulting NodeLists is the empty set, the effective memory 103policy for the file will revert to "default" policy. 104 105NUMA memory allocation policies have optional flags that can be used in 106conjunction with their modes. These optional flags can be specified 107when tmpfs is mounted by appending them to the mode before the NodeList. 108See Documentation/admin-guide/mm/numa_memory_policy.rst for a list of 109all available memory allocation policy mode flags and their effect on 110memory policy. 111 112 =static is equivalent to MPOL_F_STATIC_NODES 113 =relative is equivalent to MPOL_F_RELATIVE_NODES 114 115For example, mpol=bind=static:NodeList, is the equivalent of an 116allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES. 117 118Note that trying to mount a tmpfs with an mpol option will fail if the 119running kernel does not support NUMA; and will fail if its nodelist 120specifies a node which is not online. If your system relies on that 121tmpfs being mounted, but from time to time runs a kernel built without 122NUMA capability (perhaps a safe recovery kernel), or with fewer nodes 123online, then it is advisable to omit the mpol option from automatic 124mount options. It can be added later, when the tmpfs is already mounted 125on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'. 126 127 128To specify the initial root directory you can use the following mount 129options: 130 131mode: The permissions as an octal number 132uid: The user id 133gid: The group id 134 135These options do not have any effect on remount. You can change these 136parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem. 137 138 139So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs' 140will give you tmpfs instance on /mytmpfs which can allocate 10GB 141RAM/SWAP in 10240 inodes and it is only accessible by root. 142 143 144Author: 145 Christoph Rohland <cr@sap.com>, 1.12.01 146Updated: 147 Hugh Dickins, 4 June 2007 148Updated: 149 KOSAKI Motohiro, 16 Mar 2010 150
