|Tmpfs is a file system which keeps all files in virtual memory.
|Everything in tmpfs is temporary in the sense that no files will be
|created on your hard drive. If you unmount a tmpfs instance,
|everything stored therein is lost.
|tmpfs puts everything into the kernel internal caches and grows and
|shrinks to accommodate the files it contains and is able to swap
|unneeded pages out to swap space. It has maximum size limits which can
|be adjusted on the fly via 'mount -o remount ...'
|If you compare it to ramfs (which was the template to create tmpfs)
|you gain swapping and limit checking. Another similar thing is the RAM
|disk (/dev/ram*), which simulates a fixed size hard disk in physical
|RAM, where you have to create an ordinary filesystem on top. Ramdisks
|cannot swap and you do not have the possibility to resize them.
|Since tmpfs lives completely in the page cache and on swap, all tmpfs
|pages will be shown as "Shmem" in /proc/meminfo and "Shared" in
|free(1). Notice that these counters also include shared memory
|(shmem, see ipcs(1)). The most reliable way to get the count is
|using df(1) and du(1).
|tmpfs has the following uses:
|1) There is always a kernel internal mount which you will not see at
| all. This is used for shared anonymous mappings and SYSV shared
| This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
| set, the user visible part of tmpfs is not build. But the internal
| mechanisms are always present.
|2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
| POSIX shared memory (shm_open, shm_unlink). Adding the following
| line to /etc/fstab should take care of this:
| tmpfs /dev/shm tmpfs defaults 0 0
| Remember to create the directory that you intend to mount tmpfs on
| if necessary.
| This mount is _not_ needed for SYSV shared memory. The internal
| mount is used for that. (In the 2.3 kernel versions it was
| necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
| shared memory)
|3) Some people (including me) find it very convenient to mount it
| e.g. on /tmp and /var/tmp and have a big swap partition. And now
| loop mounts of tmpfs files do work, so mkinitrd shipped by most
| distributions should succeed with a tmpfs /tmp.
|4) And probably a lot more I do not know about :-)
|tmpfs has three mount options for sizing:
|size: The limit of allocated bytes for this tmpfs instance. The
| default is half of your physical RAM without swap. If you
| oversize your tmpfs instances the machine will deadlock
| since the OOM handler will not be able to free that memory.
|nr_blocks: The same as size, but in blocks of PAGE_SIZE.
|nr_inodes: The maximum number of inodes for this instance. The default
| is half of the number of your physical RAM pages, or (on a
| machine with highmem) the number of lowmem RAM pages,
| whichever is the lower.
|These parameters accept a suffix k, m or g for kilo, mega and giga and
|can be changed on remount. The size parameter also accepts a suffix %
|to limit this tmpfs instance to that percentage of your physical RAM:
|the default, when neither size nor nr_blocks is specified, is size=50%
|If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
|if nr_inodes=0, inodes will not be limited. It is generally unwise to
|mount with such options, since it allows any user with write access to
|use up all the memory on the machine; but enhances the scalability of
|that instance in a system with many cpus making intensive use of it.
|tmpfs has a mount option to set the NUMA memory allocation policy for
|all files in that instance (if CONFIG_NUMA is enabled) - which can be
|adjusted on the fly via 'mount -o remount ...'
|mpol=default use the process allocation policy
| (see set_mempolicy(2))
|mpol=prefer:Node prefers to allocate memory from the given Node
|mpol=bind:NodeList allocates memory only from nodes in NodeList
|mpol=interleave prefers to allocate from each node in turn
|mpol=interleave:NodeList allocates from each node of NodeList in turn
|mpol=local prefers to allocate memory from the local node
|NodeList format is a comma-separated list of decimal numbers and ranges,
|a range being two hyphen-separated decimal numbers, the smallest and
|largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15
|A memory policy with a valid NodeList will be saved, as specified, for
|use at file creation time. When a task allocates a file in the file
|system, the mount option memory policy will be applied with a NodeList,
|if any, modified by the calling task's cpuset constraints
|[See Documentation/cgroup-v1/cpusets.txt] and any optional flags, listed
|below. If the resulting NodeLists is the empty set, the effective memory
|policy for the file will revert to "default" policy.
|NUMA memory allocation policies have optional flags that can be used in
|conjunction with their modes. These optional flags can be specified
|when tmpfs is mounted by appending them to the mode before the NodeList.
|See Documentation/admin-guide/mm/numa_memory_policy.rst for a list of
|all available memory allocation policy mode flags and their effect on
| =static is equivalent to MPOL_F_STATIC_NODES
| =relative is equivalent to MPOL_F_RELATIVE_NODES
|For example, mpol=bind=static:NodeList, is the equivalent of an
|allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
|Note that trying to mount a tmpfs with an mpol option will fail if the
|running kernel does not support NUMA; and will fail if its nodelist
|specifies a node which is not online. If your system relies on that
|tmpfs being mounted, but from time to time runs a kernel built without
|NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
|online, then it is advisable to omit the mpol option from automatic
|mount options. It can be added later, when the tmpfs is already mounted
|on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
|To specify the initial root directory you can use the following mount
|mode: The permissions as an octal number
|uid: The user id
|gid: The group id
|These options do not have any effect on remount. You can change these
|parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
|So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
|will give you tmpfs instance on /mytmpfs which can allocate 10GB
|RAM/SWAP in 10240 inodes and it is only accessible by root.
| Christoph Rohland <firstname.lastname@example.org>, 1.12.01
| Hugh Dickins, 4 June 2007
| KOSAKI Motohiro, 16 Mar 2010