linux/Documentation/filesystems/tmpfs.txt
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   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 currently in memory will show up as cached. It will not show up
  21as shared or something like that. Further on you can check the actual
  22RAM+swap use of a tmpfs instance with df(1) and du(1).
  23
  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_CACHE_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/cgroups/cpusets.txt] 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/vm/numa_memory_policy.txt for a list of all available
 109memory allocation policy mode flags and their effect on memory policy.
 110
 111        =static         is equivalent to        MPOL_F_STATIC_NODES
 112        =relative       is equivalent to        MPOL_F_RELATIVE_NODES
 113
 114For example, mpol=bind=static:NodeList, is the equivalent of an
 115allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
 116
 117Note that trying to mount a tmpfs with an mpol option will fail if the
 118running kernel does not support NUMA; and will fail if its nodelist
 119specifies a node which is not online.  If your system relies on that
 120tmpfs being mounted, but from time to time runs a kernel built without
 121NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
 122online, then it is advisable to omit the mpol option from automatic
 123mount options.  It can be added later, when the tmpfs is already mounted
 124on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
 125
 126
 127To specify the initial root directory you can use the following mount
 128options:
 129
 130mode:   The permissions as an octal number
 131uid:    The user id 
 132gid:    The group id
 133
 134These options do not have any effect on remount. You can change these
 135parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
 136
 137
 138So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
 139will give you tmpfs instance on /mytmpfs which can allocate 10GB
 140RAM/SWAP in 10240 inodes and it is only accessible by root.
 141
 142
 143Author:
 144   Christoph Rohland <cr@sap.com>, 1.12.01
 145Updated:
 146   Hugh Dickins, 4 June 2007
 147Updated:
 148   KOSAKI Motohiro, 16 Mar 2010
 149
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