NAME

       tmpfs - a virtual memory filesystem

DESCRIPTION

       The  tmpfs  facility  allows  the creation of filesystems whose contents reside in virtual
       memory.  Since the files on such filesystems typically  reside  in  RAM,  file  access  is
       extremely fast.

       The filesystem is automatically created when mounting a filesystem with the type tmpfs via
       a command such as the following:

           $ sudo mount -t tmpfs -o size=10M tmpfs /mnt/mytmpfs

       A tmpfs filesystem has the following properties:

       •  The filesystem can employ swap space when physical memory pressure demands it.

       •  The filesystem consumes only as much physical memory and swap space as is  required  to
          store the current contents of the filesystem.

       •  During  a  remount  operation  (mount -o remount),  the  filesystem size can be changed
          (without losing the existing contents of the filesystem).

       If a tmpfs filesystem is unmounted, its contents are discarded (lost).

   Mount options
       The tmpfs filesystem supports the following mount options:

       size=bytes
              Specify an upper limit on the size of the filesystem.  The size is given in  bytes,
              and rounded up to entire pages.

              The  size  may  have a k, m, or g suffix for Ki, Mi, Gi (binary kilo (kibi), binary
              mega (mebi), and binary giga (gibi)).

              The size may also have a % suffix  to  limit  this  instance  to  a  percentage  of
              physical RAM.

              The default, when neither size nor nr_blocks is specified, is size=50%.

       nr_blocks=blocks
              The same as size, but in blocks of PAGE_CACHE_SIZE.

              Blocks may be specified with k, m, or g suffixes like size, but not a % suffix.

       nr_inodes=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 smaller.

              Inodes may be specified with k, m, or g suffixes like size, but not a % suffix.

       mode=mode
              Set initial permissions of the root directory.

       gid=gid (since Linux 2.5.7)
              Set the initial group ID of the root directory.

       uid=uid (since Linux 2.5.7)
              Set the initial user ID of the root directory.

       huge=huge_option (since Linux 4.7.0)
              Set  the  huge  table  memory  allocation policy for all files in this instance (if
              CONFIG_TRANSPARENT_HUGE_PAGECACHE is enabled).

              The huge_option value is one of the following:

              never  Do not allocate huge pages.  This is the default.

              always Attempt to allocate huge pages every time a new page is needed.

              within_size
                     Only allocate huge page if it will be fully  within  i_size.   Also  respect
                     fadvise(2) and madvise(2) hints

              advise Only allocate huge pages if requested with fadvise(2) or madvise(2).

              deny   For use in emergencies, to force the huge option off from all mounts.

              force  Force the huge option on for all mounts; useful for testing.

       mpol=mpol_option (since Linux 2.6.15)
              Set  the  NUMA  memory  allocation  policy  for  all  files  in  this  instance (if
              CONFIG_NUMA is enabled).

              The mpol_option value is one of the following:

              default
                     Use the process allocation policy (see set_mempolicy(2)).

              prefer:node
                     Preferably allocate memory from the given node.

              bind:nodelist
                     Allocate memory only from nodes in nodelist.

              interleave
                     Allocate from each node in turn.

              interleave:nodelist
                     Allocate from each node of in turn.

              local  Preferably allocate memory from the local node.

              In the above, nodelist is a comma-separated list of decimal numbers and ranges that
              specify  NUMA  nodes.   A  range is a pair of hyphen-separated decimal numbers, the
              smallest   and   largest   node   numbers   in    the    range.     For    example,
              mpol=bind:0-3,5,7,9-15.

VERSIONS

       The  tmpfs  facility  was  added in Linux 2.4, as a successor to the older ramfs facility,
       which did not provide limit checking or allow for the use of swap space.

NOTES

       In order for user-space tools and applications to create  tmpfs  filesystems,  the  kernel
       must be configured with the CONFIG_TMPFS option.

       The  tmpfs  filesystem  supports  extended  attributes  (see  xattr(7)), but user extended
       attributes are not permitted.

       An internal shared memory filesystem is used for System V shared  memory  (shmget(2))  and
       shared  anonymous  mappings  (mmap(2)  with the MAP_SHARED and MAP_ANONYMOUS flags).  This
       filesystem is  available  regardless  of  whether  the  kernel  was  configured  with  the
       CONFIG_TMPFS option.

       A  tmpfs  filesystem  mounted  at  /dev/shm is used for the implementation of POSIX shared
       memory (shm_overview(7)) and POSIX semaphores (sem_overview(7)).

       The amount of memory consumed by all tmpfs filesystems is shown  in  the  Shmem  field  of
       /proc/meminfo and in the shared field displayed by free(1).

       The tmpfs facility was formerly called shmfs.

SEE ALSO

       df(1), du(1), memfd_create(2), mmap(2), set_mempolicy(2), shm_open(3), mount(8)

       The       kernel       source      files      Documentation/filesystems/tmpfs.txt      and
       Documentation/admin-guide/mm/transhuge.rst.