man-pages/man5/tmpfs.5

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.TH TMPFS 5 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
tmpfs \- a virtual memory filesystem
.SH DESCRIPTION
The
.B 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.
.PP
The filesystem is automatically created when mounting
a filesystem with the type
.BR tmpfs
via a command such as the following:
.PP
.in +4n
.EX
$ sudo mount \-t tmpfs \-o size=10M tmpfs /mnt/mytmpfs
.EE
.in
.PP
A
.B tmpfs
filesystem has the following properties:
.IP * 3
The filesystem can employ swap space when physical memory pressure
demands it.
.IP *
The filesystem consumes only as much physical memory and swap space
as is required to store the current contents of the filesystem.
.IP *
During a remount operation
.RI ( "mount\ \-o\ remount" ),
the filesystem size can be changed
(without losing the existing contents of the filesystem).
.PP
If a
.B tmpfs
filesystem is unmounted, its contents are discarded (lost).
.\" See mm/shmem.c:shmem_parse_options for options it supports.
.SS Mount options
The
.B tmpfs
filesystem supports the following mount options:
.TP
.BR size "=\fIbytes\fP"
Specify an upper limit on the size of the filesystem.
The size is given in bytes, and rounded up to entire pages.
.IP
The size may have a
.BR k ,
.BR m ,
or
.B g
suffix for Ki, Mi, Gi (binary kilo (kibi), binary mega (mebi), and binary giga
(gibi)).
.IP
The size may also have a % suffix to limit this instance to a percentage of
physical RAM.
.IP
The default, when neither
.B size
nor
.B nr_blocks
is specified, is
.IR size=50% .
.TP
.BR nr_blocks "=\fIblocks\fP"
The same as
.BR size ,
but in blocks of
.BR PAGE_CACHE_SIZE .
.IP
Blocks may be specified with
.BR k ,
.BR m ,
or
.B g
suffixes like
.BR size ,
but not a % suffix.
.TP
.BR nr_inodes "=\fIinodes\fP"
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.
.IP
Inodes may be specified with
.BR k ,
.BR m ,
or
.B g
suffixes like
.BR size ,
but not a % suffix.
.TP
.BR mode "=\fImode\fP"
Set initial permissions of the root directory.
.TP
.BR gid "=\fIgid\fP (since Linux 2.5.7)"
.\" Technically this is also in some version of Linux 2.4.
.\" commit 099445b489625b80b1d6687c9b6072dbeaca4096
Set the initial group ID of the root directory.
.TP
.BR uid "=\fIuid\fP (since Linux 2.5.7)"
.\" Technically this is also in some version of Linux 2.4.
.\" commit 099445b489625b80b1d6687c9b6072dbeaca4096
Set the initial user ID of the root directory.
.TP
.BR huge "=\fIhuge_option\fR (since Linux 4.7.0)"
.\" commit 5a6e75f8110c97e2a5488894d4e922187e6cb343
Set the huge table memory allocation policy for all files in this instance (if
.B CONFIG_TRANSPARENT_HUGE_PAGECACHE
is enabled).
.IP
The
.I huge_option
value is one of the following:
.RS
.TP
.B never
Do not allocate huge pages.
This is the default.
.TP
.B always
Attempt to allocate huge pages every time a new page is needed.
.TP
.B within_size
Only allocate huge page if it will be fully within
.IR i_size .
Also respect
.BR fadvise (2)/ madvise (2)
hints
.TP
.B advise
Only allocate huge pages if requested with
.BR fadvise (2)/ madvise (2).
.TP
.B deny
For use in emergencies, to force the huge option off from all mounts.
.TP
.B force
Force the huge option on for all mounts; useful for testing.
.RE
.TP
.BR mpol "=\fImpol_option\fR (since Linux 2.6.15)"
.\" commit 7339ff8302fd70aabf5f1ae26e0c4905fa74a495
Set the NUMA memory allocation policy for all files in this instance (if
.B CONFIG_NUMA
is enabled).
.IP
The
.I mpol_option
value is one of the following:
.RS
.TP
.B default
Use the process allocation policy (see
.BR set_mempolicy (2)).
.TP
.BR prefer ":\fInode\fP"
Preferably allocate memory from the given
.IR node .
.TP
.BR bind ":\fInodelist\fP"
Allocate memory only from nodes in
.IR nodelist .
.TP
.B interleave
Allocate from each node in turn.
.TP
.BR interleave ":\fInodelist\fP"
Allocate from each node of
.I in
turn.
.TP
.B local
Preferably allocate memory from the local node.
.RE
.IP
In the above,
.I 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,
.IR mpol=bind:0\-3,5,7,9\-15 .
.SH VERSIONS
The
.B tmpfs
facility was added in Linux 2.4, as a successor to the older
.B ramfs
facility, which did not provide limit checking or
allow for the use of swap space.
.SH NOTES
In order for user-space tools and applications to create
.B tmpfs
filesystems, the kernel must be configured with the
.B CONFIG_TMPFS
option.
.PP
The
.BR tmpfs
filesystem supports extended attributes (see
.BR xattr (7)),
but
.I user
extended attributes are not permitted.
.PP
An internal shared memory filesystem is used for
System V shared memory
.RB ( shmget (2))
and shared anonymous mappings
.RB ( mmap (2)
with the
.B MAP_SHARED
and
.BR MAP_ANONYMOUS
flags).
This filesystem is available regardless of whether
the kernel was configured with the
.B CONFIG_TMPFS
option.
.PP
A
.B tmpfs
filesystem mounted at
.IR /dev/shm
is used for the implementation of POSIX shared memory
.RB ( shm_overview (7))
and POSIX semaphores
.RB ( sem_overview (7)).
.PP
The amount of memory consumed by all
.B tmpfs
filesystems is shown in the
.I Shmem
field of
.IR /proc/meminfo
and in the
.I shared
field displayed by
.BR free (1).
.PP
The
.B tmpfs
facility was formerly called
.BR shmfs .
.SH SEE ALSO
.BR df (1),
.BR du (1),
.BR memfd_create (2),
.BR mmap (2),
.BR set_mempolicy (2),
.BR shm_open (3),
.BR mount (8)
.PP
The kernel source files
.IR Documentation/filesystems/tmpfs.txt
and
.IR Documentation/admin\-guide/mm/transhuge.rst .