docs: filesystems: convert tmpfs.txt to ReST

- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Use :field: markup;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/30397a47a78ca59760fbc0fc5f50c5f1002d487a.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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+.. SPDX-License-Identifier: GPL-2.0
+
+=====
+Tmpfs
+=====
+
+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
+   memory.
+
+   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/admin-guide/cgroup-v1/cpusets.rst] 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
+memory policy.
+
+::
+
+	=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
+options:
+
+====	==================================
+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.
+
+
+:Author:
+   Christoph Rohland <cr@sap.com>, 1.12.01
+:Updated:
+   Hugh Dickins, 4 June 2007
+:Updated:
+   KOSAKI Motohiro, 16 Mar 2010