[Patch] tmpfs documentation

[Christoph Rohland <cr@sap.com>]

Hi Linus and Marcelo,

The appended patch adds and reorganizes the documentation for
tmpfs.

It is against 2.4.16 but should apply to 2.5 also.

Please apply to both versions

		Christoph

diff -uNr 2.4.16/Documentation/Configure.help m2.4.16/Documentation/Configure.help
--- 2.4.16/Documentation/Configure.help	Mon Nov 26 16:21:44 2001
+++ m2.4.16/Documentation/Configure.help	Sat Dec  1 17:58:42 2001
@@ -13839,30 +13839,11 @@
 CONFIG_TMPFS
   Tmpfs is a file system which keeps all files in virtual memory.
 
-  In contrast to RAM disks, which get allocated a fixed amount of
-  physical RAM, tmpfs grows and shrinks to accommodate the files it
-  contains and is able to swap unneeded pages out to swap space.
+  Everything in tmpfs is temporary in the sense that no files will be
+  created on your hard drive. If you reboot, everything in tmpfs will
+  be lost.
 
-  Everything is "virtual" in the sense that no files will be created
-  on your hard drive; if you reboot, everything in tmpfs will be
-  lost.
-
-  You should mount the file system somewhere to be able to use
-  POSIX shared memory. Adding the following line to /etc/fstab should
-  take care of things:
-
-  tmpfs		/dev/shm	tmpfs		defaults	0 0
-
-  Remember to create the directory that you intend to mount tmpfs on
-  if necessary (/dev/shm is automagically created if you use devfs).
-
-  You can set limits for the number of blocks and inodes used by the
-  file system with the mount options "size", "nr_blocks" and
-  "nr_inodes". These parameters accept a suffix k, m or g for kilo,
-  mega and giga and can be changed on remount.
-
-  The initial permissions of the root directory can be set with the
-  mount option "mode".
+  See <file:Documentation/filesystems/tmpfs.txt> for details
 
 Simple RAM-based file system support
 CONFIG_RAMFS
diff -uNr 2.4.16/Documentation/filesystems/tmpfs.txt m2.4.16/Documentation/filesystems/tmpfs.txt
--- 2.4.16/Documentation/filesystems/tmpfs.txt	Thu Jan  1 01:00:00 1970
+++ m2.4.16/Documentation/filesystems/tmpfs.txt	Sat Dec  1 18:02:26 2001
@@ -0,0 +1,96 @@
+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 reboot, everything in tmpfs will be
+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 currently in memory will show up as cached. It will not show up
+as shared or something like that. Further on you can check the actual
+RAM+swap use of a tmpfs instance with 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 (/dev/shm is automagically created if you use devfs).
+
+   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. But be
+   aware: loop mounts of tmpfs files do not work due to the internal
+   design. So mkinitrd shipped by most distributions will fail with a
+   tmpfs /tmp.
+
+4) And probably a lot more I do not know about :-)
+
+
+tmpfs has a couple of mount options:
+
+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 PAGECACHE_SIZE.
+nr_inodes: The maximum number of inodes for this instance. The default
+           is half of the number of your physical RAM pages.
+
+These parameters accept a suffix k, m or g for kilo, mega and giga and
+can be changed on remount.
+
+The initial permissions of the root directory can be set with the
+mount option "mode". Later on you can change the permissions of the
+root directory with chmod.
+
+So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=0700 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.
+
+
+TODOs:
+
+1) give the size option a percent semantic: If you give a mount option
+   size=50% the tmpfs instance should be able to grow to 50 percent of
+   RAM + swap. So the instance should adapt autatically if you add or
+   remove swap space.
+2) loop mounts: This is difficult since loop.c relies on the readpage
+   operation. This operation gets a page from the caller to be filled
+   with the content of the file at that position. But tmpfs always has
+   the page and thus cannot copy the content to the given page. So it
+   cannot provide this operation. The VM had to be changed seriously
+   to achieve this.
+3) Show the number of tmpfs RAM pages. (As shared?)
+
+
+Author:
+   Christoph Rohland <cr@sap.com>, 1.12.01