make shm filesystem part configurable

Hi,

The appended patch (additional to my read/write support patch) makes the shm filesystem configurable and renames it to the more sensible name swapfs. Since the fs type "shm" is quite established with 2.4 I register that name also.

It also updates the documentation.

Greetings
Christoph

diff -uNr 2.4.0-shm_vm_locked-truncate-rw/Documentation/Changes 2.4.0-shm_vm_locked-truncate-rw-swapfs/Documentation/Changes
@@ -114,20 +114,6 @@
 DevFS is now in the kernel.  See Documentation/filesystems/devfs/* in
 the kernel source tree for all the gory details.
 
-System V shared memory is now implemented via a virtual filesystem.
-You do not have to mount it to use it. SYSV shared memory limits are
-set via /proc/sys/kernel/shm{max,all,mni}.  You should mount the
-filesystem under /dev/shm to be able to use POSIX shared
-memory. Adding the following line to /etc/fstab should take care of
-things:
-
-none		/dev/shm	shm		defaults	0 0
-
-Remember to create the directory that you intend to mount shm on if
-necessary (The entry is automagically created if you use devfs). You
-can set limits for the number of blocks and inodes used by the
-filesystem with the mount options nr_blocks and nr_inodes.
-
 The Logical Volume Manager (LVM) is now in the kernel.  If you want to
 use this, you'll need to install the necessary LVM toolset.
 

diff -uNr 2.4.0-shm_vm_locked-truncate-rw/fs/Config.in 2.4.0-shm_vm_locked-truncate-rw-swapfs/fs/Config.in
@@ -29,6 +29,7 @@
 	int 'JFFS debugging verbosity (0 = quiet, 3 = noisy)' CONFIG_JFFS_FS_VERBOSE 0
 fi
 tristate 'Compressed ROM file system support' CONFIG_CRAMFS
+bool 'Swap file system support (former shm fs)' CONFIG_SWAPFS
 tristate 'Simple RAM-based file system support' CONFIG_RAMFS
 
 tristate 'ISO 9660 CDROM file system support' CONFIG_ISO9660_FS

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Now...is this shared memory or swap? If it's swap, why is it different than a swapfile? If you are intending the shmem be called swapfs, I personally thing that it'll cause a significant amount of confusion.

-d

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It is a filesystem which lives in RAM and can swap out. SYSV shm and shared anonymous maps are still build on top of this (The config option only disables the part not needed for this).

I am quite open about naming, but "shm" is not appropriate any more since the fs does a lot more than shared memory. Solaris calles this "tmpfs" but I did not want to 'steal' their name and I also do not think that it's a very good name.

So any suggestions for a better name?

Greetings
Christoph

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Hmm, ok, what are the activities that use this other than shm?

-d

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You can e.g. use it for your /tmp filesystem. there seem to be some people out there which used ramfs for that...

What do you think about "vmfs"? This probably reflects its nature better than swapfs.

Greetings
Christoph

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Admins already know what "tmpfs" means, so you should just call your filesystem that. I know it isn't a pretty name, but in the interest of reducing confusion, you should use the existing name.

Don't think of it as just "for /tmp". It is for temporary storage. The name is a reminder that you shouldn't store archives in tmpfs.

Again for compatibility, Sun's size option would be useful.

-o size=111222333      Size in bytes, rounded up by page size.
-o size=111222k        Size in kilobytes (base-2 or ISO standard?)
-o size=111m           Size in megabytes (base-2 or ISO standard?)

I'd prefer k for ISO standard and K for base-2.
Of course m isn't millibytes, but that isn't horrible.
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That sounds applicable and pretty good.

-d

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Hi Albert,

OK right now I see two alternatives for the name: "tmpfs" for the SUN admins and "vmfs" for expressing what it does and to be in line with "ramfs". Any votes?

No, I would go for base-2 only. That's what we typically mean with K and M in the IT world. To be case sensitive is IMHO overkill and confusing.

Greetings
Christoph

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Christoph> OK right now I see two alternatives for the name: "tmpfs" Christoph> for the SUN admins and "vmfs" for expressing what it does Christoph> and to be in line with "ramfs". Any votes?

I think vmfs is the better choice. Not to be gratuitously incompatable with sun, but there is no guarentee linux's implementation and sun's will be or remain compatable, so it really doesn't hurt to choose a name which is more descriptive rather than that used by someone else's product -- even if the other product is widely deployed and understood.

Christoph> No, I would go for base-2 only. That's what we typically Christoph> mean with K and M in the IT world. To be case sensitive is Christoph> IMHO overkill and confusing.

Unquestionably the Right Thing. VM is after all being measured, and RAM is always measured in binary. (OK, I just *know* someone will provide a counter-example disproving that assertion -- perhaps the old 36 bit systems? Or core? But are there any modern counter-examples?)

-JimC

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So does *BSD also have the size parameter?

Greetings
Christoph

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