[PATCH 01/16 v5] pramfs: documentation

[Marco Stornelli <marco.stornelli@gmail.com>]

From: Marco Stornelli <marco.stornelli@gmail.com>

Documentation for PRAMFS.

Signed-off-by: Marco Stornelli <marco.stornelli@gmail.com>
---
diff -Nurp linux-2.6.36-orig/Documentation/filesystems/pramfs.txt linux-2.6.36/Documentation/filesystems/pramfs.txt
--- linux-2.6.36-orig/Documentation/filesystems/pramfs.txt	1970-01-01 01:00:00.000000000 +0100
+++ linux-2.6.36/Documentation/filesystems/pramfs.txt	2010-12-15 19:41:58.000000000 +0100
@@ -0,0 +1,179 @@
+
+PRAMFS Overview
+===============
+
+Many embedded systems have a block of non-volatile RAM separate from
+normal system memory, i.e. of which the kernel maintains no memory page
+descriptors. For such systems it would be beneficial to mount a
+fast read/write filesystem over this "I/O memory", for storing frequently
+accessed data that must survive system reboots and power cycles. An
+example usage might be system logs under /var/log, or a user address
+book in a cell phone or PDA.
+
+Linux traditionally had no support for a persistent, non-volatile RAM-based
+filesystem, persistent meaning the filesystem survives a system reboot
+or power cycle intact. The RAM-based filesystems such as tmpfs and ramfs
+have no actual backing store but exist entirely in the page and buffer
+caches, hence the filesystem disappears after a system reboot or
+power cycle.
+
+A relatively straightforward solution is to write a simple block driver
+for the non-volatile RAM, and mount over it any disk-based filesystem such
+as ext2, ext3, ext4, etc.
+
+But the disk-based fs over non-volatile RAM block driver approach has
+some drawbacks:
+
+1. Complexity of disk-based fs: disk-based filesystems such as ext2/ext3/ext4
+   were designed for optimum performance on spinning disk media, so they
+   implement features such as block groups, which attempts to group inode data
+   into a contiguous set of data blocks to minimize disk seeking when accessing
+   files. For RAM there is no such concern; a file's data blocks can be
+   scattered throughout the media with no access speed penalty at all. So block
+   groups in a filesystem mounted over RAM just adds unnecessary
+   complexity. A better approach is to use a filesystem specifically
+   tailored to RAM media which does away with these disk-based features.
+   This increases the efficient use of space on the media, i.e. more
+   space is dedicated to actual file data storage and less to meta-data
+   needed to maintain that file data.
+
+2. Different problems between disks and RAM: Because PRAMFS attempts to avoid
+   filesystem corruption caused by kernel bugs, dirty pages in the page cache
+   are not allowed to be written back to the backing-store RAM. This way, an
+   errant write into the page cache will not get written back to the filesystem.
+   However, if the backing-store RAM is comparable in access speed to system
+   memory, the penalty of not using caching is minimal. With this consideration
+   it's better to move file data directly between the user buffers and the backing
+   store RAM, i.e. use direct I/O. This prevents the unnecessary populating of
+   the page cache with dirty pages. However direct I/O has to be enabled at
+   every file open. To enable direct I/O at all times for all regular files
+   requires either that applications be modified to include the O_DIRECT flag on
+   all file opens, or that the filesystem used performs direct I/O by default.
+
+The Persistent/Protected RAM Special Filesystem (PRAMFS) is a read/write
+filesystem that has been designed to address these issues. PRAMFS is targeted
+to fast I/O memory, and if the memory is non-volatile, the filesystem will be
+persistent.
+
+In PRAMFS, direct I/O is enabled across all files in the filesystem, in other
+words the O_DIRECT flag is forced on every open of a PRAMFS file. Also, file
+I/O in the PRAMFS is always synchronous. There is no need to block the current
+process while the transfer to/from the PRAMFS is in progress, since one of
+the requirements of the PRAMFS is that the filesystem exists in fast RAM. So
+file I/O in PRAMFS is always direct, synchronous, and never blocks.
+
+The data organization in PRAMFS can be thought of as an extremely simplified
+version of ext2, such that the ratio of data to meta-data is very high.
+
+PRAMFS supports the execute-in-place. With XIP, instead of keeping data in the
+page cache, the need to have a page cache copy is eliminated completely.
+Read&write type operations are performed directly from/to the memory. For file
+mappings, the RAM itself is mapped directly into userspace. XIP, in addition,
+speed up the applications start-up time because it removes the needs of any
+copies.
+
+PRAMFS is write protected. The page table entries that map the backing-store
+RAM are normally marked read-only. Write operations into the filesystem
+temporarily mark the affected pages as writeable, the write operation is
+carried out with locks held, and then the page table entries is
+marked read-only again.
+This feature provides protection against filesystem corruption caused by errant
+writes into the RAM due to kernel bugs for instance. In case there are systems
+where the write protection is not possible (for instance the RAM cannot be
+mapped with page tables), this feature can be disabled via the
+CONFIG_PRAMFS_WRITE_PROTECT config option.
+
+PRAMFS supports extended attributes, ACLs and security labels.
+
+In summary, PRAMFS is a light-weight, space-efficient special filesystem that
+is ideal for systems with a block of fast non-volatile RAM that need to access
+data on it using a standard filesytem interface.
+
+Supported mount options
+=======================
+
+The PRAMFS currently requires one mount option, and there are several
+optional mount options:
+
+physaddr=	Required. It tells PRAMFS the physical address of the
+		start of the RAM that makes up the filesystem. The
+		physical address must be located on a page boundary.
+
+init=		Optional. It is used to initialize the memory to an
+		empty filesystem. Any data in an existing filesystem
+		will be lost if this option is given. The parameter to
+		"init=" is the RAM in kilo/mega/giga bytes.
+
+bs=		Optional. It is used to specify a block size. It is
+		ignored if the "init=" option is not specified, since
+		otherwise the block size is read from the PRAMFS
+		super-block. The default blocksize is 2048 bytes,
+		and the allowed block sizes are 512, 1024, 2048, and
+		4096.
+
+bpi=		Optional. It is used to specify the bytes per inode
+		ratio, i.e. for every N bytes in the filesystem, an
+		inode will be created. This behaves the same as the "-i"
+		option to mke2fs. It is ignored if the "init=" option is
+		not specified.
+
+N=		Optional. It is used to specify the number of inodes to
+		allocate in the inode table. If the option is not
+		specified, the bytes-per-inode ratio is used to
+		calculate the number of inodes. If neither the "N=" or
+		"bpi=" options are specified, the default behavior is to
+		reserve 5% of the total space in the filesystem for the
+		inode table. This option behaves the same as the "-N"
+		option to mke2fs. It is ignored if the "init=" option is
+		not specified.
+
+errors=		Optional. It can be "cont", "remount-ro" and "panic". With the
+		first value no action is done in case of error. With the second
+		one the fs is mounted read-only. with the third one a kernel
+		panic happens. Default action is to continue on error.
+
+acl,noacl	Optional. Enable/disable the support for access control lists
+		(disabled by default).
+
+user_xattr,	Optional. Enable/disable the support for the user extended
+user_noxattr	attributes (disabled by default).
+
+noprotect	Optional. Disable the memory protection (enabled by default).
+
+xip		Optional. Enable the execute-in-place (disabled by default).
+
+Examples:
+
+mount -t pramfs -o physaddr=0x20000000,init=1M,bs=1k none /mnt/pram
+
+This example locates the filesystem at physical address 0x20000000, and
+also requests an empty filesystem be initialized, of total size of one
+megabyte and blocksize of one kilobyte. The mount point is /mnt/pram.
+
+mount -t pramfs -o physaddr=0x20000000 none /mnt/pram
+
+This example locates the filesystem at physical address 0x20000000 as in
+the first example, but uses the intact filesystem that already exists.
+
+Current Limitations
+===================
+
+- The RAM used for PRAMFS must be directly addressable.
+
+- PRAMFS does not support hard links.
+
+- PRAMFS supports only private memory mappings. This allows most
+  executables to run, but programs that attempt shared memory
+  mappings, such as X apps that use X shared memory, will fail.
+
+- PRAMFS does not support quota settings.
+
+Further Documentation
+=====================
+
+If you are interested in the internal design of PRAMFS, there is
+documentation available at the Sourceforge PRAMFS home page at
+http://pramfs.sourceforge.net/.
+
+Please send bug reports/comments/feedback to the pramfs development
+list at sourceforge: pramfs-devel@lists.sourceforge.net.
diff -Nurp linux-2.6.36-orig/Documentation/filesystems/xip.txt linux-2.6.36/Documentation/filesystems/xip.txt
--- linux-2.6.36-orig/Documentation/filesystems/xip.txt	2010-10-20 22:30:22.000000000 +0200
+++ linux-2.6.36/Documentation/filesystems/xip.txt	2010-10-31 09:25:05.000000000 +0100
@@ -49,6 +49,8 @@ This address space operation is mutually
 do page cache read/write operations.
 The following filesystems support it as of today:
 - ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
+- pramfs: persistent and protected RAM filesystem, see
+  Documentation/filesystems/pramfs.txt
 
 A set of file operations that do utilize get_xip_page can be found in
 mm/filemap_xip.c . The following file operation implementations are provided: