[PATCH] xip: description

nobh				Do not attach buffer_heads to file pagecache.

xip				Use execute in place (no caching) if possible

grpquota,noquota,quota,usrquota	Quota options are silently ignored by ext2.

Execute-in-place for file mappings

----------------------------------

Motivation

----------

File mappings are performed by mapping page cache pages to userspace. In

addition, read&write type file operations also transfer data from/to the page

cache.

For memory backed storage devices that use the block device interface, the page

cache pages are in fact copies of the original storage. Various approaches

exist to work around the need for an extra copy. The ramdisk driver for example

does read the data into the page cache, keeps a reference, and discards the

original data behind later on.

Execute-in-place solves this issue the other way around: instead of keeping

data in the page cache, the need to have a page cache copy is eliminated

completely. With execute-in-place, read&write type operations are performed

directly from/to the memory backed storage device. For file mappings, the

storage device itself is mapped directly into userspace.

This implementation was initialy written for shared memory segments between

different virtual machines on s390 hardware to allow multiple machines to

share the same binaries and libraries.

Implementation

--------------

Execute-in-place is implemented in three steps: block device operation,

address space operation, and file operations.

A block device operation named direct_access is used to retrieve a

reference (pointer) to a block on-disk. The reference is supposed to be

cpu-addressable, physical address and remain valid until the release operation

is performed. A struct block_device reference is used to address the device,

and a sector_t argument is used to identify the individual block. As an

alternative, memory technology devices can be used for this.

The block device operation is optional, these block devices support it as of

today:

- dcssblk: s390 dcss block device driver

An address space operation named get_xip_page is used to retrieve reference

to a struct page. To address the target page, a reference to an address_space,

and a sector number is provided. A 3rd argument indicates whether the

function should allocate blocks if needed.

This address space operation is mutually exclusive with readpage&writepage that

do page cache read/write operations.

The following filesystems support it as of today:

- ext2: the second extended filesystem, see Documentation/filesystems/ext2.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:

- aio_read/aio_write

- readv/writev

- sendfile

The generic file operations do_sync_read/do_sync_write can be used to implement

classic synchronous IO calls.

Shortcomings

------------

This implementation is limited to storage devices that are cpu addressable at

all times (no highmem or such). It works well on rom/ram, but enhancements are

needed to make it work with flash in read+write mode.

Putting the Linux kernel and/or its modules on a xip filesystem does not mean

they are not copied.