view Documentation/filesystems/xip.txt @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
line source
1 Execute-in-place for file mappings
2 ----------------------------------
4 Motivation
5 ----------
6 File mappings are performed by mapping page cache pages to userspace. In
7 addition, read&write type file operations also transfer data from/to the page
8 cache.
10 For memory backed storage devices that use the block device interface, the page
11 cache pages are in fact copies of the original storage. Various approaches
12 exist to work around the need for an extra copy. The ramdisk driver for example
13 does read the data into the page cache, keeps a reference, and discards the
14 original data behind later on.
16 Execute-in-place solves this issue the other way around: instead of keeping
17 data in the page cache, the need to have a page cache copy is eliminated
18 completely. With execute-in-place, read&write type operations are performed
19 directly from/to the memory backed storage device. For file mappings, the
20 storage device itself is mapped directly into userspace.
22 This implementation was initialy written for shared memory segments between
23 different virtual machines on s390 hardware to allow multiple machines to
24 share the same binaries and libraries.
26 Implementation
27 --------------
28 Execute-in-place is implemented in three steps: block device operation,
29 address space operation, and file operations.
31 A block device operation named direct_access is used to retrieve a
32 reference (pointer) to a block on-disk. The reference is supposed to be
33 cpu-addressable, physical address and remain valid until the release operation
34 is performed. A struct block_device reference is used to address the device,
35 and a sector_t argument is used to identify the individual block. As an
36 alternative, memory technology devices can be used for this.
38 The block device operation is optional, these block devices support it as of
39 today:
40 - dcssblk: s390 dcss block device driver
42 An address space operation named get_xip_page is used to retrieve reference
43 to a struct page. To address the target page, a reference to an address_space,
44 and a sector number is provided. A 3rd argument indicates whether the
45 function should allocate blocks if needed.
47 This address space operation is mutually exclusive with readpage&writepage that
48 do page cache read/write operations.
49 The following filesystems support it as of today:
50 - ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
52 A set of file operations that do utilize get_xip_page can be found in
53 mm/filemap_xip.c . The following file operation implementations are provided:
54 - aio_read/aio_write
55 - readv/writev
56 - sendfile
58 The generic file operations do_sync_read/do_sync_write can be used to implement
59 classic synchronous IO calls.
61 Shortcomings
62 ------------
63 This implementation is limited to storage devices that are cpu addressable at
64 all times (no highmem or such). It works well on rom/ram, but enhancements are
65 needed to make it work with flash in read+write mode.
66 Putting the Linux kernel and/or its modules on a xip filesystem does not mean
67 they are not copied.