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author | Carsten Otte <cotte@freenet.de> | 2005-06-23 22:05:31 -0700 |
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committer | Linus Torvalds <torvalds@ppc970.osdl.org> | 2005-06-24 00:06:42 -0700 |
commit | d763b7a4736e219528f77bf6bc75dd78b1d75c03 (patch) | |
tree | 022b661841982e7b34ee5e6579df0c3d7989aa04 /Documentation/filesystems/xip.txt | |
parent | fe77ba6f4f97690baa4c756611a07f3cc033f6ae (diff) | |
download | kernel_samsung_espresso10-d763b7a4736e219528f77bf6bc75dd78b1d75c03.zip kernel_samsung_espresso10-d763b7a4736e219528f77bf6bc75dd78b1d75c03.tar.gz kernel_samsung_espresso10-d763b7a4736e219528f77bf6bc75dd78b1d75c03.tar.bz2 |
[PATCH] xip: description
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'Documentation/filesystems/xip.txt')
-rw-r--r-- | Documentation/filesystems/xip.txt | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/Documentation/filesystems/xip.txt b/Documentation/filesystems/xip.txt new file mode 100644 index 0000000..6c0cef1 --- /dev/null +++ b/Documentation/filesystems/xip.txt @@ -0,0 +1,67 @@ +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. |