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Diffstat (limited to 'Documentation/filesystems/xip.txt')
| -rw-r--r-- | Documentation/filesystems/xip.txt | 71 |
1 files changed, 0 insertions, 71 deletions
diff --git a/Documentation/filesystems/xip.txt b/Documentation/filesystems/xip.txt deleted file mode 100644 index b77472949ede..000000000000 --- a/Documentation/filesystems/xip.txt +++ /dev/null @@ -1,71 +0,0 @@ -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 initially 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 translate the -block device sector number to a page frame number (pfn) that identifies -the physical page for the memory. It also returns a kernel virtual -address that can be used to access the memory. - -The direct_access method takes a 'size' parameter that indicates the -number of bytes being requested. The function should return the number -of bytes that can be contiguously accessed at that offset. It may also -return a negative errno if an error occurs. - -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_mem is used to retrieve references -to a page frame number and a kernel address. To obtain these values a reference -to an address_space is provided. This function assigns values to the kmem and -pfn parameters. The third 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. |