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Diffstat (limited to 'Documentation/filesystems/xip.txt')
| -rw-r--r-- | Documentation/filesystems/xip.txt | 68 |
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diff --git a/Documentation/filesystems/xip.txt b/Documentation/filesystems/xip.txt new file mode 100644 index 0000000..0466ee5 --- /dev/null +++ b/Documentation/filesystems/xip.txt @@ -0,0 +1,68 @@ +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 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_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. |
