|Execute-in-place for file mappings
|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
|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.
|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
|- 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:
|The generic file operations do_sync_read/do_sync_write can be used to implement
|classic synchronous IO calls.
|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.