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Possible License(s): GPL-2.0, LGPL-2.0, AGPL-1.0
1Execute-in-place for file mappings 2---------------------------------- 3 4Motivation 5---------- 6File mappings are performed by mapping page cache pages to userspace. In 7addition, read&write type file operations also transfer data from/to the page 8cache. 9 10For memory backed storage devices that use the block device interface, the page 11cache pages are in fact copies of the original storage. Various approaches 12exist to work around the need for an extra copy. The ramdisk driver for example 13does read the data into the page cache, keeps a reference, and discards the 14original data behind later on. 15 16Execute-in-place solves this issue the other way around: instead of keeping 17data in the page cache, the need to have a page cache copy is eliminated 18completely. With execute-in-place, read&write type operations are performed 19directly from/to the memory backed storage device. For file mappings, the 20storage device itself is mapped directly into userspace. 21 22This implementation was initially written for shared memory segments between 23different virtual machines on s390 hardware to allow multiple machines to 24share the same binaries and libraries. 25 26Implementation 27-------------- 28Execute-in-place is implemented in three steps: block device operation, 29address space operation, and file operations. 30 31A block device operation named direct_access is used to retrieve a 32reference (pointer) to a block on-disk. The reference is supposed to be 33cpu-addressable, physical address and remain valid until the release operation 34is performed. A struct block_device reference is used to address the device, 35and a sector_t argument is used to identify the individual block. As an 36alternative, memory technology devices can be used for this. 37 38The block device operation is optional, these block devices support it as of 39today: 40- dcssblk: s390 dcss block device driver 41 42An address space operation named get_xip_mem is used to retrieve references 43to a page frame number and a kernel address. To obtain these values a reference 44to an address_space is provided. This function assigns values to the kmem and 45pfn parameters. The third argument indicates whether the function should allocate 46blocks if needed. 47 48This address space operation is mutually exclusive with readpage&writepage that 49do page cache read/write operations. 50The following filesystems support it as of today: 51- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt 52 53A set of file operations that do utilize get_xip_page can be found in 54mm/filemap_xip.c . The following file operation implementations are provided: 55- aio_read/aio_write 56- readv/writev 57- sendfile 58 59The generic file operations do_sync_read/do_sync_write can be used to implement 60classic synchronous IO calls. 61 62Shortcomings 63------------ 64This implementation is limited to storage devices that are cpu addressable at 65all times (no highmem or such). It works well on rom/ram, but enhancements are 66needed to make it work with flash in read+write mode. 67Putting the Linux kernel and/or its modules on a xip filesystem does not mean 68they are not copied.