69 lines
3.0 KiB
Plaintext
69 lines
3.0 KiB
Plaintext
Execute-in-place for file mappings
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----------------------------------
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Motivation
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----------
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File mappings are performed by mapping page cache pages to userspace. In
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addition, read&write type file operations also transfer data from/to the page
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cache.
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For memory backed storage devices that use the block device interface, the page
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cache pages are in fact copies of the original storage. Various approaches
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exist to work around the need for an extra copy. The ramdisk driver for example
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does read the data into the page cache, keeps a reference, and discards the
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original data behind later on.
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Execute-in-place solves this issue the other way around: instead of keeping
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data in the page cache, the need to have a page cache copy is eliminated
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completely. With execute-in-place, read&write type operations are performed
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directly from/to the memory backed storage device. For file mappings, the
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storage device itself is mapped directly into userspace.
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This implementation was initially written for shared memory segments between
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different virtual machines on s390 hardware to allow multiple machines to
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share the same binaries and libraries.
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Implementation
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--------------
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Execute-in-place is implemented in three steps: block device operation,
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address space operation, and file operations.
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A block device operation named direct_access is used to retrieve a
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reference (pointer) to a block on-disk. The reference is supposed to be
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cpu-addressable, physical address and remain valid until the release operation
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is performed. A struct block_device reference is used to address the device,
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and a sector_t argument is used to identify the individual block. As an
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alternative, memory technology devices can be used for this.
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The block device operation is optional, these block devices support it as of
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today:
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- dcssblk: s390 dcss block device driver
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An address space operation named get_xip_mem is used to retrieve references
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to a page frame number and a kernel address. To obtain these values a reference
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to an address_space is provided. This function assigns values to the kmem and
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pfn parameters. The third argument indicates whether the function should allocate
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blocks if needed.
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This address space operation is mutually exclusive with readpage&writepage that
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do page cache read/write operations.
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The following filesystems support it as of today:
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- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
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A set of file operations that do utilize get_xip_page can be found in
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mm/filemap_xip.c . The following file operation implementations are provided:
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- aio_read/aio_write
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- readv/writev
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- sendfile
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The generic file operations do_sync_read/do_sync_write can be used to implement
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classic synchronous IO calls.
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Shortcomings
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------------
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This implementation is limited to storage devices that are cpu addressable at
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all times (no highmem or such). It works well on rom/ram, but enhancements are
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needed to make it work with flash in read+write mode.
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Putting the Linux kernel and/or its modules on a xip filesystem does not mean
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they are not copied.
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