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    SubjectRe: [PATCH 2.6.21] cramfs: add cramfs Linear XIP
    Andrew Morton wrote:
    > But we'd expected and hoped that flash-based XIP would be able to use
    > the existing xip infrastructure, in mm/filemap_xip.c. Not possible?
    Thanks for the heads up Andrew. Reading the cramfs patch, I've found a
    lot of similarities between the current mainline xip stack and this
    nice piece of work. I also observed interresting differences:

    The current xip stack does use the block device abstraction added with
    an extra direct memory access operation. The cramfs patch does not
    abstract between the memory segment and the file system, it rather
    uses an address as mount parameter.
    Both methods can coexist, and a file system that does not use the
    block device abstraction may still provide get_xip_page address space
    operation and benefit from the xip stack infrastructure. Each method
    makes sense in the context of the corresponding file systems.

    The current xip stack relies on having struct page behind the memory
    segment. This causes few impact on memory management, but occupies
    some more memory. The cramfs patch chose to modify copy on write in
    order to deal with vmas that don't have struct page behind.
    So far, Hugh and Linus have shown strong opposition against copy on
    write with no struct page behind. If this implementation is acceptable
    to the them, it seems preferable to me over wasting memory. The xip
    stack should be modified to use this vma flag in that case.

    This implementation extends cramfs while the current xip stack extends
    ext2. I would love to see a cramfs implementation based on
    filemap_xip, choice is always a good thing. To me, it looks like this
    work can be modified to use filemap_xip.

    so long,
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      Last update: 2007-05-23 09:53    [W:0.038 / U:7.092 seconds]
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