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    SubjectRe: [PATCH] powerpc/mm: setting mmaped page cache property through device tree
    On Tue, Dec 1, 2009 at 6:58 PM, Benjamin Herrenschmidt
    <> wrote:
    > On Tue, 2009-12-01 at 18:30 +0800, Li Yang wrote:
    >> The patch adds the ability for powerpc architecture to set page cache
    >> property of mmaped area through device tree.  This is useful for two
    >> cases.  First, for memory shared with other OS'es to have the same cache
    >> property to avoid cache paradoxes.  Second, enabling application to map
    >> memory which is not managed by kernel as cacheable for better performance.
    > But that doesn't solve the problem of those same pages being mapped
    > cachable as part of the linear mapping does it ?

    I think that it doesn't has this problem. Only regions out of
    lmb.memory are configurable through device tree.

    > Can you tell us more about your precise usage scenario ? What are you

    The scenario for the first case is that in a multicore system running
    ASMP which means different OS runs on different cores. They might
    communicate through a shared memory region. The region on every OS
    need to be mapped with the same cache perperty to avoid cache paradox.

    The scenario for the second case is to pre-allocate some memory to a
    certain application or device (probably through mem=XXX kernel
    parameter or limit through device tree). The memory is not known to
    kernel, but fully managed by the application/device. We need being
    able to map the region cachable for better performance.

    > trying to achieve here ? We can find a solution though it might involve
    > a specific driver to handle that memory.

    Right, but what the user to kernel API should be used? Is it ok to
    use the O_SYNC flag as I previously proposed?

    - Leo
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