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    Subject2.6.29 pat issue
    Hi!

    The function in include/linux/mm.h:
    is_linear_pfn_mapping()

    doesn't seem valid to me.

    In particular, we have VMAs to graphics devices in which vma->vm_pgoff
    is non-zero (Points to an offset in the drm device node), and the VMA is
    sparsely populated with pfns pointing to uncached discontigous RAM pages.

    This causes the X86 PAT code to hit the optimized path when it
    shouldn't, and issue a warning.

    Also a question about the philosofy behind this strict checking that all
    PTEs have the same caching attributes: I guess this is only to catch
    bugs in kernel drivers that don't get this right. At the same time, now
    that also user-space VMAs are checked this will probably have a
    significant performance impact. Shouldn't this checking really live
    behind a debug define?

    Thanks,
    Thomas





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