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    SubjectRe: 4MB pages and framebuffer access, x11perf results, 2.1.125

    On Thu, 29 Oct 1998, Andrea Arcangeli wrote:
    > This should be clean doable adding a pmd_only() to tell if the pmd does
    > not need a pte to get the info about phys space. Other arch (and <P5) can
    > #define pmd_only() 0.

    There is no way this can be cleanly done.

    The problem occurs whenever you then try to map or unmap parts of the area
    to something else:

    p = mmap(fd, 4M)

    which anything with this approach will just completely die off, even once
    you removed the ugly special casing.

    Basically, anything that wants to do "large page" mappings of a frame
    buffer needs to use something that isn't standard mmap(). Whether that's a
    special flag to mmap() or something else is immaterial, but it needs to be
    done on a VMA basis, not on a page table basis. Because we need to
    disallow partial unmaps etc.

    Trying to do this with a standard mmap() and doing page table only tricks
    is doomed. Don't even try.


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