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    SubjectRe: [PATCH] NOMMU: Fix MAP_PRIVATE mmap() of objects where the data can be mapped directly
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    On Fri, Sep 25, 2009 at 4:24 PM, David Howells <dhowells@redhat.com> wrote:
    > graff yang <graff.yang@gmail.com> wrote:
    >
    >> > +       if (capabilities & BDI_CAP_MAP_DIRECT) {
    >>
    >> This will breaks many drivers, e.g. some frame-buffer drivers, on NOMMU
    >> system.  Because they don't have get_unmapped_area().  These drivers depend
    >> on it's mmap() to return the frame-buffer base address.
    >
    > Then they won't work on NOMMU.  Read Documentation/nommu-mmap.txt:
    >
    >        ============================================
    >        PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
    >        ============================================
    >
    >        To provide shareable character device support, a driver must provide a
    >        file->f_op->get_unmapped_area() operation. The mmap() routines will
    >        call this to get a proposed address for the mapping. This may return an
    >        error if it doesn't wish to honour the mapping because it's too long,
    >        at a weird offset, under some unsupported combination of flags or
    >        whatever.
    >
    > The chardev driver doesn't provide the virtual address through its mmap() fop
    > in MMU-mode either - that's provided by do_mmap_pgoff().
    >
    > David
    >
    OK, I think do_mmap_pgoff() now looks nice with your patch. Merely,
    at least the alsa/fb driver
    will crash due to the vm_region changes.
    The alsa guys are fixing this issue:
    http://mailman.alsa-project.org/pipermail/alsa-devel/2009-March/015802.html

    Is there any other things be affected?

    --
    -Graff
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