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    SubjectRe: why choose 896MB to the start point of ZONE_HIGHMEM
    On Tue, Apr 6, 2010 at 12:44 PM, H. Peter Anvin <> wrote:
    > On 04/06/2010 12:20 PM, Frank Hu wrote:
    >>> The ELF ABI specifies that user space has 3 GB available to it.  That
    >>> leaves 1 GB for the kernel.  The kernel, by default, uses 128 MB for I/O
    >>> mapping, vmalloc, and kmap support, which leaves 896 MB for LOWMEM.
    >>> All of these boundaries are configurable; with PAE enabled the user
    >>> space boundary has to be on a 1 GB boundary.
    >> the VM split is also configurable when building the kernel (for 32-bit
    >> processors).
    > I did say "all these boundaries are configurable".  Rather explicitly.
    >        -hpa

    thought that you can only configure how to split the VM like 1G/3G or
    2G/2G. But the DMA zone size, the 128MB space for I/O is not
    configurable. The NORMAL zone size will be deducted based on the VM
    Split and some hard coded DMA zone and 128 MB space size.

    I am not a guru in this space... so I might be wrong.
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      Last update: 2010-04-06 22:19    [W:0.022 / U:49.792 seconds]
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