Re: why choose 896MB to the start point of ZONE_HIGHMEM

[Frank Hu <frank.hu.2001@gmail.com>]

On Tue, Apr 6, 2010 at 12:44 PM, H. Peter Anvin <hpa@zytor.com> 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.