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

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

On Tue, Apr 6, 2010 at 11:17 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 04/06/2010 08:02 AM, Joel Fernandes wrote:
>> Hi Hayfeng,
>>
>> On Tue, Apr 6, 2010 at 8:07 PM, hayfeng Lee <teklife.kernel@gmail.com> wrote:
>>> hello,every one.
>>> I have a question:
>>> Why does linux choose 896MB to do a start point of ZONE_HIGHMEM and
>>> the end point of ZONE_NORMAL. Just for experience?
>>> What is the advantages?
>>
>> This is not an advantage but a limitation of 32 bit processor and
>> architecture. Only physical memory in first 896MB  is directly mapped
>> to the kernel virtual memory address space. This is called
>> ZONE_NORMAL. To access any physical memory in ZONE_HIGHMEM, the kernel
>> has to set up page table entries to indirectly map the physical memory
>> into a virtual memory address (I think around 128MB or so worth page
>> table entries are reused for this purpose). On the other hand, on 64
>> bit architectures, the entire physical memory is directly mapped and
>> accessible to the kernel. ZONE_HIGHMEM doesn't exist on 64 bit.
>>
>> Take the above with a grain of salt, someone with a better knowledge
>> about this intrusive topic can be give a more detailed explanation :)
>>
>
> 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.
>
>        -hpa
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the VM split is also configurable when building the kernel (for 32-bit
processors).