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

From: H. Peter Anvin
Date: Tue Apr 06 2010 - 14:27:50 EST

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@xxxxxxxxx> 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.

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