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Date: Tue, 6 Apr 2010 13:15:49 -0700
Message-ID: <p2q8f84f9951004061315z7d2be34aj41b5052cf20879f3@mail.gmail.com>
Subject: Re: why choose 896MB to the start point of ZONE_HIGHMEM
From: Frank Hu <frank.hu.2001@gmail.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Joel Fernandes <agnel.joel@gmail.com>,
       hayfeng Lee <teklife.kernel@gmail.com>, linux-kernel@vger.kernel.org,
       linux-kernel@zh-kernel.org, kernelnewbies@nl.linux.org
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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.
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