linux-kernel - Re: why choose 896MB to the start point of ZONE

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Message-ID: <w2p9ff7a3bc1004061301v6cc76e1g1e573468b1b8807d@mail.gmail.com>
Date:	Wed, 7 Apr 2010 01:31:46 +0530
From:	Joel Fernandes <agnel.joel@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Frank Hu <frank.hu.2001@...il.com>,
	hayfeng Lee <teklife.kernel@...il.com>,
	linux-kernel@...r.kernel.org, linux-kernel@...kernel.org,
	kernelnewbies@...linux.org
Subject: Re: why choose 896MB to the start point of ZONE_HIGHMEM

Hi Peter,

On Wed, Apr 7, 2010 at 1:14 AM, H. Peter Anvin <hpa@...or.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.
>

I thought the 896 MB was a hardware limitation on 32 bit architectures
and something that cannot be configured? Or am I missing something
here? Also the vm-splits refer to "virtual memory" . While ZONE_* and
the 896MB we were discussing refers to "physical memory". How then is
discussing about vm splits pertinent here?

Thanks,
-Joel
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