linux-kernel - question about identiry map(x86)

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Date:	Fri, 19 Jan 2007 18:02:36 +0300
From:	asgard <just.asgard@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: question about identiry map(x86)

Hi, all.

On early boot stage linux maps only 8M(arch/i386/kernel/head.S).
It creates identity map and kernel map like this:
0x00000000 - 0x007FFFFF -> 0x00000000 -> 0x007FFFFF /* identity map */
0xC0000000 - 0xC07FFFFF -> 0x00000000 -> 0x007FFFFF /* kernel map */

On early boot stage identity map is needed for jumping to 0xC0000000.
Intel manual says, that after such jumping identity map can be deleted.

(arch/i386/mm/init.c)
Linux deletes early boot identity map, but when it maps 1G(all kernel
space) in kernel_physical_map_init, it creates another one identity map:
0x00000000 - 0x01FFFFFF -> 0x00000000 - 0x01FFFFFF /* NEW identity map
*/
0xC0000000 - 0xC1FFFFFF -> 0x00000000 - 0x01FFFFFF /* 1G kernel map */

so, question is: are there any significant reasons to create identity
map after jumping was done? As I know, after jumping kernel doesn't need
in identity map. So, if, for example, PAE is enabled for creation middle
directory kernel allocates 4K. And when identity map creates, kernel
loses some mem. So, why identity map is created in this case.

Thanks and good luck. 

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