linux-kernel - Re: brk patches..

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Date:	Sun, 01 Mar 2009 17:02:41 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: brk patches..

Jeremy Fitzhardinge wrote:
> Yinghai Lu wrote:
>>> But its no different from what i386 does now to allocate its initial
>>> pagetables.  How does this not break now?
>>>
>>>     
>>
>> it will try to use initial page table af first, and it is not big
>> enough, it will according to e820 and other reserved_early areas to
>> find good positions.
>>   
> 
> head_32.S has no such logic.  It just starts building the kernel
> mappings directly after _end, starting at pg0, and uses as much space as
> it needs.  For a !PSE CPU with a large kernel, that can be quite a lot
> of space.  Only later, when its creating the linear memory mappings,
> does it search around in the e820 tables (which it now has access to)
> for space.

yes, we should have some way to only make initial_pgtable only cover to _end
and put that initial_pagbe before _end.

> 
> The whole point of the brk segment was to have a way of allocating some
> memory very early, before e820 is even available.  If you really think
> this is dangerous, then we can easily extend the bss in the linker
> script to include the brk memory, and release any leftover when we do
> the normal bootmem freeup.  That would also give us a well-defined upper
> limit on how much brk memory can be allocated; its a bit undefined at
> the moment, as it depends on how much slop there is after the kernel
> mapping.

hope later boot loader could check vmlinux size in bzImage (according uncompressed size)
and find good position in RAM for bzImage.

we should find good position for brk with find_e820_area().

YH
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