linux-kernel - Re: [crash] Re: Latest brk patchset

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Message-ID: <49BE9756.6070401@zytor.com>
Date:	Mon, 16 Mar 2009 11:15:50 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [crash] Re: Latest brk patchset

Jeremy Fitzhardinge wrote:
> Yinghai Lu wrote:
>> could be max_pfn_mapped change in head_32.S that reduce mapping range 
>> to _end only.
>>   
> 
> Yes, I would say you're right.  Trimming the mapping to _end only leaves 
> the space under the kernel avaliable for allocating pagetable from e820 
> space.  In this case we've got DEBUG_PAGEALLOC enabled, which inhibits 
> the use of PSE, and BIOS corruption checking on, which eats a chunk of 
> low memory.  In combination, there's only 0x8f000 bytes free below the 
> kernel, and it needs 0xe1000 to allocate for pagetables.
> 
> Reverting 2bd2753ff46346543ab92e80df9d96366e21baa5 fixes the problem for 
> me, though it does result in a kernel with a 73MB BSS...
> 

I did kind of think that was a bit fishy, but I somehow assumed you'd 
taken the consequences into account.  Obviously not.

The 73 MB brk isn't a problem per se as it will not be used in most 
systems.  This, in fact, is the whole reason for using brk and not just 
bss: it allows unused brk to be reclaimed at a relatively early stage.

	-hpa
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