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

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Message-ID: <49BEBD98.3090905@goop.org>
Date:	Mon, 16 Mar 2009 13:59:04 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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

H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
>> Yinghai Lu wrote:
>>>> -INIT_MAP_SIZE = (PAGE_TABLE_SIZE + ALLOCATOR_SLOP) * PAGE_SIZE_asm
>>>> -RESERVE_BRK(pagetables, INIT_MAP_SIZE)
>>>> +/* Enough space to fit pagetables for the low memory linear map */
>>>> +MAPPING_BEYOND_END = (PAGE_TABLE_SIZE(1 << (32 - PAGE_SHIFT)) * 
>>>> PAGE_SIZE)
>>>>     
>>>
>>> that is for 4g?
>>>   
>>
>> Yes, I suppose it only needs to be 1G.  But it doesn't make much 
>> difference in practice.
>>
>
> A tighter bound would probably be nice, if nothing else because 
> tighter bounds tend to catch more bugs.

Well, its a bit more complex than that; we'd need to make it conditional 
on the user/kernel split, which seems all a bit fiddly.  I guess it 
could be something like:

MAPPING_BEYOND_END = PAGE_TABLE_SIZE((1<<(32-PAGE_SHIFT)) - (PAGE_OFFSET / PAGE_SIZE)) * PAGE_SIZE

    J
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