linux-kernel - Re: Crash while mapping memory in pagetable

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Message-ID: <461FEA0B.40700@zytor.com>
Date:	Fri, 13 Apr 2007 13:37:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Zachary Amsden <zach@...are.com>,
	Chris Wright <chrisw@...s-sol.org>, Andi Kleen <ak@...e.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Crash while mapping memory in pagetable_init() (Was: Re: .config)

H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
>> H. Peter Anvin wrote:
>>> Really (pae ? 2M : 1M), in other words, plus the 128K for bootmem. 
>>> Note that this is creating page tables for, not erasing.  To map 2M,
>>> we will only use 2K of additional memory (meaning there is 50% chance
>>> we end up using an additional 4K page.)
>>>
>>> So the solution is simply to change INIT_MAP_BEYOND_END in head.S
>>> appropriately.
>> Like this?  Should we bother adding some slop pages for allocations
>> which happen before paging_init()?
> 
> Yes, although it really should be sensitive to CONFIG_X86_PAE.
> 
>>  
>>  /*
>>   * This is how much memory *in addition to the memory covered up to
>> - * and including _end* we need mapped initially.  We need one bit for
>> - * each possible page, but only in low memory, which means
>> - * 2^32/4096/8 = 128K worst case (4G/4G split.)
>> + * and including _end* we need mapped initially.
>> + * We need:
>> + *  - one bit for each possible page, but only in low memory, which 
>> means
>> + *     2^32/4096/8 = 128K worst case (4G/4G split.)
>> + *  - enough space to map all low memory, which means
>> + *     (2^32/4096) / 512 + 4 pages (worst case for PAE)
>>   *
>>   * Modulo rounding, each megabyte assigned here requires a kilobyte of
>>   * memory, which is currently unreclaimed.
>>   *
>>   * This should be a multiple of a page.
>>   */
>> -#define INIT_MAP_BEYOND_END    (128*1024)
>> +#define INIT_MAP_BEYOND_END    (128*1024 + (2048 + 4)*4096)
> 
> I suggest, for clarity and to minimize bloat:
> 
> #ifdef CONFIG_X86_PAE
> # define PAGE_TABLE_SIZE    ((2048+4)*4096)
> #else
> # define PAGE_TABLE_SIZE    ((1024+1)*4096)
> #endif
> 
> #define BOOTMEM_SIZE        (128*1024)
> 
> #define INIT_MAP_BEYOND_END    (BOOTMEM_SIZE+PAGE_TABLE_SIZE)
> 

Actually, better yet; there is no reason for these to be macros:

#ifdef CONFIG_X86_PAE
PAGE_TABLE_SIZE = (2048+4)*4096
#else
PAGE_TABLE_SIZE = (1024+1)*4096
#endif
BOOTMEM_SIZE = 128*1024

INIT_MAP_BEYOND_END = BOOTMEM_SIZE + PAGE_TABLE_SIZE

	-hpa

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