Re: Crash while mapping memory in pagetable_init() (Was: Re: .config)

H. Peter Anvin
2007-04-13T20:32:52Z

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)

-hpa