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

["H. Peter Anvin" <hpa@zytor.com>]

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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