Re: [PATCH 2/3] Setup min_low_pfn/max_low_pfn correctly

[Ralf Baechle <ralf@linux-mips.org>]

On Wed, Dec 06, 2006 at 04:48:29PM +0100, Franck Bui-Huu wrote:

> The old code was assuming that min_low_pfn was always 0. This
> means that we can't handle platforms with a big hole at start
> of memory since mem_map[] size would blew up.

It was - and IP22 was paying a high price for that.  It's currently 1MB
on 32-bit and 1.75MB on 64-bit kernels - but used to be much higher in
the past.

Btw, I think you're confusing the terms here; your patch description reads
wrong while the patch looks fine.  PFN is page frame number, not physical
frame number.  To avoid wasting dead mem_map[] entries idealy the pfn for
the first allocatable page should be zero.
So ignoring NUMA and discontig memory for a moment (those make everything
more complicated ...) PFN 0 is the first entry in mem_map[] - which usually
but not necessarily is physical address 0x0.

> This patch does not relax this constraint but it's a first
> step to achieve that.

> diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
> index 89440a0..8e58d7f 100644
> --- a/arch/mips/kernel/setup.c
> +++ b/arch/mips/kernel/setup.c
> @@ -271,7 +271,6 @@ static void __init bootmem_init(void)
>  static void __init bootmem_init(void)
>  {
>  	unsigned long reserved_end;
> -	unsigned long highest = 0;
>  	unsigned long mapstart = -1UL;

Assigning a negative number to an unsigned long variable ...

>  	unsigned long bootmap_size;
>  	int i;
> @@ -284,6 +283,13 @@ static void __init bootmem_init(void)
>  	reserved_end = max(init_initrd(), PFN_UP(__pa_symbol(&_end)));
>  
>  	/*
> +	 * max_low_pfn is not a number of pages. The number of pages
> +	 * of the system is given by 'max_low_pfn - min_low_pfn'.
> +	 */
> +	min_low_pfn = -1UL;

Assigning a negative number to an unsigned long variable ...

  Ralf