Re: [PATCH 1/1] Only use ARCH_PFN_OFFSET once during boot

[Franck Bui-Huu <vagabon.xyz@gmail.com>]

Hi Mel !

Mel Gorman wrote:
> The FLATMEM memory model assumes that memory is one contiguous region based
> at PFN 0 and uses the NODE_DATA(0)->node_mem_map as the global mem_map. As

[snip]

> 
> diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.17-mm6-clean/include/asm-generic/memory_model.h linux-2.6.17-mm6-archpfnoffset_optimise/include/asm-generic/memory_model.h
> --- linux-2.6.17-mm6-clean/include/asm-generic/memory_model.h	2006-07-05 14:31:17.000000000 +0100
> +++ linux-2.6.17-mm6-archpfnoffset_optimise/include/asm-generic/memory_model.h	2006-07-05 14:36:04.000000000 +0100
> @@ -28,9 +28,8 @@
>   */
>  #if defined(CONFIG_FLATMEM)
>  
> -#define __pfn_to_page(pfn)	(mem_map + ((pfn) - ARCH_PFN_OFFSET))
> -#define __page_to_pfn(page)	((unsigned long)((page) - mem_map) + \
> -				 ARCH_PFN_OFFSET)
> +#define __pfn_to_page(pfn)	(mem_map + (pfn))
> +#define __page_to_pfn(page)	((unsigned long)((page) - mem_map))
>  #elif defined(CONFIG_DISCONTIGMEM)
>  

ok for that part.

>  #define __pfn_to_page(pfn)			\
> diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.17-mm6-clean/mm/page_alloc.c linux-2.6.17-mm6-archpfnoffset_optimise/mm/page_alloc.c
> --- linux-2.6.17-mm6-clean/mm/page_alloc.c	2006-07-05 14:31:18.000000000 +0100
> +++ linux-2.6.17-mm6-archpfnoffset_optimise/mm/page_alloc.c	2006-07-05 17:01:01.000000000 +0100
> @@ -2157,10 +2157,14 @@ static void __init alloc_node_mem_map(st
>  	}
>  #ifdef CONFIG_FLATMEM
>  	/*
> -	 * With no DISCONTIG, the global mem_map is just set as node 0's
> +	 * With FLATMEM, the global mem_map is just set as node 0's. The
> +	 * FLATMEM memory model assumes that memory is in one contiguous area
> +	 * starting at PFN 0. Architectures that do not start NODE 0 at PFN 0
> +	 * must define ARCH_PFN_OFFSET as the offset between
> +	 * NODE_DATA(0)->node_mem_map and PFN 0.
>  	 */
>  	if (pgdat == NODE_DATA(0))
> -		mem_map = NODE_DATA(0)->node_mem_map;
> +		mem_map = NODE_DATA(0)->node_mem_map - ARCH_PFN_OFFSET;

is mem_map always aligned on MAX_ORDER ?

>  #endif
>  #endif /* CONFIG_FLAT_NODE_MEM_MAP */
>  }
>  

I'm not sure of that part. We basically make incoherent the use of
free_area_init_node()'s fourth parameter by doing this change (for
FLATMEM model of course).

When using free_area_init() which is defined as follow:

void __init free_area_init(unsigned long *zones_size)
{
	free_area_init_node(0, NODE_DATA(0), zones_size,
			__pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
}

we will end up to have 2 definitions for the mem start:

	- __pa(PAGE_OFFSET) >> PAGE_SHIFT
	- ARCH_PFN_OFFSET

The former will be used to calculate the size of mem_map and the
latter will be used to calculate the offset between
NODE_DATA(0)->node_mem_map and PFN 0. I don't think that will result
in any problem since:

	ARCH_PFN_OFFSET == __pa(PAGE_OFFSET) >> PAGE_SHIFT

That just makes the code is harder to follow.

But if some platforms were doing something like (which is unlikely)

	[...]
	free_area_init_node(0, NODE_DATA(0), zone_size, FOO_PFN_OFFSET, NULL);
	[...]

and ARCH_PFN_OFFSET != FOO_PFN_OFFSET then they may have some troubles.

What about this patch for page_alloc.c ? I think it makes more obvious
what is ARCH_PFN_OFFSET. And if someone doesn't want to use
ARCH_PFN_OFFSET, he still can use:

	free_area_init_node(0, NODE_DATA(0), zone_size, FOO_PFN_OFFSET, NULL);

		Franck

-- >8 --

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 253a450..9daee06 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2147,7 +2147,7 @@ #ifdef CONFIG_FLATMEM
 	 * With no DISCONTIG, the global mem_map is just set as node 0's
 	 */
 	if (pgdat == NODE_DATA(0))
-		mem_map = NODE_DATA(0)->node_mem_map;
+		mem_map = NODE_DATA(0)->node_mem_map - pgdat->node_start_pfn;
 #endif
 #endif /* CONFIG_FLAT_NODE_MEM_MAP */
 }
@@ -2172,10 +2172,13 @@ struct pglist_data contig_page_data = { 
 EXPORT_SYMBOL(contig_page_data);
 #endif
 
+/*
+ * This function is used only by FLATMEM. In that case the 
+ * start of physical mem is always given by ARCH_PFN_OFFSET.
+ */
 void __init free_area_init(unsigned long *zones_size)
 {
-	free_area_init_node(0, NODE_DATA(0), zones_size,
-			__pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
+	free_area_init_node(0, NODE_DATA(0), zones_size, ARCH_PFN_OFFSET, NULL);
 }
 
 #ifdef CONFIG_PROC_FS