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

[Mel Gorman <mel@csn.ul.ie>]

On Thu, 6 Jul 2006, Franck Bui-Huu wrote:

> Hi Mel !

Hi.

> 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.

Grand.

> >  #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 ?

The start of mem_map is at PFN 0 so it's always aligned. node_mem_map is 
MAX_ORDER_NR_PAGES aligned in alloc_node_mem_map().

Bear in mind that the only difference between the patched and unpatched 
kernel is that the unpatched kernel offsets NODE_DATA(0)->node_mem_map by 
NODE_DATA(0)->node_mem_map at every call to __pfn_to_page() and 
__page_to_pfn() and the patched kernel offsets just once. I'm not looking 
to alter anything fundamental here.

> >  #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

The size of the node_mem_map is determined by zones_size. mem_map does not 
have a size as such because it's not necessarily a fully populated array. 
The location of mem_map is determined by the location of 
NODE_DATA(0)->node_mem_map and ARCH_PFN_OFFSET. Neither the former nor the 
latter affect the size of mem_map in other words.

> 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.

I'm not aware of an architecture that would run into such a problem but 
that is not saying a lot.

> 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);


I think my patch does the job of moving ARCH_PFN_OFFSET out of the hot 
path in a less risky fashion. However, if you are sure that callers to 
free_area_init() and ARCH_PFN_OFFSET are ok after your patch, I'd be happy 
to go with it. If you're not sure, I reckon my patch would be the way to 
go.

> 		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

--
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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