Re: 2.6.17-mm1

[Mel Gorman]

On Mon, 26 Jun 2006, Franck Bui-Huu wrote:

> Hi Mel
>
> Mel Gorman wrote:
> > On Fri, 23 Jun 2006, Franck Bui-Huu wrote:
>
> [snip]
>
> > > -unsigned long __init init_bootmem (unsigned long start, unsigned long
> > > pages)
> > > +unsigned long __init init_bootmem(unsigned long start, unsigned long
> > > pages)
> > > {
> > >     max_low_pfn = pages;
> > >     min_low_pfn = start;
> > > -    return(init_bootmem_core(NODE_DATA(0), start, 0, pages));
> > > +    return init_bootmem_core(NODE_DATA(0), start, ARCH_PFN_OFFSET,
> > > pages);
> > > }
> >
> > Not all arches will use init_bootmem(). Arm for example uses
> > init_bootmem_node(). ARCH_PFN_OFFSET is only meant to affect mem_map,
>
> well, I don't agree here. ARCH_PFN_OFFSET is used to save the first
> page number that has physical memory. I don't see why we couldn't use
> it to correctly initialise the memory system...

Architectures will not always have a known fixed start of physical memory. 
On IA64 at least, they initialise memory as if it starts at 0 but on my 
one test machine, the beginning part is always a memory hole. I've seen 
nothing to indicate that this hole will be the same size on all IA64 
machines but I kinda doubt it. Also, arches that use init_bootmem() do not 
necessary use free_area_init().

> If we don't change init_bootmem() to use ARCH_PFN_OFFSET, then the
> kernel will initialise the start of memory to 0 which is boggus.
> IOW,
> we can't use this function without this change (except if your memory
> start at 0 of course). And I think that init_bootmem() has been
> implemented for systems which have only one node _whatever_ memory
> start value...

While you may be right, it'll only fix the problem for arches with 
ARCH_PFN_OFFSET and using init_bootmem (which is the case of MIPS I 
guess). But arches using init_bootmem_node or not using free_area_init() 
may still get kicked.

> > > #ifndef CONFIG_HAVE_ARCH_BOOTMEM_NODE
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>
> [snip]
>
> > > @@ -2174,8 +2181,7 @@ #endif
> > >
> > > 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);
> > > }
> >
> > Same comment applies as for init_bootmem(). I can't be sure it's a good
> > idea.
>
> same comment as for init_bootmem()
>
>
> 		Franck

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