Re: 2.6.17-mm1

[Mel Gorman]

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

> Mel Gorman wrote:

> > > > 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 
> > > useit 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.
>
> in that case ARCH_PFN_OFFSET is 0 which is the old behaviour, nothing
> change...

The change is that ARCH_PFN_OFFSET has a slightly different meaning when 
you alter those two initialisation functions. Currently it is used to 
offset memmap from NODE_DATA(0)->node_start_pfn. By changing 
free_area_init() and init_bootmem(), it changes to altering the value of 
NODE_DATA(0)->node_start_pfn but only when memory is initialised in a 
particular way. I believe it makes ARCH_PFN_OFFSET even more obscure than 
it currently is.

I think we should just fix the problem at hand now for which two simple 
patches have been posted and consider making further changes to 
free_area_init() and init_bootmem() as a separate issue.

> > 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().
>
> but in that case do they use ARCH_PFN_OFFSET != 0 ?

IA64 do not and I didn't check all arches.

> if so that would be
> very surprising. That would meand "I have a hole a the start of my mem,
> I don't know at compile time where it starts, but I state that my physical
> mem start at ARCH_PFN_OFFSET anyways"
>
> > > 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.
>
> Again in these cases, I doubt that they will setup ARCH_PFN_OFFSET...

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