Re: Turning on Sparsemem

[C Michael Sundius <Michael.sundius@sciatl.com>]

Dave Hansen wrote:
> On Tue, 2008-08-05 at 14:39 -0700, C Michael Sundius wrote:
>   
>> Hi Andy and Dave,
>>
>> I turned on sparsemem as you described before. I am crashing in
>> the mem_init() function when I try a call to pfn_to_page().
>>
>> I've noticed that that macro uses the sparsemem macro 
>> __pfn_to_section(pfn) and
>> that intern calls __nr_to_section(nr). That finally looks at the 
>> mem_section[] variable.
>> well.. this returns NULL since it seems as though my mem_section[] array 
>> looks
>> to be not initialized correctly.
>>
>> QUESTION: where does this array get initialized. I've looked through the 
>> code and
>> can't seem to see how that is initialized.
>>     
>
> My first guess is that you're missing a call to sparse_init() in your
> architecture-specific code.  On x86_32, we do that in paging_init(),
> just before zone_sizes_init() (arch/x86/mm/init_32.c).
>
> Before you call this, you'll also have to call memory_present(...) on
> the memory that you do have.  But, you should probably already have done
> that.
>
>   
yes I call sparse_init()..... My code might be not calling 
memory_present for some of the memory
I'm looking at that right now.
>> recall I'm using mips32 processor, but I've looked in all the processors.
>> it seems as though sparse_init() and memory present() both use 
>> __nr_to_section()
>> and thus would require mem_section[] to be set up already.
>>     
>
> __nr_to_section() is special.  It takes a section number and just gives
> you back the 'struct mem_section'.  It doesn't actually look into that
> mem_section and make sure it is valid, it just locates the data
> structure.
>
> -- Dave
>
>   
still that code is strange to me:

-------------code
static inline struct mem_section *__nr_to_section(unsigned long nr)
{
        if (!mem_section[SECTION_NR_TO_ROOT(nr)])
                return NULL;
        return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
}

--------------

on the first line of the function above, what does it mean "if not 
<struct>"?  seems that returns true if
the contents of that struct is "0"... but either way, doesn't that have 
to be initialized to something before
it is called from memory_present()?

mike



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