Re: [PATCH] ARM: mm: Regarding section when dealing with meminfo

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Thu, 2011-01-20 at 18:45 +0900, KyongHo Cho wrote:
> Sparsemem allows that a bank of memory spans over several adjacent
> sections if the start address and the end address of the bank
> belong to different sections.
> When gathering statictics of physical memory in mem_init() and
> show_mem(), this possiblity was not considered.
> 
> This patch guarantees that simple increasing the pointer to page
> descriptors does not exceed the boundary of a section
...
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 57c4c5c..6ccecbe 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -93,24 +93,38 @@ void show_mem(void)
> 
>  		pfn1 = bank_pfn_start(bank);
>  		pfn2 = bank_pfn_end(bank);
> -
> +#ifndef CONFIG_SPARSEMEM
>  		page = pfn_to_page(pfn1);
>  		end  = pfn_to_page(pfn2 - 1) + 1;
> -
> +#else
> +		pfn2--;
>  		do {
> -			total++;
> -			if (PageReserved(page))
> -				reserved++;
> -			else if (PageSwapCache(page))
> -				cached++;
> -			else if (PageSlab(page))
> -				slab++;
> -			else if (!page_count(page))
> -				free++;
> -			else
> -				shared += page_count(page) - 1;
> -			page++;
> -		} while (page < end);
> +			page = pfn_to_page(pfn1);
> +			if (pfn_to_section_nr(pfn1) < pfn_to_section_nr(pfn2)) {
> +				pfn1 += PAGES_PER_SECTION;
> +				pfn1 &= PAGE_SECTION_MASK;
> +			} else {
> +				pfn1 = pfn2;
> +			}
> +			end = pfn_to_page(pfn1) + 1;
> +#endif

This problem actually exists without sparsemem, too.  Discontigmem (at
least) does it as well.

The x86 version of show_mem() actually manages to do this without any
#ifdefs, and works for a ton of configuration options.  It uses
pfn_valid() to tell whether it can touch a given pfn.

Long-term, it might be a good idea to convert arm's show_mem() over to
use pgdat's like everything else.  But, for now, you should just be able
to do something roughly like this:

-               page = pfn_to_page(pfn1);
-               end  = pfn_to_page(pfn2 - 1) + 1;
-
-               do {
+		for (pfn = pfn1; pfn < pfn2; pfn++) {
+			if (!pfn_valid(pfn))
+				continue;
+			page = pfn_to_page(pfn);
+
                        total++;
                        if (PageReserved(page))
                                reserved++;
                        else if (PageSwapCache(page))
                                cached++;
                        else if (PageSlab(page))
                                slab++;
                        else if (!page_count(page))
                                free++;
                        else
                                shared += page_count(page) - 1;
                        page++;
-               } while (page < end);
+		}

That should work for sparsemem, or any other crazy memory models that we
come up with.  pfn_to_page() is pretty quick, especially when doing it
in a tight loop like that.

-- Dave