Re: [PATCH 2/4] mm/sparse: introduce alloc_usemap_and_memmap

[Dave Hansen <dave.hansen@intel.com>]

On 08/14/2013 05:31 PM, Wanpeng Li wrote:
> After commit 9bdac91424075("sparsemem: Put mem map for one node together."),
> vmemmap for one node will be allocated together, its logic is similiar as 
> memory allocation for pageblock flags. This patch introduce alloc_usemap_and_memmap
> to extract the same logic of memory alloction for pageblock flags and vmemmap.

Shame on whoever copy-n-pasted that in the first place.

> -
> -	for (pnum = 0; pnum < NR_MEM_SECTIONS; pnum++) {
> -		struct mem_section *ms;
> -
> -		if (!present_section_nr(pnum))
> -			continue;
> -		ms = __nr_to_section(pnum);
> -		nodeid_begin = sparse_early_nid(ms);
> -		pnum_begin = pnum;
> -		break;
> -	}
> -	usemap_count = 1;
> -	for (pnum = pnum_begin + 1; pnum < NR_MEM_SECTIONS; pnum++) {
> -		struct mem_section *ms;
> -		int nodeid;
> -
> -		if (!present_section_nr(pnum))
> -			continue;
> -		ms = __nr_to_section(pnum);
> -		nodeid = sparse_early_nid(ms);
> -		if (nodeid == nodeid_begin) {
> -			usemap_count++;
> -			continue;
> -		}
> -		/* ok, we need to take cake of from pnum_begin to pnum - 1*/
> -		sparse_early_usemaps_alloc_node(usemap_map, pnum_begin, pnum,
> -						 usemap_count, nodeid_begin);
> -		/* new start, update count etc*/
> -		nodeid_begin = nodeid;
> -		pnum_begin = pnum;
> -		usemap_count = 1;
> -	}
> -	/* ok, last chunk */
> -	sparse_early_usemaps_alloc_node(usemap_map, pnum_begin, NR_MEM_SECTIONS,
> -					 usemap_count, nodeid_begin);
> +	alloc_usemap_and_memmap(usemap_map, true);
...
> +	alloc_usemap_and_memmap((unsigned long **)map_map, false);
>  #endif

Why does alloc_usemap_and_memmap() take an 'unsigned long **'?
'unsigned long' is for the usemap and 'struct page' is for the memmap.
It's misleading to have it take an 'unsigned long **' and then just cast
it over to a 'struct page **' internally.

Also, what's the point of having a function that returns something in a
double-pointer, but that doesn't use its return value?

alloc_usemap_and_memmap() also needs a comment about what it's doing
with that pointer and its other argument.


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