Re: [PATCH v4 4/4] mm/sparse: Optimize memmap allocation during sparse_init()

[Dave Hansen]

> @@ -297,8 +298,8 @@ void __init sparse_mem_maps_populate_node(struct page **map_map,
>  		if (!present_section_nr(pnum))
>  			continue;
>  
> -		map_map[pnum] = sparse_mem_map_populate(pnum, nodeid, NULL);
> -		if (map_map[pnum])
> +		map_map[nr_consumed_maps] = sparse_mem_map_populate(pnum, nodeid, NULL);
> +		if (map_map[nr_consumed_maps++])
>  			continue;
...

This looks wonky.

This seems to say that even if we fail to sparse_mem_map_populate() (it
returns NULL), we still consume a map.  Is that right?

>  	/* fallback */
> +	nr_consumed_maps = 0;
>  	for (pnum = pnum_begin; pnum < pnum_end; pnum++) {
>  		struct mem_section *ms;
>  
>  		if (!present_section_nr(pnum))
>  			continue;
> -		map_map[pnum] = sparse_mem_map_populate(pnum, nodeid, NULL);
> -		if (map_map[pnum])
> +		map_map[nr_consumed_maps] = sparse_mem_map_populate(pnum, nodeid, NULL);
> +		if (map_map[nr_consumed_maps++])
>  			continue;

Same questionable pattern as above...

>  #ifdef CONFIG_SPARSEMEM_ALLOC_MEM_MAP_TOGETHER
> -	size2 = sizeof(struct page *) * NR_MEM_SECTIONS;
> +	size2 = sizeof(struct page *) * nr_present_sections;
>  	map_map = memblock_virt_alloc(size2, 0);
>  	if (!map_map)
>  		panic("can not allocate map_map\n");
> @@ -586,27 +594,44 @@ void __init sparse_init(void)
>  				sizeof(map_map[0]));
>  #endif
>  
> +	/* The numner of present sections stored in nr_present_sections

"number"?

Also, this is not correct comment CodingStyle.

> +	 * are kept the same since mem sections are marked as present in
> +	 * memory_present().

Are you just trying to say that we are not making sections present here?

>                         In this for loop, we need check which sections
> +	 * failed to allocate memmap or usemap, then clear its
> +	 * ->section_mem_map accordingly. During this process, we need
> +	 * increase 'alloc_usemap_and_memmap' whether its allocation of
> +	 * memmap or usemap failed or not, so that after we handle the i-th
> +	 * memory section, can get memmap and usemap of (i+1)-th section
> +	 * correctly. */

I'm really scratching my head over this comment.  For instance "increase
'alloc_usemap_and_memmap'" doesn't make any sense to me.  How do you
increase a function?

I wonder if you could give that comment another shot.