Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of pages occupied by memmap

[Jaegeuk Hanse <jaegeuk.hanse@gmail.com>]

On 11/19/2012 12:07 AM, Jiang Liu wrote:
> If SPARSEMEM is enabled, it won't build page structures for
> non-existing pages (holes) within a zone, so provide a more accurate
> estimation of pages occupied by memmap if there are big holes within
> the zone.
>
> And pages for highmem zones' memmap will be allocated from lowmem,
> so charge nr_kernel_pages for that.
>
> Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
> ---
>   mm/page_alloc.c |   22 ++++++++++++++++++++--
>   1 file changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5b327d7..eb25679 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4435,6 +4435,22 @@ void __init set_pageblock_order(void)
>   
>   #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
>   
> +static unsigned long calc_memmap_size(unsigned long spanned_pages,
> +				      unsigned long present_pages)
> +{
> +	unsigned long pages = spanned_pages;
> +
> +	/*
> +	 * Provide a more accurate estimation if there are big holes within
> +	 * the zone and SPARSEMEM is in use.
> +	 */
> +	if (spanned_pages > present_pages + (present_pages >> 4) &&
> +	    IS_ENABLED(CONFIG_SPARSEMEM))
> +		pages = present_pages;
> +
> +	return PAGE_ALIGN(pages * sizeof(struct page)) >> PAGE_SHIFT;
> +}
> +
>   /*
>    * Set up the zone data structures:
>    *   - mark all pages reserved
> @@ -4469,8 +4485,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
>   		 * is used by this zone for memmap. This affects the watermark
>   		 * and per-cpu initialisations
>   		 */
> -		memmap_pages =
> -			PAGE_ALIGN(size * sizeof(struct page)) >> PAGE_SHIFT;
> +		memmap_pages = calc_memmap_size(size, realsize);
>   		if (freesize >= memmap_pages) {
>   			freesize -= memmap_pages;
>   			if (memmap_pages)
> @@ -4491,6 +4506,9 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
>   
>   		if (!is_highmem_idx(j))
>   			nr_kernel_pages += freesize;
> +		/* Charge for highmem memmap if there are enough kernel pages */
> +		else if (nr_kernel_pages > memmap_pages * 2)
> +			nr_kernel_pages -= memmap_pages;

Since this is in else branch, if nr_kernel_pages is equal to 0 at 
initially time?

>   		nr_all_pages += freesize;
>   
>   		zone->spanned_pages = size;

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