Re: [PATCH] mm/page_alloc: break on the first hit of mem range

[Michal Hocko <mhocko@kernel.org>]

On Tue 27-03-18 11:57:07, Wei Yang wrote:
> find_min_pfn_for_node() iterate on pfn range to find the minimum pfn for a
> node. The memblock_region in memblock_type are already ordered, which means
> the first hit in iteration is the minimum pfn.

I haven't looked at the code yet but the changelog should contain the
motivation why it exists. It seems like this is an optimization. If so,
what is the impact?

> This patch returns the fist hit instead of iterating the whole regions.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> ---
>  mm/page_alloc.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 635d7dd29d7f..a65de1ec4b91 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6365,14 +6365,14 @@ unsigned long __init node_map_pfn_alignment(void)
>  /* Find the lowest pfn for a node */
>  static unsigned long __init find_min_pfn_for_node(int nid)
>  {
> -	unsigned long min_pfn = ULONG_MAX;
> -	unsigned long start_pfn;
> +	unsigned long min_pfn;
>  	int i;
>  
> -	for_each_mem_pfn_range(i, nid, &start_pfn, NULL, NULL)
> -		min_pfn = min(min_pfn, start_pfn);
> +	for_each_mem_pfn_range(i, nid, &min_pfn, NULL, NULL) {
> +		break;
> +	}
>  
> -	if (min_pfn == ULONG_MAX) {
> +	if (i == -1) {
>  		pr_warn("Could not find start_pfn for node %d\n", nid);
>  		return 0;
>  	}
> -- 
> 2.15.1
> 

-- 
Michal Hocko
SUSE Labs