Re: [PATCH v3] mm/page_alloc: simplify lowmem_reserve max calculation

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 15 Aug 2025 10:45:09 +0800 Ye Liu <ye.liu@linux.dev> wrote:

> From: Ye Liu <liuye@kylinos.cn>
> 
> Use max() to find the maximum lowmem_reserve value and min_t() to
> cap it to managed_pages in calculate_totalreserve_pages(), instead
> of open-coding the comparisons. No functional change.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6235,16 +6235,13 @@ static void calculate_totalreserve_pages(void)
>  			unsigned long managed_pages = zone_managed_pages(zone);
>  
>  			/* Find valid and maximum lowmem_reserve in the zone */
> -			for (j = i; j < MAX_NR_ZONES; j++) {
> -				if (zone->lowmem_reserve[j] > max)
> -					max = zone->lowmem_reserve[j];
> -			}
> +			for (j = i; j < MAX_NR_ZONES; j++)
> +				max = max(max, zone->lowmem_reserve[j]);
>  
>  			/* we treat the high watermark as reserved pages. */
>  			max += high_wmark_pages(zone);
>  
> -			if (max > managed_pages)
> -				max = managed_pages;
> +			max = min_t(unsigned long, max, managed_pages);
>  

Use of max_t/min_t is usually a sign that we messed up the type choices
somewhere.

In this case, I'd say that zone.lowmem_reserve[] should have been
ulong.  Oh well.