Re: [PATCH v3] page_alloc: allow migration of smaller hugepages during contig_alloc

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

On Fri, 21 Nov 2025 14:15:40 -0500 Gregory Price <gourry@gourry.net> wrote:

> We presently skip regions with hugepages entirely when trying to do
> contiguous page allocation.  Instead, if hugepage migration is enabled,
> consider regions with hugepages smaller than the target contiguous
> allocation request as valid targets for allocation.

Why?  What benefit does this have to our users?

Some runtime testing results might be helpful?

> isolate_migrate_pages_block() already expects requests with hugepages
> to originate from alloc_contig, and hugetlb code also does a migratable
> check when isolating in folio_isolate_hugetlb().
> 
> Suggested-by: David Hildenbrand <david@redhat.com>

A Link: here might be illuminating.

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6849,8 +6849,19 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>  		if (PageReserved(page))
>  			return false;
>  
> -		if (PageHuge(page))
> -			return false;
> +		if (PageHuge(page)) {
> +			unsigned int order;
> +
> +			if (!IS_ENABLED(CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION))
> +				return false;
> +
> +			/* Don't consider moving same size/larger pages */

Comment says "what" (which was fairly obvious).  Please reveal "why".

> +			page = compound_head(page);
> +			order = compound_order(page);
> +			if ((order >= MAX_FOLIO_ORDER) ||
> +			    (nr_pages <= (1 << order)))
> +				return false;
> +		}
>  	}
>  	return true;
>  }