Re: [RFC PATCH v2] page_alloc: allow migration of smaller hugepages during contig_alloc.

[SeongJae Park <sj@kernel.org>]

On Mon, 20 Oct 2025 17:08:16 -0400 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 requested allocation.
> 
> Compaction `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>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>  mm/page_alloc.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 600d9e981c23..da2e65bf63e3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7048,8 +7048,19 @@ 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 */
> +			page = compound_head(page);
> +			order = compound_order(page);
> +			if ((order >= MAX_PAGE_ORDER) ||
> +			    (nr_pages < (1 << order)))
> +				return false;

Shouldn't the comparison of 'nr_pages' against '1 << order' use '<=' instead of
'<', to match the commit description?

> +		}
>  	}
>  	return true;
>  }
> -- 
> 2.51.0


Thanks,
SJ