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

[David Hildenbrand <david@redhat.com>]

On 20.10.25 23:08, Gregory Price 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

Please, let's not talk about "compaction" here, it's just confusing to 
talk about compaction for something that is not compaction but uses some 
primitives (because not properly separated yet)

Just say "isolate_migrate_pages_block() already expects ..."

> 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;

This is roughly what we do in pageblock_skip_persistent(), just with a 
hardcoded pageblock size.

I'm not sure about the MAX_PAGE_ORDER check, though. If an arch supports 
two hugetlb sizes that exceed MAX_PAGE_ORDER, it would not work as expected.

Doesn't arm64 support that with cont-PMD vs. PUD hugetlb folios? 
MAX_FOLIO_ORDER would be better.

-- 
Cheers

David / dhildenb