Re: [PATCH v7] page_alloc: allow migration of smaller hugepages during contig_alloc

[David Hildenbrand (Red Hat)]

> -			return false;
> +		/*
> +		 * Only consider ranges containing hugepages if those pages are
> +		 * smaller than the requested contiguous region.  e.g.:
> +		 *     Move 2MB pages to free up a 1GB range.
> +		 *     Don't move 1GB pages to free up a 2MB range.
> +		 *
> +		 * This makes contiguous allocation more reliable if multiple
> +		 * hugepage sizes are used without causing needless movement.
> +		 */
> +		if (PageHuge(page)) {
> +			unsigned int order;
> +
> +			if (!IS_ENABLED(CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION))
> +				return false;
> +
> +			if (skip_hugetlb) {
> +				*skipped_hugetlb = true;
> +				return false;
> +			}
> +
> +			page = compound_head(page);
> +			order = compound_order(page);
> +			if ((order >= MAX_FOLIO_ORDER) ||
> +			    (nr_pages <= (1 << order)))
> +				return false;
> +
> +			/*
> +			 * Reaching this point means we've encounted a huge page
> +			 * smaller than nr_pages, skip all pfn's for that page.
> +			 *
> +			 * We can't get here from a tail-PageHuge, as it implies
> +			 * we started a scan in the middle of a hugepage larger
> +			 * than nr_pages - which the prior check filters for.
> +			 */
> +			i += (1 << order) - 1;

Assuming page != compound_head(page), you'd be skipping too much.

You'd only have to skip as many pages as pages are left in this compound 
page.

I think we have similar code elsewhere: see has_unmovable_pages().

Apart from that LGTM.

--
Cheers

David