linux-kernel - Re: [PATCH v5] page_alloc: allow migration of smaller hugepages during contig

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Message-ID: <aURXd7d1on5vxnnT@cmpxchg.org>
Date: Thu, 18 Dec 2025 14:35:19 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Gregory Price <gourry@...rry.net>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	akpm@...ux-foundation.org, vbabka@...e.cz, surenb@...gle.com,
	mhocko@...e.com, jackmanb@...gle.com, ziy@...dia.com,
	richard.weiyang@...il.com, osalvador@...e.de, rientjes@...gle.com,
	david@...hat.com, joshua.hahnjy@...il.com, fvdl@...gle.com
Subject: Re: [PATCH v5] page_alloc: allow migration of smaller hugepages
 during contig_alloc

On Thu, Dec 18, 2025 at 02:08:31PM -0500, Gregory Price wrote:
> @@ -7099,8 +7099,30 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>  		if (PageReserved(page))
>  			return false;
>  
> -		if (PageHuge(page))
> -			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 (!search_hugetlb)
> +				return false;
> +
> +			page = compound_head(page);
> +			order = compound_order(page);
> +			if ((order >= MAX_FOLIO_ORDER) ||
> +			    (nr_pages <= (1 << order)))
> +				return false;

If you keep searching past it, you can step over the whole page to
speed things up a bit:

			i += (1 << order) - 1;