linux-kernel - Re: [PATCH v7] page_alloc: allow migration of smaller hugepages during contig

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Message-ID: <9e8b01f9-d1b0-44a7-86c9-97622b2baec1@kernel.org>
Date: Tue, 6 Jan 2026 20:20:51 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Zi Yan <ziy@...dia.com>
Cc: Gregory Price <gourry@...rry.net>, 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, hannes@...xchg.org,
 richard.weiyang@...il.com
Subject: Re: [PATCH v7] page_alloc: allow migration of smaller hugepages
 during contig_alloc

On 1/6/26 19:56, Zi Yan wrote:
> On 6 Jan 2026, at 13:46, David Hildenbrand (Red Hat) wrote:
> 
>>> -			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.
> 
> It might not happen based on my reasoning[1], but feel free to punch poles. :)

:) hehe, I should just have read that comment more carefully.

Anyhow, I wonder whether removing 6 lines of comments by just doing it 
properly might be better?

head = compound_head(page);
order = compound_order(head);

...

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


In any case

Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>

-- 
Cheers

David