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

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Message-ID: <487730c6-423a-4a03-a668-9b9ff92a5cfb@redhat.com>
Date: Mon, 20 Oct 2025 19:24:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>, linux-mm@...ck.org
Cc: 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, ziy@...dia.com
Subject: Re: [RFC PATCH] page_alloc: allow migration of smaller hugepages
 during contig_alloc.

On 20.10.25 19:06, 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 target contiguous
> allocation request as valid targets for 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()`.
> 
> We add the migration check here to avoid calling compaction on a
> region if we know migration is not possible at all.
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Gregory Price <gourry@...rry.net>
> ---
>   mm/page_alloc.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 600d9e981c23..e0760eafe032 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7048,8 +7048,14 @@ 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)) {
> +			struct folio *folio = page_folio(page);
> +
> +			/* Don't consider moving same size/larger pages */
> +			if (!folio_test_hugetlb_migratable(folio) ||
> +			    (1 << folio_order(folio) >= nr_pages))

We have folio_nr_pages().

Do we really need the folio_hugetlb_migratable() check?

> +				return false;

This code is completely racy. folio_nr_pages() should be fine AFAIKT (no 
VM_WARN_ON() etc), not sure about folio_test_hugetlb_migratable().

If it becomes a problem we could do a snapshot_page() to take a snapshot 
we can query.

-- 
Cheers

David / dhildenb