linux-kernel - Re: [RFC PATCH v2] page_alloc: allow migration of smaller hugepages during contig

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Message-ID: <20251021091443.5ggfmvl5jwyknjnv@master>
Date: Tue, 21 Oct 2025 09:14:43 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: Wei Yang <richard.weiyang@...il.com>, 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, David Hildenbrand <david@...hat.com>
Subject: Re: [RFC PATCH v2] page_alloc: allow migration of smaller hugepages
 during contig_alloc

On Mon, Oct 20, 2025 at 09:28:18PM -0400, Zi Yan wrote:
>On 20 Oct 2025, at 21:25, Wei Yang wrote:
>
>> On Mon, Oct 20, 2025 at 05:08:16PM -0400, 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
>>
>> isolate_migratepages_block() ?
>>
>>> 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@...hat.com>
>>> Signed-off-by: Gregory Price <gourry@...rry.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);
>>
>> How about using folio_large_order(page_folio(page)) ?
>
>See the discussion at https://lore.kernel.org/linux-mm/609E7E01-33A6-4931-AC89-1F4B2944FB64@nvidia.com/ about reading folio order without locking.
>

I took another look at this.

The above code is expanded to:

  page = _compound_head(page);
  order = compound_order(page);
      folio = page;
      test_bit(PG_head, &folio->flags.f);
      return folio_large_order(folio);

If we use

  order = folio_order(page_folio(page))

It is expanded to:

  folio = _compound_head(page);
  folio_test_large(folio)
      folio_test_head(folio)
          test_bit(PG_head, &folio->flags.f)
  return folio_large_order(folio);

These two seems expanded to the same code.

Maybe we can use folio_order() here?

>>
>>> +			if ((order >= MAX_PAGE_ORDER) ||
>>> +			    (nr_pages < (1 << order)))
>>> +				return false;
>>> +		}
>>> 	}
>>> 	return true;
>>> }
>>> -- 
>>> 2.51.0
>>>
>>
>> Generally LGTM
>>
>> Reviewed-by: Wei Yang <richard.weiyang@...il.com>
>>
>> -- 
>> Wei Yang
>> Help you, Help me
>
>
>--
>Best Regards,
>Yan, Zi

-- 
Wei Yang
Help you, Help me