linux-kernel - Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <ab5f77a2-c143-485d-9006-ec45493ea56c@redhat.com>
Date: Wed, 1 Oct 2025 11:15:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Wei Yang <richard.weiyang@...il.com>, Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, baohua@...nel.org, baolin.wang@...ux.alibaba.com,
 dev.jain@....com, hughd@...gle.com, ioworker0@...il.com,
 kirill@...temov.name, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 mpenttil@...hat.com, npache@...hat.com, ryan.roberts@....com, ziy@...dia.com
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries

On 01.10.25 10:54, Wei Yang wrote:
> On Wed, Oct 01, 2025 at 11:22:51AM +0800, Lance Yang wrote:
>> From: Lance Yang <lance.yang@...ux.dev>
>>
>> Currently, special non-swap entries (like migration, hwpoison, or PTE
>> markers) are not caught early in hpage_collapse_scan_pmd(), leading to
>> failures deep in the swap-in logic.
>>
>> hpage_collapse_scan_pmd()
>> `- collapse_huge_page()
>>      `- __collapse_huge_page_swapin() -> fails!
>>
>> As David suggested[1], this patch skips any such non-swap entries
>> early. If any one is found, the scan is aborted immediately with the
>> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
>> work.
>>
>> [1] https://lore.kernel.org/linux-mm/7840f68e-7580-42cb-a7c8-1ba64fd6df69@redhat.com
>> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
>>
>> Suggested-by: David Hildenbrand <david@...hat.com>
>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>> ---
>> v1 -> v2:
>> - Skip all non-present entries except swap entries (per David) thanks!
>> - https://lore.kernel.org/linux-mm/20250924100207.28332-1-lance.yang@linux.dev/
>>
>> mm/khugepaged.c | 32 ++++++++++++++++++--------------
>> 1 file changed, 18 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 7ab2d1a42df3..d0957648db19 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1284,7 +1284,23 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>> 	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
>> 	     _pte++, addr += PAGE_SIZE) {
>> 		pte_t pteval = ptep_get(_pte);
>> -		if (is_swap_pte(pteval)) {
> 
> It looks is_swap_pte() is mis-leading?

Yeah, it nowadays includes non-swap entries as well.

Maybe we should consider renaming it at some point to something better.

-- 
Cheers

David / dhildenb