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

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Message-ID: <35c9a208-a7e6-41ab-aa3e-dbfdffaf5f44@linux.dev>
Date: Wed, 1 Oct 2025 18:48:07 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Dev Jain <dev.jain@....com>
Cc: david@...hat.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 baohua@...nel.org, baolin.wang@...ux.alibaba.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, richard.weiyang@...il.com,
 akpm@...ux-foundation.org
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries



On 2025/10/1 18:20, Dev Jain wrote:
> 
> On 01/10/25 8:52 am, 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@...hat.com
>> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680- 
>> dcd55219c8bd@...ifer.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@...ux.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)) {
>> +        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> +            ++none_or_zero;
>> +            if (!userfaultfd_armed(vma) &&
>> +                (!cc->is_khugepaged ||
>> +                 none_or_zero <= khugepaged_max_ptes_none)) {
>> +                continue;
>> +            } else {
>> +                result = SCAN_EXCEED_NONE_PTE;
>> +                count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>> +                goto out_unmap;
>> +            }
>> +        } else if (!pte_present(pteval)) {
> 
> If you are trying to merge this with the _isolate() conditions, we can do
> a micro-optimization here - is_swap_pte, (pte_none && is_zero_pfn), and 
> pte_uffd_wp
> are disjoint conditions, so we can use if-else-if-else-if to write them.

Ah, indeed, thanks!

I think it would fit better into the follow-up patch that unifies the
scanning logic, and I'll make sure to include it there ;p

> 
>> +            if (non_swap_entry(pte_to_swp_entry(pteval))) {
>> +                result = SCAN_PTE_NON_PRESENT;
>> +                goto out_unmap;
>> +            }
>> +
>>               ++unmapped;
>>               if (!cc->is_khugepaged ||
>>                   unmapped <= khugepaged_max_ptes_swap) {
>> @@ -1293,7 +1309,7 @@ static int hpage_collapse_scan_pmd(struct 
>> mm_struct *mm,
>>                    * enabled swap entries.  Please see
>>                    * comment below for pte_uffd_wp().
>>                    */
>> -                if (pte_swp_uffd_wp_any(pteval)) {
>> +                if (pte_swp_uffd_wp(pteval)) {
>>                       result = SCAN_PTE_UFFD_WP;
> 
> Could have mentioned in the changelog "while at it, convert 
> pte_swp_uffd_wp_any to
> pte_swp_uffd_wp since we are in the swap pte branch".

Right, that would have been clearer.

I'll add that if a next version is needed :)

> 
>>                       goto out_unmap;
>>                   }
>> @@ -1304,18 +1320,6 @@ static int hpage_collapse_scan_pmd(struct 
>> mm_struct *mm,
>>                   goto out_unmap;
>>               }
>>           }
>> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> -            ++none_or_zero;
>> -            if (!userfaultfd_armed(vma) &&
>> -                (!cc->is_khugepaged ||
>> -                 none_or_zero <= khugepaged_max_ptes_none)) {
>> -                continue;
>> -            } else {
>> -                result = SCAN_EXCEED_NONE_PTE;
>> -                count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>> -                goto out_unmap;
>> -            }
>> -        }
>>           if (pte_uffd_wp(pteval)) {
>>               /*
>>                * Don't collapse the page if any of the small
> 
> Otherwise LGTM
> 
> Reviewed-by: Dev Jain <dev.jain@....com>

Cheers!