linux-kernel - Re: [PATCH mm-new 1/2] mm/khugepaged: optimize PTE scanning with if-else-if-else-if chain

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Message-ID: <2ecb9ba1-8801-42b8-9575-5add856d0441@linux.dev>
Date: Sat, 4 Oct 2025 11:08:40 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Dev Jain <dev.jain@....com>
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 lorenzo.stoakes@...cle.com, david@...hat.com, ioworker0@...il.com,
 richard.weiyang@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 akpm@...ux-foundation.org
Subject: Re: [PATCH mm-new 1/2] mm/khugepaged: optimize PTE scanning with
 if-else-if-else-if chain



On 2025/10/4 00:33, Dev Jain wrote:
> 
> On 02/10/25 1:02 pm, Lance Yang wrote:
>> From: Lance Yang <lance.yang@...ux.dev>
>>
>> As pointed out by Dev, the PTE checks for disjoint conditions in the
>> scanning loops can be optimized. is_swap_pte, (pte_none && is_zero_pfn),
>> and pte_uffd_wp are mutually exclusive.
>>
>> This patch refactors the loops in both __collapse_huge_page_isolate() and
>> hpage_collapse_scan_pmd() to use a continuous if-else-if-else-if chain
>> instead of separate if blocks.
>>
>> Also, this is a preparatory step to make it easier to merge the
>> almost-duplicated scanning logic in these two functions, as suggested
>> by David.
>>
>> Suggested-by: Dev Jain <dev.jain@....com>
>> Suggested-by: David Hildenbrand <david@...hat.com>
>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>> ---
>>   mm/khugepaged.c | 12 ++++--------
>>   1 file changed, 4 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index f4f57ba69d72..808523f92c7b 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -548,8 +548,7 @@ static int __collapse_huge_page_isolate(struct 
>> vm_area_struct *vma,
>>       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
>>            _pte++, addr += PAGE_SIZE) {
>>           pte_t pteval = ptep_get(_pte);
>> -        if (pte_none(pteval) || (pte_present(pteval) &&
>> -                is_zero_pfn(pte_pfn(pteval)))) {
>> +        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> 
> Should have mentioned in the description that pte_present() is not required
> here, so removing it.

Yep, got it.

> 
> Reviewed-by: Dev Jain <dev.jain@....com>

Cheers,
Lance

> 
>>               ++none_or_zero;
>>               if (!userfaultfd_armed(vma) &&
>>                   (!cc->is_khugepaged ||
>> @@ -560,12 +559,10 @@ static int __collapse_huge_page_isolate(struct 
>> vm_area_struct *vma,
>>                   count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>>                   goto out;
>>               }
>> -        }
>> -        if (!pte_present(pteval)) {
>> +        } else if (!pte_present(pteval)) {
>>               result = SCAN_PTE_NON_PRESENT;
>>               goto out;
>> -        }
>> -        if (pte_uffd_wp(pteval)) {
>> +        } else if (pte_uffd_wp(pteval)) {
>>               result = SCAN_PTE_UFFD_WP;
>>               goto out;
>>           }
>> @@ -1316,8 +1313,7 @@ static int hpage_collapse_scan_pmd(struct 
>> mm_struct *mm,
>>                   count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
>>                   goto out_unmap;
>>               }
>> -        }
>> -        if (pte_uffd_wp(pteval)) {
>> +        } else if (pte_uffd_wp(pteval)) {
>>               /*
>>                * Don't collapse the page if any of the small
>>                * PTEs are armed with uffd write protection.