linux-kernel - Re: [PATCH 1/3] mm/khugepaged: remove unnecessary goto 'skip' label

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Message-ID: <c7c1d62c-7d27-4b73-8173-0493adc66cfc@amd.com>
Date: Fri, 19 Dec 2025 14:54:40 +0530
From: "Garg, Shivank" <shivankg@....com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Nico Pache
 <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
 Lance Yang <lance.yang@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/khugepaged: remove unnecessary goto 'skip' label

On 12/17/2025 8:18 AM, Wei Yang wrote:
> On Tue, Dec 16, 2025 at 11:11:38AM +0000, Shivank Garg wrote:
>> Replace 'goto skip' with actual logic for better code readability.
>>
>> No functional change.
>>
>> Signed-off-by: Shivank Garg <shivankg@....com>
>> ---
>> mm/khugepaged.c | 7 ++++---
>> 1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 6c8c35d3e0c9..107146f012b1 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -2442,14 +2442,15 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>> 			break;
>> 		}
>> 		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
>> -skip:
>> 			progress++;
>> 			continue;
>> 		}
>> 		hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
>> 		hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
>> -		if (khugepaged_scan.address > hend)
>> -			goto skip;
>> +		if (khugepaged_scan.address > hend) {
>> +			progress++;
>> +			continue;
>> +		}
> 
> Hi, Shivank
> 
> The change here looks good, while I come up with an question.
> 
> The @progress here seems record two things:
> 
>   * number of pages scaned
>   * number of vma skipped
> 
Three things: number of mm. It's incremented 1 for whole khugepaged_scan_mm_slot().

> While in very rare case, we may miss to count the second case.
> 
> For example, we have following vmas in a process:
> 
>      vma1             vma2
>     +----------------+------------+
>     |2M              |1M          |
>     +----------------+------------+
> 
> Let's assume vma1 is exactly HPAGE_PMD_SIZE and also HPAGE_PMD_SIZE aligned.
> But vma2 is only half of HPAGE_PMD_SIZE.
> 
> When scan finish vma1 and start on vma2, we would have hstart = hend =
> address. So we continue here but would not do real scan, since address == hend.
> 
> I am thinking whether this could handle it:
> 
> 		if (khugepaged_scan.address > hend || hend <= hstart) {
> 			progress++;
> 			continue;
> 		}
> 
> Do you thinks I am correct on this?

I think you're correct.
IIUC, @progress acts as rate limiter here.

It is increasing +1 for whole, and then increases by +1 per VMA (if skipped),
or by +HPAGE_PMD_NR (if actually scanned).

So, progress ensuring the hugepaged_do_scan run only until (progress >= pages)
at which point it yields and sleeps (wait_event_freezable).

Without your suggested fix, if a process contains a large number of small VMAs (where
round_up hstart >= round_down(hend), it will unfairly consume more CPU cycles before
yielding compared to a process with fewer or aligned VMAs.

I think your suggestion is ensuring fairness by charging 'progress' count correctly.

Thanks,
Shivank