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

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Message-ID: <63453d94-6410-43fd-8676-dbb04e423b10@amd.com>
Date: Sun, 21 Dec 2025 00:02:11 +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 12/20/2025 6:08 AM, Wei Yang wrote:
> On Fri, Dec 19, 2025 at 02:54:40PM +0530, Garg, Shivank wrote:
>>
>>
>> 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().
>>
> 
> Agree.
> 
>>
>>> 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.
> 
> You are right. While I am not sure it exists in reality, but in theory it
> could be.
> 
>>
>> I think your suggestion is ensuring fairness by charging 'progress' count correctly.
>>
> 
> Thanks for your confirmation. Would you mind add a cleanup in next version, or
> you prefer me to send it :-)

Sure, I'll add this fix patch in the next version.

Thanks,
Shivank