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

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Message-ID: <20251220003851.dzsafcxtlbgzwsm6@master>
Date: Sat, 20 Dec 2025 00:38:51 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: "Garg, Shivank" <shivankg@....com>
Cc: Wei Yang <richard.weiyang@...il.com>,
	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 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 :-)

>Thanks,
>Shivank

-- 
Wei Yang
Help you, Help me