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

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Message-ID: <20251221011634.uolx6ajnnpaevnvb@master>
Date: Sun, 21 Dec 2025 01:16:34 +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 Sun, Dec 21, 2025 at 12:02:11AM +0530, Garg, Shivank wrote:
>
>
>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

>Thanks,
>Shivank

-- 
Wei Yang
Help you, Help me