Re: [PATCH v2 4/4] mm: khugepaged: set to next mm direct when mm has MMF_DISABLE_THP_COMPLETELY

[Wei Yang]

On Tue, Dec 30, 2025 at 09:03:23PM +0100, David Hildenbrand (Red Hat) wrote:
>On 12/29/25 06:51, Vernon Yang wrote:
>> When an mm with the MMF_DISABLE_THP_COMPLETELY flag is detected during
>> scanning, directly set khugepaged_scan.mm_slot to the next mm_slot,
>> reduce redundant operation.
>> 
>> Signed-off-by: Vernon Yang <yanglincheng@xxxxxxxxxx>
>> ---
>>   mm/khugepaged.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>> 
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 2b3685b195f5..72be87ef384b 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -2439,6 +2439,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>>   		cond_resched();
>>   		if (unlikely(hpage_collapse_test_exit_or_disable(mm))) {
>> +			vma = NULL;
>>   			progress++;
>>   			break;
>>   		}
>
>I don't understand why we need changes at all.
>
>The code is
>
>	mm = slot->mm;
>	/*
>	 * Don't wait for semaphore (to avoid long wait times).  Just move to
>	 * the next mm on the list.
>	 */
>	vma = NULL;
>	if (unlikely(!mmap_read_trylock(mm)))
>		goto breakouterloop_mmap_lock;
>
>	progress++;
>	if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
>		goto breakouterloop;
>
>	...
>
>So we'll go straight to breakouterloop with vma=NULL.
>
>Do you want to optimize for skipping the MM if the flag gets toggled
>while we are scanning that MM?
>
>Is that really something we should be worrying about?
>
>Also, why can't we simply do a
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index 97d1b2824386f..af8481d4b0f4e 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -2516,7 +2516,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>         * Release the current mm_slot if this mm is about to die, or
>         * if we scanned all vmas of this mm.
>         */
>-       if (hpage_collapse_test_exit(mm) || !vma) {
>+       if (hpage_collapse_test_exit_or_disable(mm) || !vma) {
>                /*
>                 * Make sure that if mm_users is reaching zero while
>                 * khugepaged runs here, khugepaged_exit will find
>

This one looks better.

But the sad thing is we can't remove this mm from scan list, since user may
toggle this flag later.

>
>-- 
>Cheers
>
>David

-- 
Wei Yang
Help you, Help me