linux-kernel - Re: [PATCH v3 6/6] mm: khugepaged: set to next mm direct when mm has MMF_DISABLE_THP

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Message-ID: <b85dad10-3a4a-4986-80ba-f2ac055bcea1@linux.dev>
Date: Mon, 5 Jan 2026 10:09:43 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Wei Yang <richard.weiyang@...il.com>
Cc: Vernon Yang <vernon2gm@...il.com>, lorenzo.stoakes@...cle.com,
 ziy@...dia.com, dev.jain@....com, baohua@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, david@...nel.org,
 Vernon Yang <yanglincheng@...inos.cn>, akpm@...ux-foundation.org
Subject: Re: [PATCH v3 6/6] mm: khugepaged: set to next mm direct when mm has
 MMF_DISABLE_THP_COMPLETELY



On 2026/1/5 08:31, Wei Yang wrote:
> On Sun, Jan 04, 2026 at 08:20:29PM +0800, Lance Yang wrote:
>>
>>
>> On 2026/1/4 13:41, 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@...inos.cn>
>>> ---
>>>    mm/khugepaged.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 1ca034a5f653..d4ed0f397335 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -2541,7 +2541,7 @@ 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
>>
>>
>> Let's convert hpage_collapse_test_exit() in collect_mm_slot() as well,
>> otherwise the mm_slot would not be freed and will be scanned again ...
>>
>> static void collect_mm_slot(struct mm_slot *slot)
>> {
>> 	struct mm_struct *mm = slot->mm;
>>
>> 	lockdep_assert_held(&khugepaged_mm_lock);
>>
>> 	if (hpage_collapse_test_exit(mm)) { <-
>>
> 
> What if user toggle the MMF_DISABLE_THP_COMPLETELY flag again?

Maybe it's fine :)

If user sets MMF_DISABLE_THP_COMPLETELY, they probaly would not
clear it soon. Keeping the slot wastes memory.

If they do clear it later, page faults will trigger
do_huge_pmd_anonymous_page() -> khugepaged_enter_vma(), which
re-adds the mm.

Anyway, no strong opinion on that.

> 
>> 		hash_del(&slot->hash);
>> 		list_del(&slot->mm_node);
>>
>> 		mm_slot_free(mm_slot_cache, slot);
>> 		mmdrop(mm);
>> 	}
>> }
>