Re: [PATCH v3 6/6] mm: khugepaged: set to next mm direct when mm has MMF_DISABLE_THP_COMPLETELY

[Lance Yang <lance.yang@linux.dev>]

On 2026/1/5 10:06, Vernon Yang wrote:
> On Sun, Jan 4, 2026 at 8:20 PM Lance Yang <lance.yang@linux.dev> 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@kylinos.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)) { <-
>>
>>                  hash_del(&slot->hash);
>>                  list_del(&slot->mm_node);
>>
>>                  mm_slot_free(mm_slot_cache, slot);
>>                  mmdrop(mm);
>>          }
>> }
> 
> This patch just reduces redundant operation, For a detailed
> discussion[1].
> 
> You already commit 5dad604809c5 ("mm/khugepaged: keep mm in mm_slot
> without MMF_DISABLE_THP check"), I assume there is some problem here,
> e.g. not can readd? data-race? etc. Can you explain the root cause? Thanks!

Ah, I didn't fully recall that ...

Maybe I kept the slot because it's hard for khugepaged to re-add the mm 
later.

But looking at the code again, I'm not sure if that was the right call :(

> 
> [1] https://lore.kernel.org/linux-mm/CACZaFFOvDad09MUopairAoAjZG6X5gffMaQbnfy0sCHGz8xSfg@mail.gmail.com
> 
> --
> Thanks,
> Vernon