Re: [PATCH 1/1] mm/ksm: avoid missing ksmd wakeups in ksm_enter

在 2026/8/5 20:26, David Hildenbrand (Arm) 写道:
> On 8/3/26 17:21, Longlong Xia wrote:
>> From: Longlong Xia <[email protected]>
>>
>> __ksm_enter() decides whether ksmd needs a wakeup by checking if the
>> mm slot list is empty before inserting the new slot.
>>
>> The empty check is currently outside ksm_mmlist_lock.  Another CPU can
>> remove the last slot and let ksmd go back to sleep after the unlocked
>> check, while this CPU inserts a new slot and skips the wakeup based on
>> the stale result.
>>
>> Take ksm_mmlist_lock before checking the list so the empty-to-nonempty
>> transition and the insertion are observed as one critical section.
>>
>> Fixes: 6e15838425ac ("ksm: keep quiet while list empty")
>> Signed-off-by: Longlong Xia <[email protected]>
>> ---
>>   mm/ksm.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index 7d5b76478f0b..3f2f4707ca9e 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -3019,7 +3019,7 @@ int __ksm_enter(struct mm_struct *mm)
>>   {
>>   	struct ksm_mm_slot *mm_slot;
>>   	struct mm_slot *slot;
>> -	int needs_wakeup;
>> +	bool needs_wakeup;
> Unrelated change that should not be included here.
>
>>   
>>   	mm_slot = mm_slot_alloc(mm_slot_cache);
>>   	if (!mm_slot)
>> @@ -3027,10 +3027,9 @@ int __ksm_enter(struct mm_struct *mm)
>>   
>>   	slot = &mm_slot->slot;
>>   
>> +	spin_lock(&ksm_mmlist_lock);
>>   	/* Check ksm_run too?  Would need tighter locking */
>>   	needs_wakeup = list_empty(&ksm_mm_head.slot.mm_node);
>> -
>> -	spin_lock(&ksm_mmlist_lock);
>>   	mm_slot_insert(mm_slots_hash, mm, slot);
>>   	/*
>>   	 * When KSM_RUN_MERGE (or KSM_RUN_STOP),
>
> Makes sense to me. With the above variable change removed:
>
> Acked-by: David Hildenbrand (Arm) <[email protected]>


   Thanks for the review.  I will keep `needs_wakeup` as an `int` and send
   v2 with your Acked-by.

   Best regards,
   Longlong