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

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]>

-- 
Cheers,

David