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

[Longlong Xia]

在 2026/8/5 20:26, David Hildenbrand (Arm) 写道:
> On 8/3/26 17:21, Longlong Xia wrote:
> > From: Longlong Xia <xialonglong@xxxxxxxxxx>
> >
> > __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 <xialonglong@xxxxxxxxxx>
> > ---
> >   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) <david@xxxxxxxxxx>


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

  Best regards,
  Longlong