mm/ksm: avoid missing ksmd wakeups in ksm_enter

__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.

Link: https://lore.kernel.org/20260805132736.1063408-1-xialonglong2025@163.com
Fixes: 6e15838425 ("ksm: keep quiet while list empty")
Signed-off-by: Longlong Xia <xialonglong@kylinos.cn>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/ksm.c

@@ -3060,10 +3060,9 @@ 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),