From 3c37cac718fa407b6656d925c4437ed5410fcdda Mon Sep 17 00:00:00 2001 From: Longlong Xia Date: Wed, 5 Aug 2026 21:27:34 +0800 Subject: [PATCH] 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: 6e15838425ac ("ksm: keep quiet while list empty") Signed-off-by: Longlong Xia Acked-by: David Hildenbrand (Arm) Reviewed-by: Andrew Morton Cc: Chengming Zhou Cc: Izik Eidus Cc: xu xin Signed-off-by: Andrew Morton --- mm/ksm.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index b5854dc14a2e..14dd6a6e8e6d 100644 --- a/mm/ksm.c +++ b/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),