+ mm-ksm-avoid-missing-ksmd-wakeups-in-ksm_enter.patch added to mm-unstable branch

The patch titled
     Subject: mm/ksm: avoid missing ksmd wakeups in ksm_enter
has been added to the -mm mm-unstable branch.  Its filename is
     mm-ksm-avoid-missing-ksmd-wakeups-in-ksm_enter.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-avoid-missing-ksmd-wakeups-in-ksm_enter.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Longlong Xia <[email protected]>
Subject: mm/ksm: avoid missing ksmd wakeups in ksm_enter
Date: Wed, 5 Aug 2026 21:27:34 +0800

__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/[email protected]
Fixes: 6e15838425ac ("ksm: keep quiet while list empty")
Signed-off-by: Longlong Xia <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Chengming Zhou <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: xu xin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 mm/ksm.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/mm/ksm.c~mm-ksm-avoid-missing-ksmd-wakeups-in-ksm_enter
+++ a/mm/ksm.c
@@ -3061,10 +3061,9 @@ 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),
_

Patches currently in -mm which might be from [email protected] are

mm-ksm-avoid-missing-ksmd-wakeups-in-ksm_enter.patch
zram-fix-out-of-bounds-access-in-writeback_store.patch
zram-fix-out-of-bounds-access-in-read_block_state.patch