From 4a724bcf5d703e18957397914d79156fa2cf1174 Mon Sep 17 00:00:00 2001 From: "Harry Yoo (Meta)" Date: Thu, 3 Sep 2026 15:32:26 +0100 Subject: [PATCH] mm/slab: take n->list_lock in __slab_try_return_freelist() to avoid race Commit ba7425312607 ("mm, slab: add an optimistic __slab_try_return_freelist()") incorrectly assumed that nobody has freed an object to the slab as long as slab->freelist is NULL and cmpxchg succeeds. However, as reported by Hyunwoo Kim [1], other CPUs might have freed an object to the slab, insert the slab to the partial list, then allocated an object from the slab, and be in the middle of removing the slab from the list under n->list_lock. Since __refill_objects_node() puts the slab back on pc.slabs outside n->list_lock, it might insert the slab into that list while the slab is concurrently being removed from n->partial. This led to a list corruption [1]: list_add corruption. next->prev should be prev (ffff888100000248), but was dead000000000122. (next=ffffea000416e410). kernel BUG at lib/list_debug.c:29! Oops: invalid opcode: 0000 [#1] SMP NOPTI CPU: 1 UID: 65534 PID: 144 Comm: poc Not tainted 7.2.0-16172-gcf72cbb39da8-dirty #1 PREEMPT(lazy) RIP: 0010:__list_add_valid_or_report+0x80/0xd0 ... Call Trace: alloc_from_new_slab+0x183/0x300 ___slab_alloc+0x31c/0x890 __kmalloc_noprof+0x3d4/0x800 lsm_blob_alloc+0x2d/0x50 security_msg_msg_alloc+0x26/0x90 load_msg+0x1aa/0x210 do_msgsnd+0x91/0x800 do_syscall_64+0x109/0x5d0 entry_SYSCALL_64_after_hwframe+0x77/0x7f ... Kernel panic - not syncing: Fatal exception This is a classic ABA problem where cmpxchg succeeds but the state has changed since __refill_objects_node() took the freelist from the slab. As Vlastimil Babka mentioned [2], it should be rare to return more than one slab (due to the racy read of slab->counters in get_partial_node_bulk()). Therefore, instead of introducing additional complexity, acquire and release n->list_lock twice in the worst case. Return the slab directly to the partial list and hold n->list_lock across the cmpxchg and add_partial(). This is similar to the initial version of commit ba7425312607 [3]. This is enough to avoid the race as the list manipulation is serialized by n->list_lock. While at it, bring back unlikely() hint now that the condition is unlikely. Reported-by: Hyunwoo Kim Closes: https://lore.kernel.org/linux-mm/apPa-cGLcyt90l-E@v4bel [1] Link: https://lore.kernel.org/linux-mm/ae25c193-b95f-40c1-83b6-1c2546467e41@kernel.org [2] Link: https://lore.kernel.org/all/20260421-b4-refill-optimistic-return-v1-1-24f0bfc1acff@kernel.org [3] Fixes: ba7425312607 ("mm, slab: add an optimistic __slab_try_return_freelist()") Cc: stable@vger.kernel.org Signed-off-by: Harry Yoo (Meta) Link: https://patch.msgid.link/20260903-slab-fix-aba-v3-1-b44cb6badd54@kernel.org Reviewed-by: Hao Li Signed-off-by: Vlastimil Babka (SUSE) --- mm/slub.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 7a7e906a0e44..54ec12503357 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -5680,10 +5680,12 @@ static noinline void free_to_partial_list( * * Fail if the slab isn't full anymore due to a concurrent free. */ -static bool __slab_try_return_freelist(struct kmem_cache *s, struct slab *slab, - void *head, int cnt) +static bool __slab_try_return_freelist(struct kmem_cache *s, + struct kmem_cache_node *n, + struct slab *slab, void *head, int cnt) { struct freelist_counters old, new; + unsigned long flags; old.freelist = slab->freelist; old.counters = slab->counters; @@ -5695,9 +5697,15 @@ static bool __slab_try_return_freelist(struct kmem_cache *s, struct slab *slab, new.counters = old.counters; new.inuse -= cnt; - if (!slab_update_freelist(s, slab, &old, &new, "__slab_try_return_freelist")) - return false; + spin_lock_irqsave(&n->list_lock, flags); + if (!slab_update_freelist(s, slab, &old, &new, "__slab_try_return_freelist")) { + spin_unlock_irqrestore(&n->list_lock, flags); + return false; + } + + add_partial(n, slab, ADD_TO_TAIL); + spin_unlock_irqrestore(&n->list_lock, flags); return true; } @@ -7297,10 +7305,8 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi void *head = object; void *tail; - if (__slab_try_return_freelist(s, slab, head, count)) { - list_add(&slab->slab_list, &pc.slabs); + if (__slab_try_return_freelist(s, n, slab, head, count)) break; - } do { tail = object; @@ -7313,7 +7319,7 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi break; } - if (!list_empty(&pc.slabs)) { + if (unlikely(!list_empty(&pc.slabs))) { spin_lock_irqsave(&n->list_lock, flags); list_for_each_entry(slab, &pc.slabs, slab_list)