From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D7F764AF9F5 for ; Wed, 2 Sep 2026 16:57:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788368279; cv=none; b=VA8tio9ZuiPXeoYfuMqcXNZFgmdrp2ON2+5mIDjfee84LFXbZfgd7sSFNSsJppFm7g676l//yO73hEsfJbMlZqNQwoMi4Wk4Oq/U27xJ2lk6YCTO9v9YoYHBuM5LPw67fzkINZBjlqF1J+0H4P/w6YN0IJccwN/L8csCsUZ8Faw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788368279; c=relaxed/simple; bh=0D7k1lbgq23J0I06xSHMmP9Sb1yNyraYmkx9govjxeM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jl7pzV7/mrztSXF8UhtU+dzmpg3qxVtVsbaiDX34GE7VBRVk1xE1engUc8icYmYzI1IcL0IEJrI5b1eohwpHjyek00l+tJvixx6HsdoP7xRC3NYd9MWKiCjxDsn3on7VO7RSjR6AMd3l9luUOhP7hdSZK5atF46yXTvb0Cp8/gY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bRC3J1nx; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="bRC3J1nx" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 97D381F00A3A; Wed, 2 Sep 2026 16:57:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788368277; bh=egVdb9UoIL1aFUo42c4ZwNrUAMjWH0tjK9izmK9H+8E=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=bRC3J1nxrhByXxV1AzoxvxllYIAt9IFtTY8uFYbNNIEThdUlLVwzKclQGJsQgyqKu wW3d/ZWibebIPxjn2Qj7i54hBJ9j3hiRHFD1tsk+2FEYBMqZH1n91FgrZ1xZ8GJDlQ p5J9cld0PMCG7FKqipexJezTLfyw3+JrG5RVTU1HvYLAnmatcb9BBFsWmR+eYiB8w+ cTRa64QUk18pJFpJCRnHG7O+1gU8Ordfg0g+cDtKi9uVV2jQROhougTPl1hpbuDRpy I1EKZrdWjGt7P1AWZ+65QJSuygNTsHiDSEHNNBNRyr9U1hWk2sS8PvA2Hqo9VATkXe ud+QOgnfWYejg== Message-ID: Date: Wed, 2 Sep 2026 18:57:53 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/slab: take n->list_lock in __slab_try_return_freelist() to avoid race Content-Language: en-US To: "Harry Yoo (Meta)" , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hyunwoo Kim References: <20260902-slab-fix-aba-v2-1-d1ece15a8417@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/2/26 18:41, Harry Yoo (Meta) wrote: > 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, as reported by Hyunwoo Kim [1]: "as reported by ..." was already said above, we could keep just the [1] or drop completely? > 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. Ah nicely spotted. > 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) > --- > Changes in v2: > - EDITME: describe what is new in this series revision. > - EDITME: use bulletpoints and terse descriptions. > - Link to v1: https://lore.kernel.org/r/apPa-cGLcyt90l-E@v4bel > --- > mm/slub.c | 17 ++++++++++++----- > 1 file changed, 12 insertions(+), 5 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index f9b56cb439e7..5cbbacb8ee32 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -5684,6 +5684,8 @@ static bool __slab_try_return_freelist(struct kmem_cache *s, struct slab *slab, > void *head, int cnt) > { > struct freelist_counters old, new; > + struct kmem_cache_node *n; > + unsigned long flags; > > old.freelist = slab->freelist; > old.counters = slab->counters; > @@ -5695,9 +5697,16 @@ 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")) > + n = get_node(s, slab_nid(slab)); Wonder if we could just pass the 'n' to __slab_try_return_freelist() that we already have in the caller. > + 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; > } > > @@ -7296,10 +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, slab, head, count)) > break; > - } > > do { > tail = object; > @@ -7312,7 +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) > > --- > base-commit: cee9395acd8043be0644b25c34bfa86623f2b935 > change-id: 20260902-slab-fix-aba-d60f39fc9dee > > Best regards,