Re: [PATCH net v4] net: do not let defer_count outlive an empty defer_list

[kuniyu@google.com]

On Thu, 15 Oct 2026 09:22:09 -0700, Joseph Kain wrote:
> __skb_defer_free_flush() can leave defer_count at or above skb_defer_max
> with an empty defer_list. That state is absorbing: every producer then
> takes the nodefer path without queueing, so the list never refills.
> Deferred freeing stays off for that (cpu, node) until reboot, and
> cross-CPU frees fall back to the page allocator, which is the contention
> deferral exists to avoid.
> 
> skb_attempt_defer_free() increments before testing the limit and does not
> undo the increment when it rejects, so a rejected free leaves the counter
> raised with nothing queued.
> 
> Nothing brings it back down. The reject path returns before
> kick_defer_list_purge(), so once the counter is stuck no producer
> schedules a flush on the owner CPU. A CPU that never raises NET_RX itself
> -- an application CPU doing TX while NIC IRQs are pinned elsewhere and
> RPS is off -- then never runs the flush at all, so clearing the counter
> inside the flush cannot help it either. proc_do_skb_defer_max() neither
> resets the counters nor schedules a flush when the limit changes.
> 
> It is reachable with no race at all:
> 
>   sysctl -w net.core.skb_defer_max=1    # frees increment, then reject
>   sysctl -w net.core.skb_defer_max=128  # does not recover
> 
> skb_defer_max has .extra1 = SYSCTL_ZERO and no .extra2, so 1 is settable.
> From v7.1 on, 0 is safe because proc_do_skb_defer_max() flips
> skb_defer_disable_key and the producer returns before incrementing. On
> 6.18 through 7.0, which carry the llist conversion but not commit
> 08dc30de1a40 ("net: add skb_defer_disable_key static key"), setting 0 and
> restoring the previous value strands the counter the same way -- and 0 is
> the documented way to turn the feature off.
> 
> Test the limit before incrementing, so the counter is only ever raised by
> a producer that goes on to queue. It then cannot outlive the skbs it
> counts, and the absorbing state is unreachable without depending on a
> flush ever running on the owner CPU.

I think the state is still reachable when reducing the knob
if the new value >> 1 is smaller than the current defer_count
due to the kick condition (==).  This can be fixed by bumping
the knob to defer_count * 2 though.


> 
> This makes the limit advisory. The test no longer reserves the slot it
> checks, and the recvmsg callers run in preemptible context, so the
> overshoot is bounded by the number of producers in flight rather than by
> skb_defer_max; the post-increment test reserved the slot and bounded the
> queue exactly. The counter is already approximate -- the flush zeroes it
> without regard for producers in flight -- and the next flush erases the
> overshoot, where the latch it replaces never recovered at all.
> 
> Undoing the increment on the reject path instead would be wrong: a flush
> that zeroes the counter between a producer's increment and its undo
> leaves the counter negative, and the producer's local is an unsigned
> long, so every later test reads it as enormous and rejects. That is the
> same dead end, reached more easily.
> 
> While here, drain before clearing in the flush. A producer that queues
> between the clear and the drain currently has its skb drained but its
> increment kept, which over-counts and costs deferral until the next
> flush. Draining first matches the ordering __dev_xmit_skb() already uses
> for the qdisc defer_list.
> 
> Found by manual code inspection, LLM-assisted, while backporting the
> llist conversion to 6.1 after hitting the sd->defer_lock contention it
> removes. The latch itself was not hit in production; it was reproduced by
> lowering net.core.skb_defer_max.
> 
> Fixes: 844c9db7f7f5 ("net: use llist for sd->defer_list")
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Joseph Kain <jkain@nuro.ai>
> ---
> v4:
>  - v3's "undo the increment on reject" was wrong: a flush that zeroes the
>    counter between a producer's increment and its undo leaves the counter
>    negative, and the producer's local is an unsigned long, so every later
>    test reads it as enormous and rejects forever. Test the limit before
>    incrementing instead, so there is no decrement to underflow
>    (netdev-bot sashiko).
> 
> Note this changes the effective limit from skb_defer_max - 1 to
> skb_defer_max, because the test now runs before the increment rather than
> after it. That matches what Documentation/admin-guide/sysctl/net.rst
> describes. It also means the half-capacity kick can now fire at
> skb_defer_max = 1, which commit c09b0cd2cc6c ("net: avoid strange
> behavior with skb_defer_max == 1") set out to make possible.
> 
> Built for x86_64 and arm64, with and without CONFIG_PREEMPT_RT. On
> v7.3-rc6 with skb_defer_max=1 and a cross-CPU TCP load, defer_count
> reached 1918118 unpatched and did not recover when the default was
> restored; with this patch it stays within the limit.
> 
> v3: https://lore.kernel.org/all/20261013205416.3417296-1-jkain@nuro.ai/
> v2: https://lore.kernel.org/all/20261008220256.1844072-1-jkain@nuro.ai/
> v1: https://lore.kernel.org/all/20261007180056.1394868-1-jkain@nuro.ai/
>  net/core/dev.c    | 5 ++++-
>  net/core/skbuff.c | 9 ++++++---
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 18dc88990510..60d86bc21632 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6940,8 +6940,11 @@ static void __skb_defer_free_flush(struct skb_defer_node *sdn, int budget)
>  
>  	if (llist_empty(&sdn->defer_list))
>  		return;
> -	atomic_long_set(&sdn->defer_count, 0);
>  	free_list = llist_del_all(&sdn->defer_list);
> +	/* Racy on purpose, as in __dev_xmit_skb(): a producer landing between
> +	 * the del_all and this store stays queued but uncounted.
> +	 */
> +	atomic_long_set(&sdn->defer_count, 0);
>  
>  	llist_for_each_entry_safe(skb, next, free_list, ll_node) {
>  		prefetch(next);
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 4aea06d5167d..25f82b7dcf01 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -7386,11 +7386,14 @@ nodefer:	kfree_skb_napi_cache(skb);
>  	sdn = per_cpu_ptr(net_hotdata.skb_defer_nodes, cpu) + cpu_to_node(my_cpu);
>  
>  	defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
> -	defer_count = atomic_long_inc_return(&sdn->defer_count);
> -
> -	if (defer_count >= defer_max)
> +	/* Test before incrementing: the reject path returns below without
> +	 * reaching kick_defer_list_purge(), so a kept increment never flushes.
> +	 */
> +	if (atomic_long_read(&sdn->defer_count) >= defer_max)
>  		goto nodefer;
>  
> +	defer_count = atomic_long_inc_return(&sdn->defer_count);
> +

IIUC, atomic_long_read() before atomic_long_inc_return() needs
two-step coherency transaction, and we could avoid that and
the corner case above by fixing the off-by-one and calling
kick_defer_list_purge() in the slow path:

if (unlikely(defer_count > defer_max)) {
    if (!llist_empty(&sdn->defer_list))
        kick_defer_list_purge(cpu);
    goto nodefer;
}


>  	llist_add(&skb->ll_node, &sdn->defer_list);
>  
>  	if (unlikely(!cpu_online(cpu) || my_cpu != raw_smp_processor_id())) {
> -- 
> 2.55.0
>