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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id 00721157ae682-88486086224sm41718997b3.10.2026.09.14.14.42.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Sep 2026 14:42:34 -0700 (PDT) From: Willem de Bruijn To: netdev@vger.kernel.org Cc: davem@davemloft.net, kuba@kernel.org, edumazet@google.com, pabeni@redhat.com, horms@kernel.org, andrew+netdev@lunn.ch, Willem de Bruijn , stable@vger.kernel.org, mst@redhat.com, jasowangio@gmail.com Subject: [PATCH net 1/2] virtio_net: copy zerocopy frags in start_xmit without NAPI Date: Mon, 14 Sep 2026 17:37:42 -0400 Message-ID: <20260914214229.1674102-2-willemdebruijn.kernel@gmail.com> X-Mailer: git-send-email 2.55.0.1032.g73a4cd73de-goog In-Reply-To: <20260914214229.1674102-1-willemdebruijn.kernel@gmail.com> References: <20260914214229.1674102-1-willemdebruijn.kernel@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Willem de Bruijn Virtio-net without NAPI frees completed skbs lazily on the next start_xmit. Senders waiting for in-flight zerocopy buffers can deadlock if they cannot transmit more packets, as then no commpleted packets will be freed. When !use_napi, virtio-net already calls skb_orphan to avoid waiting up for transmitted skbs to be freed. For zerocopy packets that require deep copying on orphan (i.e. those that do not set SKBFL_DONT_ORPHAN, such as PACKET_TX_RING), call skb_orphan_frags before orphaning to release the buffers. This fixes the tpacket_snd slot reuse bug on skb_orphan for virtio-net, and prevents PACKET_TX_RING from running out of slots. This fix also touches vhost_net zerocopy packets, which also do not set SKBFL_DONT_ORPHAN. This is fine: vhost_net packets only encounter virtio-net in nested virtualization, and only if napi_tx is explicitly disabled (it has been default-enabled since Linux 4.12). In that rare case, copying the frags is desirable anyway to prevent holding guest descriptors pinned across unbounded intervals. This is a prerequisite for the next patch, which converts PACKET_TX_RING to standard zerocopy completion. Without this patch first, a bounded ring sender can stall indefinitely behind a virtio-net virtqueue that cannot reclaim. Fixes: 5cd8d46ea156 ("packet: copy user buffers before orphan or clone") Cc: stable@vger.kernel.org Cc: mst@redhat.com Cc: jasowangio@gmail.com Signed-off-by: Willem de Bruijn --- drivers/net/virtio_net.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index e34c52d059d3..1ce528c18f9e 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -3349,6 +3349,13 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) else virtqueue_disable_cb(sq->vq); + if (!use_napi && + unlikely(skb_orphan_frags(skb, GFP_ATOMIC))) { + DEV_STATS_INC(dev, tx_dropped); + dev_kfree_skb_any(skb); + return NETDEV_TX_OK; + } + /* timestamp packet in software */ skb_tx_timestamp(skb); -- 2.55.0.1032.g73a4cd73de-goog