netdev - [PATCH net v2 1/2] vsock/virtio: Coalesce only linear skb

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Message-Id: <20260113-vsock-recv-coalescence-v2-1-552b17837cf4@rbox.co>
Date: Tue, 13 Jan 2026 16:08:18 +0100
From: Michal Luczaj <mhal@...x.co>
To: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
 Eugenio Pérez <eperezma@...hat.com>, 
 Stefan Hajnoczi <stefanha@...hat.com>, 
 Stefano Garzarella <sgarzare@...hat.com>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, 
 Arseniy Krasnov <avkrasnov@...utedevices.com>
Cc: kvm@...r.kernel.org, virtualization@...ts.linux.dev, 
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Michal Luczaj <mhal@...x.co>
Subject: [PATCH net v2 1/2] vsock/virtio: Coalesce only linear skb

vsock/virtio common tries to coalesce buffers in rx queue: if a linear skb
(with a spare tail room) is followed by a small skb (length limited by
GOOD_COPY_LEN = 128), an attempt is made to join them.

Since the introduction of MSG_ZEROCOPY support, assumption that a small skb
will always be linear is incorrect. In the zerocopy case, data is lost and
the linear skb is appended with uninitialized kernel memory.

Of all 3 supported virtio-based transports, only loopback-transport is
affected. G2H virtio-transport rx queue operates on explicitly linear skbs;
see virtio_vsock_alloc_linear_skb() in virtio_vsock_rx_fill(). H2G
vhost-transport may allocate non-linear skbs, but only for sizes that are
not considered for coalescence; see PAGE_ALLOC_COSTLY_ORDER in
virtio_vsock_alloc_skb().

Ensure only linear skbs are coalesced. Note that skb_tailroom(last_skb) > 0
guarantees last_skb is linear.

Fixes: 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY flag support")
Signed-off-by: Michal Luczaj <mhal@...x.co>
---
 net/vmw_vsock/virtio_transport_common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index dcc8a1d5851e..26b979ad71f0 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1359,9 +1359,11 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,

 	/* Try to copy small packets into the buffer of last packet queued,
 	 * to avoid wasting memory queueing the entire buffer with a small
-	 * payload.
+	 * payload. Skip non-linear (e.g. zerocopy) skbs; these carry payload
+	 * in skb_shinfo.
 	 */
-	if (len <= GOOD_COPY_LEN && !skb_queue_empty(&vvs->rx_queue)) {
+	if (len <= GOOD_COPY_LEN && !skb_queue_empty(&vvs->rx_queue) &&
+	    !skb_is_nonlinear(skb)) {
 		struct virtio_vsock_hdr *last_hdr;
 		struct sk_buff *last_skb;

-- 
2.52.0