Re: [PATCH net v2 1/2] vsock/virtio: Coalesce only linear skb
From: Stefano Garzarella <sgarzare@redhat.com>
Date: 2026-01-14 09:57:58
Also in:
kvm, lkml, virtualization
On Tue, Jan 13, 2026 at 04:08:18PM +0100, Michal Luczaj wrote:
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 <redacted>
---
net/vmw_vsock/virtio_transport_common.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)Thank you for enriching the commit message! LGTM! Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
quoted hunk ↗ jump to hunk
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