On Tue, May 12, 2026 at 04:54:34AM -0400, Michael S. Tsirkin wrote:
On Tue, May 12, 2026 at 10:07:37AM +0200, Stefano Garzarella wrote:
From: Stefano Garzarella <[email protected]>
After commit 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb
queue"), virtio_transport_inc_rx_pkt() subtracts per-skb overhead from
buf_alloc when checking whether a new packet fits. This reduces the
effective receive buffer below what the user configured via
SO_VM_SOCKETS_BUFFER_SIZE, causing legitimate data packets to be
silently dropped and applications that rely on the full buffer size
to deadlock.
Also, the reduced space is not communicated to the remote peer, so
its credit calculation accounts more credit than the receiver will
actually accept, causing data loss (there is no retransmission).
With this approach we currently have failures in
tools/testing/vsock/vsock_test.c. Test 18 sometimes fails, while
test 22 always fails in this way:
18 - SOCK_STREAM MSG_ZEROCOPY...hash mismatch
22 - SOCK_STREAM virtio credit update + SO_RCVLOWAT...send failed:
Resource temporarily unavailable
Fix this by using `buf_alloc * 2` as the total budget for payload plus
skb overhead in virtio_transport_inc_rx_pkt(), similar to how SO_RCVBUF
is doubled to reserve space for sk_buff metadata. This preserves the
full buf_alloc for payload under normal operation, while still bounding
the skb queue growth.
When the total budget (buf_alloc * 2) is exceeded (e.g. under small-packet
flooding where overhead dominates), the connection is reset and local
socket error set to ENOBUFS, so both peers are explicitly notified of
the failure rather than silently losing data.
With this patch, all tests in tools/testing/vsock/vsock_test.c are
now passing again.
A solution to handle small-packet overhead efficiently also for
SEQPACKET (we already do that for STREAM) is planned as follow-up work.
This patch is needed in any case to prevent silent data loss, because
even if we reduce the overhead, we can't eliminate it entirely.
Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue")
Signed-off-by: Stefano Garzarella <[email protected]>
Thanks for the patch! I'd like to split this:
1. buf alloc boost
2. reset when out of credits
Good point, also the reset maybe should have an other fixes tag (i.e.
when we introduced that check)
this way we can revert 2 easier later.
I'm not sure if we should revert them at some point, even though we'll
be able to handle the overhead better, but I agree that we should split
them.
I'll wait for a few more comments and then send v3 with the split.
Thanks,
Stefano
