From: Stefano Garzarella When many small packets accumulate in the receive queue, the skb overhead can exceed buf_alloc even while the payload is within bounds. This causes virtio_transport_inc_rx_pkt() to reject packets, leading to connection resets during large transfers under backpressure. The issue was reported by Brien, who has a reproducer, but it is also easily reproducible with iperf-vsock [1] using a small packet size: iperf3 --vsock -c $CID -l 129 which fails immediately without this patch but with commit 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue"). Inspired by TCP's tcp_collapse() which solves a similar problem, add virtio_transport_collapse_rx_queue() that walks the receive queue and re-copies data into compact linear skbs to reduce the overhead. The collapse is triggered from virtio_transport_recv_enqueue() when virtio_transport_inc_rx_pkt() fails. A pre-scan counts the eligible bytes to size each allocation precisely, avoiding waste for isolated small packets. Partially consumed skbs are kept as-is to preserve buf_used/fwd_cnt accounting, EOM-marked skbs to maintain SEQPACKET message boundaries, and skbs already larger than the collapse target because they already have a good data-to-overhead ratio. [1] https://github.com/stefano-garzarella/iperf-vsock Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue") Cc: stable@vger.kernel.org Reported-by: Brien Oberstein Closes: https://lore.kernel.org/netdev/618701dd023e$063de350$12b9a9f0$@gmail.com/ Tested-by: Brien Oberstein Signed-off-by: Stefano Garzarella --- net/vmw_vsock/virtio_transport_common.c | 148 +++++++++++++++++++++++- 1 file changed, 146 insertions(+), 2 deletions(-) diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c index 09475007165b..304ea424995d 100644 --- a/net/vmw_vsock/virtio_transport_common.c +++ b/net/vmw_vsock/virtio_transport_common.c @@ -420,6 +420,137 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk, return ret; } +static bool virtio_transport_can_collapse(struct sk_buff *skb, + unsigned int size) +{ + /* skbs that are partially consumed, mark a SEQPACKET message boundary, + * or are already large enough should not be collapsed: they either + * need special accounting, carry protocol state, or already have a + * good data-to-overhead ratio. + */ + if (VIRTIO_VSOCK_SKB_CB(skb)->offset) + return false; + if (le32_to_cpu(virtio_vsock_hdr(skb)->flags) & VIRTIO_VSOCK_SEQ_EOM) + return false; + if (skb->len >= size) + return false; + return true; +} + +/* Iterate through the packets in the queue starting from the current skb to + * count the number of bytes we can collapse. + */ +static unsigned int +virtio_transport_collapse_size(struct sk_buff *skb, + struct sk_buff_head *queue, + unsigned int max_size) +{ + unsigned int target = skb->len - VIRTIO_VSOCK_SKB_CB(skb)->offset; + + while ((skb = skb_peek_next(skb, queue)) && + virtio_transport_can_collapse(skb, max_size)) { + unsigned int len = skb->len - VIRTIO_VSOCK_SKB_CB(skb)->offset; + + if (len > max_size - target) + return target; + + target += len; + } + + return target; +} + +/* Called under lock_sock when skb overhead exceeds the budget. */ +static void virtio_transport_collapse_rx_queue(struct virtio_vsock_sock *vvs) +{ + /* Use the same linear allocation threshold as virtio_vsock_alloc_skb() + * to avoid adding pressure on the page allocator. + */ + unsigned int collapse_max = SKB_MAX_ORDER(VIRTIO_VSOCK_SKB_HEADROOM, + PAGE_ALLOC_COSTLY_ORDER); + struct sk_buff *skb, *next_skb, *new_skb = NULL; + struct sk_buff_head new_queue; + + __skb_queue_head_init(&new_queue); + + skb_queue_walk_safe(&vvs->rx_queue, skb, next_skb) { + struct virtio_vsock_hdr *hdr = virtio_vsock_hdr(skb); + u32 src_off = VIRTIO_VSOCK_SKB_CB(skb)->offset; + u32 src_len = skb->len - src_off; + bool keep = false; + + if (!virtio_transport_can_collapse(skb, collapse_max)) { + /* Finalize pending collapsed skb to preserve packet + * ordering. + */ + if (new_skb) { + __skb_queue_tail(&new_queue, new_skb); + new_skb = NULL; + } + keep = true; + goto next; + } + + /* Finalize if this packet won't fit in the remaining tailroom, + * so we can allocate a right-sized new_skb. + */ + if (new_skb && src_len > skb_tailroom(new_skb)) { + __skb_queue_tail(&new_queue, new_skb); + new_skb = NULL; + } + + if (!new_skb) { + unsigned int alloc_size; + + alloc_size = virtio_transport_collapse_size(skb, &vvs->rx_queue, + collapse_max); + + /* Only this skb's data is eligible, nothing to merge + * with. Keep as-is. + */ + if (alloc_size <= src_len) { + keep = true; + goto next; + } + + new_skb = virtio_vsock_alloc_linear_skb(alloc_size + + VIRTIO_VSOCK_SKB_HEADROOM, GFP_KERNEL); + if (!new_skb) + goto out; + + memcpy(virtio_vsock_hdr(new_skb), hdr, + sizeof(struct virtio_vsock_hdr)); + virtio_vsock_hdr(new_skb)->len = 0; + } + + /* Cannot fail since src_off/src_len are within bounds, but if + * it does, discard new_skb to avoid queuing corrupted data. + */ + if (WARN_ON_ONCE(skb_copy_bits(skb, src_off, + skb_put(new_skb, src_len), + src_len))) { + kfree_skb(new_skb); + new_skb = NULL; + goto out; + } + + le32_add_cpu(&virtio_vsock_hdr(new_skb)->len, src_len); + virtio_vsock_hdr(new_skb)->flags |= hdr->flags; + +next: + __skb_unlink(skb, &vvs->rx_queue); + if (keep) + __skb_queue_tail(&new_queue, skb); + else + consume_skb(skb); + } +out: + if (new_skb) + __skb_queue_tail(&new_queue, new_skb); + + skb_queue_splice(&new_queue, &vvs->rx_queue); +} + static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs, u32 len) { @@ -1363,8 +1494,21 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk, spin_lock_bh(&vvs->rx_lock); can_enqueue = virtio_transport_inc_rx_pkt(vvs, len); - if (!can_enqueue) - goto out; + if (!can_enqueue) { + /* Try to collapse the receive queue to reduce skb overhead and + * make room for this packet. + * Unlock rx_lock since the collapse may sleep or, in any case, + * take some time to collapse the skbs, but this is safe, since + * sk_lock is held by caller so no one else can enqueue or + * dequeue. + */ + spin_unlock_bh(&vvs->rx_lock); + virtio_transport_collapse_rx_queue(vvs); + spin_lock_bh(&vvs->rx_lock); + can_enqueue = virtio_transport_inc_rx_pkt(vvs, len); + if (!can_enqueue) + goto out; + } if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM) vvs->msg_count++; -- 2.54.0 From: Stefano Garzarella Add a test that sends 2 MB of data using randomly sized small packets (129-512 bytes) over a SOCK_STREAM connection. Packets above GOOD_COPY_LEN (128) bypass the in-place coalescing in recv_enqueue(), forcing each one into its own skb. Without receive queue collapsing, the per-skb overhead eventually exceeds buf_alloc and the connection is reset. The test verifies that all data arrives and that content integrity is preserved. Signed-off-by: Stefano Garzarella --- tools/testing/vsock/vsock_test.c | 87 ++++++++++++++++++++++++++++++++ 1 file changed, 87 insertions(+) diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c index 76be0e4a7f0e..b4ff9f946565 100644 --- a/tools/testing/vsock/vsock_test.c +++ b/tools/testing/vsock/vsock_test.c @@ -2347,6 +2347,88 @@ static void test_stream_tx_credit_bounds_server(const struct test_opts *opts) close(fd); } +/* Test that many small packets don't cause a connection reset under pressure + * and that data integrity is preserved. Packet sizes vary randomly between + * 129 and 512 bytes, above GOOD_COPY_LEN (128) to bypass in-place coalescing + * in recv_enqueue, forcing each one into its own skb. Without receive queue + * collapsing, the per-skb overhead eventually exceeds buf_alloc and the + * connection is reset. + */ +#define COLLAPSE_PKT_MIN 129 +#define COLLAPSE_PKT_MAX 512 +#define COLLAPSE_TOTAL (2 * 1024 * 1024) + +static void test_stream_collapse_client(const struct test_opts *opts) +{ + unsigned char *data; + unsigned long hash; + size_t offset = 0; + int i, fd; + + data = malloc(COLLAPSE_TOTAL); + if (!data) { + perror("malloc"); + exit(EXIT_FAILURE); + } + + for (i = 0; i < COLLAPSE_TOTAL; i++) + data[i] = rand() & 0xff; + + fd = vsock_stream_connect(opts->peer_cid, opts->peer_port); + if (fd < 0) { + perror("connect"); + exit(EXIT_FAILURE); + } + + while (offset < COLLAPSE_TOTAL) { + size_t pkt_size = COLLAPSE_PKT_MIN + + rand() % (COLLAPSE_PKT_MAX - COLLAPSE_PKT_MIN + 1); + + pkt_size = min(pkt_size, COLLAPSE_TOTAL - offset); + + send_buf(fd, data + offset, pkt_size, 0, pkt_size); + offset += pkt_size; + } + + hash = hash_djb2(data, COLLAPSE_TOTAL); + control_writeulong(hash); + + free(data); + close(fd); +} + +static void test_stream_collapse_server(const struct test_opts *opts) +{ + unsigned long hash, remote_hash; + unsigned char *data; + int fd; + + data = malloc(COLLAPSE_TOTAL); + if (!data) { + perror("malloc"); + exit(EXIT_FAILURE); + } + + fd = vsock_stream_accept(VMADDR_CID_ANY, opts->peer_port, NULL); + if (fd < 0) { + perror("accept"); + exit(EXIT_FAILURE); + } + + recv_buf(fd, data, COLLAPSE_TOTAL, 0, COLLAPSE_TOTAL); + + hash = hash_djb2(data, COLLAPSE_TOTAL); + remote_hash = control_readulong(); + if (hash != remote_hash) { + fprintf(stderr, "hash mismatch: local %lu remote %lu\n", + hash, remote_hash); + exit(EXIT_FAILURE); + } + + free(data); + close(fd); +} + static struct test_case test_cases[] = { { .name = "SOCK_STREAM connection reset", @@ -2546,6 +2628,11 @@ static struct test_case test_cases[] = { .run_client = test_stream_msg_peek_client, .run_server = test_stream_peek_after_recv_server, }, + { + .name = "SOCK_STREAM small packets backpressure", + .run_client = test_stream_collapse_client, + .run_server = test_stream_collapse_server, + }, {}, }; -- 2.54.0