Re: [PATCH net] vsock/virtio: fix zerocopy completion for multi-skb sends

[Stefano Garzarella <sgarzare@redhat.com>]

On Sat, May 16, 2026 at 12:53:29PM +0100, David Laight wrote:
>On Thu, 14 May 2026 11:29:48 +0200
>Stefano Garzarella <sgarzare@redhat.com> wrote:
>
>> From: Stefano Garzarella <sgarzare@redhat.com>
>>
>> When a large message is fragmented into multiple skbs, the zerocopy
>> uarg is only allocated and attached to the last skb in the loop.
>> Non-final skbs carry pinned user pages with no completion tracking,
>> so the kernel has no way to notify userspace when those pages are safe
>> to reuse. If the loop breaks early the uarg is never allocated at all,
>> leaking pinned pages with no completion notification.
>>
>> Fix this by following the approach used by TCP: allocate the zerocopy
>> uarg (if not provided by the caller) before the send loop and attach
>> it to every skb via skb_zcopy_set(), which takes a reference per skb.
>> Each skb's completion properly decrements the refcount, and the
>> notification only fires after the last skb is freed.
>> On failure, if no data was sent, the uarg is cleanly aborted via
>> net_zcopy_put_abort().
>>
>> This issue was initially discovered by sashiko while reviewing commit
>> 1cb36e252211 ("vsock/virtio: fix MSG_ZEROCOPY pinned-pages accounting")
>> but was pre-existing.
>>
>> Fixes: 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY flag support")
>> Cc: Arseniy Krasnov <avkrasnov@salutedevices.com>
>> Closes: https://sashiko.dev/#/patchset/20260420132051.217589-1-sgarzare%40redhat.com
>> Reported-by: Maher Azzouzi <maherazz04@gmail.com>
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>>  net/vmw_vsock/virtio_transport_common.c | 83 ++++++++++---------------
>>  1 file changed, 34 insertions(+), 49 deletions(-)
>>
>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>> index 989cc252d3d3..1e3409d28164 100644
>> --- a/net/vmw_vsock/virtio_transport_common.c
>> +++ b/net/vmw_vsock/virtio_transport_common.c
>> @@ -70,34 +70,6 @@ static bool virtio_transport_can_zcopy(const struct virtio_transport *t_ops,
>>  	return true;
>>  }
>>
>> -static int virtio_transport_init_zcopy_skb(struct vsock_sock *vsk,
>> -					   struct sk_buff *skb,
>> -					   struct msghdr *msg,
>> -					   size_t pkt_len,
>> -					   bool zerocopy)
>> -{
>> -	struct ubuf_info *uarg;
>> -
>> -	if (msg->msg_ubuf) {
>> -		uarg = msg->msg_ubuf;
>> -		net_zcopy_get(uarg);
>> -	} else {
>> -		struct ubuf_info_msgzc *uarg_zc;
>> -
>> -		uarg = msg_zerocopy_realloc(sk_vsock(vsk),
>> -					    pkt_len, NULL, false);
>> -		if (!uarg)
>> -			return -1;
>> -
>> -		uarg_zc = uarg_to_msgzc(uarg);
>> -		uarg_zc->zerocopy = zerocopy ? 1 : 0;
>> -	}
>> -
>> -	skb_zcopy_init(skb, uarg);
>> -
>> -	return 0;
>> -}
>> -
>>  static int virtio_transport_fill_skb(struct sk_buff *skb,
>>  				     struct virtio_vsock_pkt_info *info,
>>  				     size_t len,
>> @@ -317,8 +289,10 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>  	u32 src_cid, src_port, dst_cid, dst_port;
>>  	const struct virtio_transport *t_ops;
>>  	struct virtio_vsock_sock *vvs;
>> +	struct ubuf_info *uarg = NULL;
>>  	u32 pkt_len = info->pkt_len;
>>  	bool can_zcopy = false;
>> +	bool have_uref = false;
>>  	u32 rest_len;
>>  	int ret;
>>
>> @@ -360,6 +334,25 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>  		if (can_zcopy)
>>  			max_skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE,
>>  					    (MAX_SKB_FRAGS * PAGE_SIZE));
>> +
>> +		if (info->msg->msg_flags & MSG_ZEROCOPY &&
>> +		    info->op == VIRTIO_VSOCK_OP_RW) {
>> +			uarg = info->msg->msg_ubuf;
>> +
>> +			if (!uarg) {
>> +				uarg = msg_zerocopy_realloc(sk_vsock(vsk),
>> +							    pkt_len, NULL, false);
>> +				if (!uarg) {
>> +					virtio_transport_put_credit(vvs, pkt_len);
>> +					return -ENOMEM;
>> +				}
>> +
>> +				if (!can_zcopy)
>> +					uarg_to_msgzc(uarg)->zerocopy = 0;
>> +
>> +				have_uref = true;
>> +			}
>> +		}
>
>Surely that block should only be done if can_zcopy is true?
>And shouldn't something unset it if info->op != VIRTIO_VSOCK_OP_RW ?
>If the msg_zerocopy_realloc() fails then can't you just set can_zcopy to false.
>
>It info->msg->msg_buf is already set then I think you have to disable zero-copy.
>The caller has already requested a callback - and you can't add another.
>
>In any case by the end of this can_zcopy and have_uref are really the same flag.

I kept the same approach we had before, trying to make as few changes as 
possible.

All these potential issues seem to be pre-existing and should be 
eventually  addressed in other patches IMHO. This patch one only 
resolves the main issue of calling `skb_zcopy_set()` for every skb to 
avoid leaking pages, etc.

@Arseniy can you help on this?

>
>>  	}
>>
>>  	rest_len = pkt_len;
>> @@ -378,27 +371,7 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>  			break;
>>  		}
>>
>> -		/* We process buffer part by part, allocating skb on
>> -		 * each iteration. If this is last skb for this buffer
>> -		 * and MSG_ZEROCOPY mode is in use - we must allocate
>> -		 * completion for the current syscall.
>> -		 *
>> -		 * Pass pkt_len because msg iter is already consumed
>> -		 * by virtio_transport_fill_skb(), so iter->count
>> -		 * can not be used for RLIMIT_MEMLOCK pinned-pages
>> -		 * accounting done by msg_zerocopy_realloc().
>> -		 */
>> -		if (info->msg && info->msg->msg_flags & MSG_ZEROCOPY &&
>> -		    skb_len == rest_len && info->op == VIRTIO_VSOCK_OP_RW) {
>> -			if (virtio_transport_init_zcopy_skb(vsk, skb,
>> -							    info->msg,
>> -							    pkt_len,
>> -							    can_zcopy)) {
>> -				kfree_skb(skb);
>> -				ret = -ENOMEM;
>> -				break;
>> -			}
>> -		}
>> +		skb_zcopy_set(skb, uarg, NULL);
>>
>>  		virtio_transport_inc_tx_pkt(vvs, skb);
>>
>> @@ -422,6 +395,18 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>
>>  	virtio_transport_put_credit(vvs, rest_len);
>>
>> +	/* msg_zerocopy_realloc() initializes the ubuf_info refcnt to 1.
>> +	 * skb_zcopy_set() increases it for each skb, so we can drop that
>                                                            ^ must
>
>> +	 * initial reference to keep it balanced.
>> +	 */
>> +	if (have_uref) {
>> +		if (rest_len == pkt_len)
>> +			/* No data sent, abort the notification. */
>> +			net_zcopy_put_abort(uarg, true);
>
>Is it worth optimising for the 'nothing sent' case ?

What do you suggest doing?

I followed what TCP does.

Thanks,
Stefano

>
>-- David
>
>> +		else
>> +			net_zcopy_put(uarg);
>> +	}
>> +
>>  	/* Return number of bytes, if any data has been sent. */
>>  	if (rest_len != pkt_len)
>>  		ret = pkt_len - rest_len;
>