linux-kernel - Re: [MASSMAIL KLMS] Re: [PATCH v10 04/18] af_vsock: implement SEQPACKET receive loop

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Date:   Mon, 7 Jun 2021 14:28:16 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseny Krasnov <arseny.krasnov@...persky.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jorgen Hansen <jhansen@...are.com>,
        Colin Ian King <colin.king@...onical.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Norbert Slusarek <nslusarek@....net>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "oxffffaa@...il.com" <oxffffaa@...il.com>
Subject: Re: [MASSMAIL KLMS] Re: [PATCH v10 04/18] af_vsock: implement
 SEQPACKET receive loop

On Mon, Jun 07, 2021 at 02:29:28PM +0300, Arseny Krasnov wrote:
>
>On 07.06.2021 13:48, Stefano Garzarella wrote:
>> On Fri, Jun 04, 2021 at 09:00:14PM +0300, Arseny Krasnov wrote:
>>> On 04.06.2021 18:06, Stefano Garzarella wrote:
>>>> On Thu, May 20, 2021 at 10:16:08PM +0300, Arseny Krasnov wrote:
>>>>> Add receive loop for SEQPACKET. It looks like receive loop for
>>>>> STREAM, but there are differences:
>>>>> 1) It doesn't call notify callbacks.
>>>>> 2) It doesn't care about 'SO_SNDLOWAT' and 'SO_RCVLOWAT' values, because
>>>>>   there is no sense for these values in SEQPACKET case.
>>>>> 3) It waits until whole record is received or error is found during
>>>>>   receiving.
>>>>> 4) It processes and sets 'MSG_TRUNC' flag.
>>>>>
>>>>> So to avoid extra conditions for two types of socket inside one loop, two
>>>>> independent functions were created.
>>>>>
>>>>> Signed-off-by: Arseny Krasnov <arseny.krasnov@...persky.com>
>>>>> ---
>>>>> v9 -> v10:
>>>>> 1) Use 'msg_data_left()' instead of direct access to 'msg_hdr'.
>>>>>
>>>>> include/net/af_vsock.h   |  4 +++
>>>>> net/vmw_vsock/af_vsock.c | 72 +++++++++++++++++++++++++++++++++++++++-
>>>>> 2 files changed, 75 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
>>>>> index b1c717286993..5175f5a52ce1 100644
>>>>> --- a/include/net/af_vsock.h
>>>>> +++ b/include/net/af_vsock.h
>>>>> @@ -135,6 +135,10 @@ struct vsock_transport {
>>>>> 	bool (*stream_is_active)(struct vsock_sock *);
>>>>> 	bool (*stream_allow)(u32 cid, u32 port);
>>>>>
>>>>> +	/* SEQ_PACKET. */
>>>>> +	ssize_t (*seqpacket_dequeue)(struct vsock_sock *vsk, struct msghdr *msg,
>>>>> +				     int flags, bool *msg_ready);
>>>>> +
>>>>> 	/* Notification. */
>>>>> 	int (*notify_poll_in)(struct vsock_sock *, size_t, bool *);
>>>>> 	int (*notify_poll_out)(struct vsock_sock *, size_t, bool *);
>>>>> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>>>>> index c4f6bfa1e381..aede474343d1 100644
>>>>> --- a/net/vmw_vsock/af_vsock.c
>>>>> +++ b/net/vmw_vsock/af_vsock.c
>>>>> @@ -1974,6 +1974,73 @@ static int __vsock_stream_recvmsg(struct sock *sk, struct msghdr *msg,
>>>>> 	return err;
>>>>> }
>>>>>
>>>>> +static int __vsock_seqpacket_recvmsg(struct sock *sk, struct msghdr *msg,
>>>>> +				     size_t len, int flags)
>>>>> +{
>>>>> +	const struct vsock_transport *transport;
>>>>> +	bool msg_ready;
>>>>> +	struct vsock_sock *vsk;
>>>>> +	ssize_t record_len;
>>>>> +	long timeout;
>>>>> +	int err = 0;
>>>>> +	DEFINE_WAIT(wait);
>>>>> +
>>>>> +	vsk = vsock_sk(sk);
>>>>> +	transport = vsk->transport;
>>>>> +
>>>>> +	timeout = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
>>>>> +	msg_ready = false;
>>>>> +	record_len = 0;
>>>>> +
>>>>> +	while (1) {
>>>>> +		ssize_t fragment_len;
>>>>> +
>>>>> +		if (vsock_wait_data(sk, &wait, timeout, NULL, 0) <= 0) {
>>>>> +			/* In case of any loop break(timeout, signal
>>>>> +			 * interrupt or shutdown), we report user that
>>>>> +			 * nothing was copied.
>>>>> +			 */
>>>>> +			err = 0;
>>>> Why we report that nothing was copied?
>>>>
>>>> What happen to the bytes already copied in `msg`?
>>> Seems i need to return result of vsock_wait_data()...
>> I'm not sure.
>>
>> My biggest concern is if we reach timeout or get a signal while waiting
>> for the other pieces of a message.
>> I believe that we should not start copying a message if we have not
>> received all the fragments. Otherwise we have this problem.
>>
>> When we are sure that we have all the pieces, then we should copy them
>> without interrupting.
>>
>> IIRC this was done in previous versions.
>
>As i remember, previous versions also returned 0, because i thought,
>that for interrupted read we can copy piece of data to user's buffer,
>but we must return that nothing copied or error. In this way user

This can also be fine, but we should remove packet form the rx_queue 
only when we are sure that we delivered the entire message.

>
>won't read part of message, because syscall returned that there is
>nothing to copy. So as i understand, it is not enough - user's buffer
>must be touched only when whole message is copied?

The important thing is to not remove packets from the rx_queue unless we 
are sure that everything went well and we are returning the entire 
message to the user.

Stefano