Re: [RFC PATCH v3 15/20] io_uring: add io_recvzc request

[Jens Axboe <axboe@kernel.dk>]

On 12/20/23 10:04 AM, Pavel Begunkov wrote:
> On 12/20/23 16:27, Jens Axboe wrote:
>> On 12/19/23 2:03 PM, David Wei wrote:
>>> diff --git a/io_uring/net.c b/io_uring/net.c
>>> index 454ba301ae6b..7a2aadf6962c 100644
>>> --- a/io_uring/net.c
>>> +++ b/io_uring/net.c
>>> @@ -637,7 +647,7 @@ static inline bool io_recv_finish(struct io_kiocb *req, int *ret,
>>>       unsigned int cflags;
>>>         cflags = io_put_kbuf(req, issue_flags);
>>> -    if (msg->msg_inq && msg->msg_inq != -1)
>>> +    if (msg && msg->msg_inq && msg->msg_inq != -1)
>>>           cflags |= IORING_CQE_F_SOCK_NONEMPTY;
>>>         if (!(req->flags & REQ_F_APOLL_MULTISHOT)) {
>>> @@ -652,7 +662,7 @@ static inline bool io_recv_finish(struct io_kiocb *req, int *ret,
>>>               io_recv_prep_retry(req);
>>>               /* Known not-empty or unknown state, retry */
>>>               if (cflags & IORING_CQE_F_SOCK_NONEMPTY ||
>>> -                msg->msg_inq == -1)
>>> +                (msg && msg->msg_inq == -1))
>>>                   return false;
>>>               if (issue_flags & IO_URING_F_MULTISHOT)
>>>                   *ret = IOU_ISSUE_SKIP_COMPLETE;
>>
>> These are a bit ugly, just pass in a dummy msg for this?
>>
>>> +int io_recvzc(struct io_kiocb *req, unsigned int issue_flags)
>>> +{
>>> +    struct io_recvzc *zc = io_kiocb_to_cmd(req, struct io_recvzc);
>>> +    struct socket *sock;
>>> +    unsigned flags;
>>> +    int ret, min_ret = 0;
>>> +    bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
>>> +    struct io_zc_rx_ifq *ifq;
>>
>> Eg
>>     struct msghdr dummy_msg;
>>
>>     dummy_msg.msg_inq = -1;
>>
>> which will eat some stack, but probably not really an issue.
>>
>>
>>> +    if (issue_flags & IO_URING_F_UNLOCKED)
>>> +        return -EAGAIN;
>>
>> This seems odd, why? If we're called with IO_URING_F_UNLOCKED set, then
> 
> It's my addition, let me explain.
> 
> io_recvzc() -> io_zc_rx_recv() -> ... -> zc_rx_recv_frag()
> 
> This chain posts completions to a buffer completion queue, and
> we don't want extra locking to share it with io-wq threads. In
> some sense it's quite similar to the CQ locking, considering
> we restrict zc to DEFER_TASKRUN. And doesn't change anything
> anyway because multishot cannot post completions from io-wq
> and are executed from the poll callback in task work.
> 
>> it's from io-wq. And returning -EAGAIN there will not do anything to
> 
> It will. It's supposed to just requeue for polling (it's not
> IOPOLL to keep retrying -EAGAIN), just like multishots do.

It definitely needs a good comment, as it's highly non-obvious when
reading the code!

> Double checking the code, it can actually terminate the request,
> which doesn't make much difference for us because multishots
> should normally never end up in io-wq anyway, but I guess we
> can improve it a liitle bit.

Right, assumptions seems to be that -EAGAIN will lead to poll arm, which
seems a bit fragile.

> And it should also use IO_URING_F_IOWQ, forgot I split out
> it from *F_UNLOCK.

Yep, that'd be clearer.

>>> +static int zc_rx_recv_frag(struct io_zc_rx_ifq *ifq, const skb_frag_t *frag,
>>> +               int off, int len, unsigned sock_idx)
>>> +{
>>> +    off += skb_frag_off(frag);
>>> +
>>> +    if (likely(page_is_page_pool_iov(frag->bv_page))) {
>>> +        struct io_uring_rbuf_cqe *cqe;
>>> +        struct io_zc_rx_buf *buf;
>>> +        struct page_pool_iov *ppiov;
>>> +
>>> +        ppiov = page_to_page_pool_iov(frag->bv_page);
>>> +        if (ppiov->pp->p.memory_provider != PP_MP_IOU_ZCRX ||
>>> +            ppiov->pp->mp_priv != ifq)
>>> +            return -EFAULT;
>>> +
>>> +        cqe = io_zc_get_rbuf_cqe(ifq);
>>> +        if (!cqe)
>>> +            return -ENOBUFS;
>>> +
>>> +        buf = io_iov_to_buf(ppiov);
>>> +        io_zc_rx_get_buf_uref(buf);
>>> +
>>> +        cqe->region = 0;
>>> +        cqe->off = io_buf_pgid(ifq->pool, buf) * PAGE_SIZE + off;
>>> +        cqe->len = len;
>>> +        cqe->sock = sock_idx;
>>> +        cqe->flags = 0;
>>> +    } else {
>>> +        return -EOPNOTSUPP;
>>> +    }
>>> +
>>> +    return len;
>>> +}
>>
>> I think this would read a lot better as:
>>
>>     if (unlikely(!page_is_page_pool_iov(frag->bv_page)))
>>         return -EOPNOTSUPP;
> 
> That's a bit of oracle coding, this branch is implemented in
> a later patch.

Oracle coding?

Each patch stands separately, there's no reason not to make this one as
clean as it can be. And an error case with the main bits inline is a lot
nicer imho than two separate indented parts. For the latter addition
instead of the -EOPNOTSUPP, would probably be nice to have it in a
separate function. Probably ditto for the page pool case here now, would
make the later patch simpler too.

-- 
Jens Axboe