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Mon, 06 Sep 2021 08:32:34 -0700 (PDT) To: Hao Xu , Jens Axboe Cc: io-uring@vger.kernel.org, Joseph Qi References: <20210903110049.132958-1-haoxu@linux.alibaba.com> <20210903110049.132958-7-haoxu@linux.alibaba.com> <95387504-3986-77df-7cb4-d136dd4be1ec@linux.alibaba.com> <701e50f5-2444-5b56-749b-1c1affc26ce9@gmail.com> From: Pavel Begunkov Subject: Re: [PATCH 6/6] io_uring: enable multishot mode for accept Message-ID: Date: Mon, 6 Sep 2021 16:32:01 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 9/6/21 1:35 PM, Hao Xu wrote: > 在 2021/9/6 上午3:44, Jens Axboe 写道: >> On 9/4/21 4:46 PM, Pavel Begunkov wrote: >>> On 9/4/21 7:40 PM, Jens Axboe wrote: >>>> On 9/4/21 9:34 AM, Hao Xu wrote: >>>>> 在 2021/9/4 上午12:29, Jens Axboe 写道: >>>>>> On 9/3/21 5:00 AM, Hao Xu wrote: >>>>>>> Update io_accept_prep() to enable multishot mode for accept operation. >>>>>>> >>>>>>> Signed-off-by: Hao Xu >>>>>>> --- >>>>>>>    fs/io_uring.c | 14 ++++++++++++-- >>>>>>>    1 file changed, 12 insertions(+), 2 deletions(-) >>>>>>> >>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c >>>>>>> index eb81d37dce78..34612646ae3c 100644 >>>>>>> --- a/fs/io_uring.c >>>>>>> +++ b/fs/io_uring.c >>>>>>> @@ -4861,6 +4861,7 @@ static int io_recv(struct io_kiocb *req, unsigned int issue_flags) >>>>>>>    static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe) >>>>>>>    { >>>>>>>        struct io_accept *accept = &req->accept; >>>>>>> +    bool is_multishot; >>>>>>>           if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL)) >>>>>>>            return -EINVAL; >>>>>>> @@ -4872,14 +4873,23 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe) >>>>>>>        accept->flags = READ_ONCE(sqe->accept_flags); >>>>>>>        accept->nofile = rlimit(RLIMIT_NOFILE); >>>>>>>    +    is_multishot = accept->flags & IORING_ACCEPT_MULTISHOT; >>>>>>> +    if (is_multishot && (req->flags & REQ_F_FORCE_ASYNC)) >>>>>>> +        return -EINVAL; >>>>>> >>>>>> I like the idea itself as I think it makes a lot of sense to just have >>>>>> an accept sitting there and generating multiple CQEs, but I'm a bit >>>>>> puzzled by how you pass it in. accept->flags is the accept4(2) flags, >>>>>> which can currently be: >>>>>> >>>>>> SOCK_NONBLOCK >>>>>> SOCK_CLOEXEC >>>>>> >>>>>> While there's not any overlap here, that is mostly by chance I think. A >>>>>> cleaner separation is needed here, what happens if some other accept4(2) >>>>>> flag is enabled and it just happens to be the same as >>>>>> IORING_ACCEPT_MULTISHOT? >>>>> Make sense, how about a new IOSQE flag, I saw not many >>>>> entries left there. >>>> >>>> Not quite sure what the best approach would be... The mshot flag only >>>> makes sense for a few request types, so a bit of a shame to have to >>>> waste an IOSQE flag on it. Especially when the flags otherwise passed in >>>> are so sparse, there's plenty of bits there. >>>> >>>> Hence while it may not be the prettiest, perhaps using accept->flags is >>>> ok and we just need some careful code to ensure that we never have any >>>> overlap. >>> >>> Or we can alias with some of the almost-never-used fields like >>> ->ioprio or ->buf_index. >> >> It's not a bad idea, as long as we can safely use flags from eg ioprio >> for cases where ioprio would never be used. In that sense it's probably >> safer than using buf_index. >> >> The alternative is, as has been brougt up before, adding a flags2 and >> reserving the last flag in ->flags to say "there are flags in flags2". >> Not exactly super pretty either, but we'll need to extend them at some >> point. > I'm going to do it in this way, there is another thing we have to do: > extend req->flags too, since flags we already used > 32 if we add > sqe->ext_flags We still have 2 bits left, and IIRC you wanted to take only 1 of them. We don't need extending it at the moment, it sounded to me like a plan for the future. No extra trouble for now Anyway, I can't think of many requests working in this mode, and I think sqe_flags should be taken only for features applicable to all (~most) of requests. Maybe we'd better to fit it individually into accept in the end? Sounds more plausible tbh p.s. yes, there is IOSQE_BUFFER_SELECT, but I don't think that was the best solution, but in any case it's history. -- Pavel Begunkov