Re: [PATCH 6/6] io_uring: enable multishot mode for accept

[Pavel Begunkov <asml.silence@gmail.com>]

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 <haoxu@linux.alibaba.com>
>>>>>>> ---
>>>>>>>    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