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

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/9/5 上午6:46, Pavel Begunkov 写道:
> 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.
I think leverage the highest bit of ioprio may be a good idea?
> 
>> Probably best to solve that issue in include/linux/net.h, ala:
>>
>> /* Flags for socket, socketpair, accept4 */
>> #define SOCK_CLOEXEC	O_CLOEXEC
>> #ifndef SOCK_NONBLOCK
>> #define SOCK_NONBLOCK	O_NONBLOCK
>> #endif
>>
>> /*
>>   * Only used for io_uring accept4, and deliberately chosen to overlap
>>   * with the O_* file bits for read/write mode so we won't risk overlap
>>   * other flags added for socket/socketpair/accept4 use in the future.
>>   */
>> #define SOCK_URING_MULTISHOT	00000001
>>
>> which should be OK, as these overlap with the O_* filespace and the
>> read/write bits are at the start of that space.
>>
>> Should be done as a prep patch and sent out to netdev as well, so we can
>> get their sign-off on this "hack". If we can get that done, then we have
>> our flag and we can just stuff it in accept->flags as long as we clear
>> it before calling into accept from io_uring.
>>
>