Re: [PATCH] io_uring: don't mark S_ISBLK async work as unbounded

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

在 2021/4/2 下午6:48, Pavel Begunkov 写道:
> On 02/04/2021 11:32, Pavel Begunkov wrote:
>> On 02/04/2021 09:52, Hao Xu wrote:
>>> 在 2021/4/1 下午10:57, Jens Axboe 写道:
>>>> S_ISBLK is marked as unbounded work for async preparation, because it
>>>> doesn't match S_ISREG. That is incorrect, as any read/write to a block
>>>> device is also a bounded operation. Fix it up and ensure that S_ISBLK
>>>> isn't marked unbounded.
>>>>
>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>
>>> Hi Jens, I saw a (un)bounded work is for a (un)bounded worker to
>>> execute. What is the difference between bounded and unbounded?
>>
>> Unbounded works are not bounded in execution time, i.e. they may take
>> forever to complete. E.g. recv depends on the other end to send something,
>> that not necessarily will ever happen.
> 
> To elaborate a bit, one example of how it's used: because unbounded may
> stay for long, it always spawns a new worker thread for each of them.
> 
> If app submits SQEs as below, and send's are not actually sent for execution
> but stashed somewhere internally in a list, e.g. waiting for a worker thread
> to get free, it would just hang from the userspace perspective.
> 
> recv(fd1), recv(fd1), send(fd1), send(fd1)
> 
Hi Pavel, thank you for the patient explanation, I got the meaning of
  bound/unbond now, but it seems there is no difference handling bounded 
and unbounded work in the current code?
>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>> index 6d7a1b69712b..a16b7df934d1 100644
>>>> --- a/fs/io_uring.c
>>>> +++ b/fs/io_uring.c
>>>> @@ -1213,7 +1213,7 @@ static void io_prep_async_work(struct io_kiocb *req)
>>>>        if (req->flags & REQ_F_ISREG) {
>>>>            if (def->hash_reg_file || (ctx->flags & IORING_SETUP_IOPOLL))
>>>>                io_wq_hash_work(&req->work, file_inode(req->file));
>>>> -    } else {
>>>> +    } else if (!req->file || !S_ISBLK(file_inode(req->file)->i_mode)) {
>>>>            if (def->unbound_nonreg_file)
>>>>                req->work.flags |= IO_WQ_WORK_UNBOUND;
>>>>        }
>