Re: WRITEV with IOSQE_ASYNC broken?

[Jens Axboe <axboe@kernel.dk>]

On 9/4/20 11:50 PM, Pavel Begunkov wrote:
> On 05/09/2020 07:35, Jens Axboe wrote:
>> On 9/4/20 9:57 PM, Jens Axboe wrote:
>>> On 9/4/20 9:53 PM, Jens Axboe wrote:
>>>> On 9/4/20 9:22 PM, nick@nickhill.org wrote:
>>>>> Hi,
>>>>>
>>>>> I am helping out with the netty io_uring integration, and came across 
>>>>> some strange behaviour which seems like it might be a bug related to 
>>>>> async offload of read/write iovecs.
>>>>>
>>>>> Basically a WRITEV SQE seems to fail reliably with -BADADDRESS when the 
>>>>> IOSQE_ASYNC flag is set but works fine otherwise (everything else the 
>>>>> same). This is with 5.9.0-rc3.
>>>>
>>>> Do you see it just on 5.9-rc3, or also 5.8? Just curious... But that is
>>>> very odd in any case, ASYNC writev is even part of the regular tests.
>>>> Any sort of deferral, be it explicit via ASYNC or implicit through
>>>> needing to retry, saves all the needed details to retry without
>>>> needing any of the original context.
>>>>
>>>> Can you narrow down what exactly is being written - like file type,
>>>> buffered/O_DIRECT, etc. What file system, what device is hosting it.
>>>> The more details the better, will help me narrow down what is going on.
>>>
>>> Forgot, also size of the IO (both total, but also number of iovecs in
>>> that particular request.
>>>
>>> Essentially all the details that I would need to recreate what you're
>>> seeing.
>>
>> Turns out there was a bug in the explicit handling, new in the current
>> -rc series. Can you try and add the below?
> 
> Hah, absolutely the same patch was in a series I was going to send
> today, but with a note that it works by luck so not a bug. Apparently,
> it is :)> 
> BTW, const in iter->iov is guarding from such cases, yet another proof
> that const casts are evil.

Definitely, not a great idea to begin with...

-- 
Jens Axboe