Re: WRITEV with IOSQE_ASYNC broken?

[nick@nickhill.org]

On 2020-09-04 22:50, 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.
> 
>> 
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 0d7be2e9d005..000ae2acfd58 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -2980,14 +2980,15 @@ static inline int io_rw_prep_async(struct 
>> io_kiocb *req, int rw,
>>  				   bool force_nonblock)
>>  {
>>  	struct io_async_rw *iorw = &req->io->rw;
>> +	struct iovec *iov;
>>  	ssize_t ret;
>> 
>> -	iorw->iter.iov = iorw->fast_iov;
>> -	ret = __io_import_iovec(rw, req, (struct iovec **) &iorw->iter.iov,
>> -				&iorw->iter, !force_nonblock);
>> +	iorw->iter.iov = iov = iorw->fast_iov;
>> +	ret = __io_import_iovec(rw, req, &iov, &iorw->iter, 
>> !force_nonblock);
>>  	if (unlikely(ret < 0))
>>  		return ret;
>> 
>> +	iorw->iter.iov = iov;
>>  	io_req_map_rw(req, iorw->iter.iov, iorw->fast_iov, &iorw->iter);
>>  	return 0;
>>  }
>> 

Thanks for the speedy replies and finding/fixing this so fast! I'm new 
to kernel dev and haven't built my own yet but I think Norman is going 
to try out your patch soon.

Nick