Re: WRITEV with IOSQE_ASYNC broken?

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

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;
>  }
> 

-- 
Pavel Begunkov