Re: WRITEV with IOSQE_ASYNC broken?

[Norman Maurer <norman.maurer@googlemail.com>]

Yes … I will :) I am already compiling the kernel as we speak with the patch applied. Will report back later today. 



> On 5. Sep 2020, at 10:24, nick@nickhill.org wrote:
> 
> 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