Re: [RFC 2/2] io_uring: acquire ctx->uring_lock before calling io_issue_sqe()

From: Bijan Mottahedeh <bijan.mottahedeh@oracle.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Keith Busch <keith.busch@gmail.com>
Subject: Re: [RFC 2/2] io_uring: acquire ctx->uring_lock before calling io_issue_sqe()
Date: Tue, 28 Jan 2020 12:34:49 -0800	[thread overview]
Message-ID: <f56a8767-c754-b2e9-bfea-1ced197a05d7@oracle.com> (raw)
In-Reply-To: <a316d3fe-4162-8274-a74a-2d13a4caf011@kernel.dk>

On 1/16/2020 1:26 PM, Jens Axboe wrote:
> On 1/16/20 2:04 PM, Bijan Mottahedeh wrote:
>> On 1/16/2020 12:02 PM, Jens Axboe wrote:
>>> On 1/16/20 12:08 PM, Bijan Mottahedeh wrote:
>>>> On 1/16/2020 8:22 AM, Jens Axboe wrote:
>>>>> On 1/15/20 9:42 PM, Jens Axboe wrote:
>>>>>> On 1/15/20 9:34 PM, Jens Axboe wrote:
>>>>>>> On 1/15/20 7:37 PM, Bijan Mottahedeh wrote:
>>>>>>>> io_issue_sqe() calls io_iopoll_req_issued() which manipulates poll_list,
>>>>>>>> so acquire ctx->uring_lock beforehand similar to other instances of
>>>>>>>> calling io_issue_sqe().
>>>>>>> Is the below not enough?
>>>>>> This should be better, we have two that set ->in_async, and only one
>>>>>> doesn't hold the mutex.
>>>>>>
>>>>>> If this works for you, can you resend patch 2 with that? Also add a:
>>>>>>
>>>>>> Fixes: 8a4955ff1cca ("io_uring: sqthread should grab ctx->uring_lock for submissions")
>>>>>>
>>>>>> to it as well. Thanks!
>>>>> I tested and queued this up:
>>>>>
>>>>> https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.5&id=11ba820bf163e224bf5dd44e545a66a44a5b1d7a
>>>>>
>>>>> Please let me know if this works, it sits on top of the ->result patch you
>>>>> sent in.
>>>>>
>>>> That works, thanks.
>>>>
>>>> I'm however still seeing a use-after-free error in the request
>>>> completion path in nvme_unmap_data().  It happens only when testing with
>>>> large block sizes in fio, typically > 128k, e.g. bs=256k will always hit it.
>>>>
>>>> This is the error:
>>>>
>>>> DMA-API: nvme 0000:00:04.0: device driver tries to free DMA memory it
>>>> has not allocated [device address=0x6b6b6b6b6b6b6b6b] [size=1802201963
>>>> bytes]
>>>>
>>>> and this warning occasionally:
>>>>
>>>> WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_IDLE);
>>>>
>>>> It seems like a request might be issued multiple times but I can't see
>>>> anything in io_uring code that would account for it.
>>> Both of them indicate reuse, and I agree I don't think it's io_uring. It
>>> really feels like an issue with nvme when a poll queue is shared, but I
>>> haven't been able to pin point what it is yet.
>>>
>>> The 128K is interesting, that would seem to indicate that it's related to
>>> splitting of the IO (which would create > 1 IO per submitted IO).
>>>
>> Where does the split take place?  I had suspected that it might be
>> related to the submit_bio() loop in __blkdev_direct_IO() but I don't
>> think I saw multiple submit_bio() calls or maybe I missed something.
> See the path from blk_mq_make_request() -> __blk_queue_split() ->
> blk_bio_segment_split(). The bio is built and submitted, then split if
> it violates any size constraints. The splits are submitted through
> generic_make_request(), so that might be why you didn't see multiple
> submit_bio() calls.
>

I think the problem is in __blkdev_direct_IO() and not related to 
request size:

                         qc = submit_bio(bio);

                         if (polled)
                                 WRITE_ONCE(iocb->ki_cookie, qc);

The first call to submit_bio() when dio->is_sync is not set won't have 
acquired a bio ref through bio_get() and so the bio/dio could be freed 
when ki_cookie is set.

With the specific io_uring test, this happens because 
blk_mq_make_request()->blk_mq_get_request() fails and so terminates the 
request.

As for the fix for polled io (!is_sync) case, I'm wondering if 
dio->multi_bio is really necessary in __blkdev_direct_IO(). Can we call 
bio_get() unconditionally after the call to bio_alloc_bioset(), set 
dio->ref = 1, and increment it for additional submit bio calls?  Would 
it make sense to do away with multi_bio?

Also, I'm not clear on how is_sync + mult_bio case is supposed to work.  
__blkdev_direct_IO() polls for *a* completion in the request's hctx and 
not *the* request completion itself, so what does that tell us for 
multi_bio + is_sync? Is the polling supposed to guarantee that all 
constituent bios for a mult_bio request have completed before return?

--bijan

PS I couldn't see 256k requests being split via __blk_queue_split(), 
still not sure how that works.