Re: Bug? CQE.res = -EAGAIN with nvme multipath driver

[Jens Axboe <axboe@kernel.dk>]

On 1/6/25 7:33 PM, Jens Axboe wrote:
> On 1/6/25 4:53 PM, Jens Axboe wrote:
>> On 1/6/25 1:03 PM, Haeuptle, Michael wrote:
>>> Hello,
>>>
>>> I?m using the nvme multipath driver (NVMF/RDMA) and io-uring. When a
>>> path goes away, I sometimes get a CQE.res = -EAGAIN in user space.
>>> This is unexpected since the nvme multipath driver should handle this
>>> transparently. It?s somewhat workload related but easy to reproduce
>>> with fio.
>>>
>>> The multipath driver uses kblockd worker to re-queue the failed NVME
>>> bios
>>> (https://github.com/torvalds/linux/blob/13563da6ffcf49b8b45772e40b35f96926a7ee1e/drivers/nvme/host/multipath.c#L126).
>>> The original request is ended. 
>>>
>>> When the nvme_requeue_work callback is executed, the blk layer tries
>>> to allocate a new request for the bios but that fails and the bio
>>> status is set to BLK_STS_AGAIN
>>> (https://elixir.bootlin.com/linux/v6.12.6/source/block/blk-mq.c#L2987).
>>> The failure to allocate a new req seems to be due to all tags for the
>>> queue being used up.
>>>
>>> Eventually, this makes it into io_uring?s io_rw_should_reissue and
>>> hits same_thread_group(req->tctx->task, current) = false (in
>>> https://github.com/torvalds/linux/blob/13563da6ffcf49b8b45772e40b35f96926a7ee1e/io_uring/rw.c#L437).
>>> As a result, CQE.res = -EAGAIN and thrown back to the user space
>>> program.
>>>
>>> Here?s a stack dump when we hit same_thread_group(req->tctx->task,
>>> current) = false 
>>>
>>> kernel: [237700.098733]  dump_stack_lvl+0x44/0x5c
>>> kernel: [237700.098737]  io_rw_should_reissue.cold+0x5d/0x64
>>> kernel: [237700.098742]  io_complete_rw+0x9a/0xc0
>>> kernel: [237700.098745]  blkdev_bio_end_io_async+0x33/0x80
>>> kernel: [237700.098749]  blk_mq_submit_bio+0x5b5/0x620
>>> kernel: [237700.098756]  submit_bio_noacct_nocheck+0x163/0x370
>>> kernel: [237700.098760]  ? submit_bio_noacct+0x79/0x4b0
>>> kernel: [237700.098764]  nvme_requeue_work+0x4b/0x60 [nvme_core]
>>> kernel: [237700.098776]  process_one_work+0x1c7/0x380
>>> kernel: [237700.098782]  worker_thread+0x4d/0x380
>>> kernel: [237700.098786]  ? _raw_spin_lock_irqsave+0x23/0x50
>>> kernel: [237700.098791]  ? rescuer_thread+0x3a0/0x3a0
>>> kernel: [237700.098794]  kthread+0xe9/0x110
>>> kernel: [237700.098798]  ? kthread_complete_and_exit+0x20/0x20
>>> kernel: [237700.098802]  ret_from_fork+0x22/0x30
>>> kernel: [237700.098811]  </TASK>
>>>
>>> Is the same_thread_group() check really needed in this case? The
>>> thread groups are certainly different? Any side effects if this check
>>> is being removed?
>>
>> It's their for safety reasons - across all request types, it's not
>> always safe. For this case, absolutely the check does not need to be
>> there. So probably best to ponder ways to bypass it selectively.
>>
>> Let me ponder a bit what the best approach would be here...
> 
> Actually I think we can just remove it. The actual retry will happen out
> of context anyway, and the comment about the import is no longer valid
> as the import will have been done upfront since 6.10.
> 
> Do you want to send a patch for that, or do you want me to send one out
> referencing this report?

Also see:

commit 039a2e800bcd5beb89909d1a488abf3d647642cf
Author: Jens Axboe <axboe@kernel.dk>
Date:   Thu Apr 25 09:04:32 2024 -0600

    io_uring/rw: reinstate thread check for retries

let me take a closer look tomorrow...

-- 
Jens Axboe