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

[Jens Axboe <axboe@kernel.dk>]

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...

-- 
Jens Axboe