[dm-devel] [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

[Bryn M. Reeves]

On Wed, Apr 06, 2022 at 12:52:11PM -0400, Mike Snitzer wrote:
> On Tue, Apr 05 2022 at 10:09P -0400, Ming Lei <ming.lei at redhat.com> wrote:
> > Because it is null block...
> 
> Ha, yes.. very good point. I was expecting null_blk capability (read
> back written data) that it was never intended to provide. Sorry ;)

You can configure a memory backed nullblk instance using the configfs
interface that will return written data on reads:

  # modprobe null_blk nr_devices=0
  # mkdir -p /sys/kernel/config/nullb/nullb0
  # echo 500 > /sys/kernel/config/nullb/nullb0/size
  # echo 1 > /sys/kernel/config/nullb/nullb0/memory_backed
  # echo 1 > /sys/kernel/config/nullb/nullb0/power 
  # lsblk | grep null
  nullb0               250:0    0  500M  0 disk 

It's good enough to make and mount an ext4 file system; the contents
persist across umount as you'd expect and the image will pass an fsck:

  # mkfs.ext4 /dev/nullb0 
  mke2fs 1.46.3 (27-Jul-2021)
  ...

However there's still something in null_blk that causes fio's verify
mode some indigestion:

  # fio --bs=4096 --ioengine=io_uring --fixedbufs --registerfiles --hipri=1 --iodepth=2 --iodepth_batch_submit=2 --iodepth_batch_complete_min=2 --filename=/dev/nullb0 --direct=1 --runtime=20 --numjobs=2 --rw=randread --name=test --group_reporting --norandommap --do_verify=1 --verify=crc32c --verify_async=1
  test: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=io_uring, iodepth=2
  ...
  fio-3.26
  Starting 2 processes
  fio: filehash.c:64: __lookup_file_hash: Assertion `f->fd != -1' failed.
  verify: bad magic header 0, wanted acca at file /dev/nullb0 offset 31621120, length 4096 (requested block: offset=31621120, length=4096)
  verify: bad magic header 0, wanted acca at file /dev/nullb0 offset 388124672, length 4096 (requested block: offset=388124672, length=4096)
  fio: pid=2074, err=84/file:verify.c:1444, func=async_verify, error=Invalid or incomplete multibyte or wide character
  fio: pid=2075, got signal=6
  Jobs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  Jobs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  Jobs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  
  ^Cbs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  fio: terminating on signal 2

I haven't looked into it any further at this point (I was looking at
null_blk to evaluate it for use in stacker, as a backing device for
test stacks).

Regards,
Bryn.