Re: [ext4 io hang] buffered write io hang in balance_dirty_pages

Keith Busch
2023-05-04T15:59:56Z

On Thu, Apr 27, 2023 at 10:20:28AM +0800, Ming Lei wrote:
> Hello Guys,
>
> I got one report in which buffered write IO hangs in balance_dirty_pages,
> after one nvme block device is unplugged physically, then umount can't
> succeed.
>
> Turns out it is one long-term issue, and it can be triggered at least
> since v5.14 until the latest v6.3.
>
> And the issue can be reproduced reliably in KVM guest:
>
> 1) run the following script inside guest:
>
> mkfs.ext4 -F /dev/nvme0n1
> mount /dev/nvme0n1 /mnt
> dd if=/dev/zero of=/mnt/z.img&
> sleep 10
> echo 1 > /sys/block/nvme0n1/device/device/remove
>
> 2) dd hang is observed and /dev/nvme0n1 is gone actually

Sorry to jump in so late.

For an ungraceful nvme removal, like a surpirse hot unplug, the driver
sets the capacity to 0 and that effectively ends all dirty page writers
that could stall forward progress on the removal. And that 0 capacity
should also cause 'dd' to exit.

But this is not an ungraceful removal, so we're not getting that forced
behavior. Could we use the same capacity trick here after flushing any
outstanding dirty pages?