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

Ming Lei
2023-05-05T02:07:36Z

On Thu, May 04, 2023 at 09:59:52AM -0600, Keith Busch wrote:
> 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.

Actually nvme device has been gone, and the hang just happens in
balance_dirty_pages() from generic_perform_write().

The issue should be triggered on all kinds of disks which can be hot-unplug,
and it can be duplicated on both ublk and nvme easily.

>
> 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?

set_capacity(0) has been called in del_gendisk() after fsync_bdev() &
__invalidate_device(), but I understand FS code just try best to flush dirty
pages. And when the bdev is gone, these un-flushed dirty pages need cleanup,
otherwise they can't be used any more.

Thanks,
Ming