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Date:   Fri, 5 May 2023 10:06:28 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Keith Busch <kbusch@kernel.org>
Cc:     Theodore Ts'o <tytso@mit.edu>, linux-ext4@vger.kernel.org,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        linux-block@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        Dave Chinner <dchinner@redhat.com>,
        Eric Sandeen <sandeen@redhat.com>,
        Christoph Hellwig <hch@lst.de>, Zhang Yi <yi.zhang@redhat.com>,
        ming.lei@redhat.com
Subject: Re: [ext4 io hang] buffered write io hang in balance_dirty_pages
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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