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Date:   Thu, 27 Apr 2023 12:50:48 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Matthew Wilcox <willy@infradead.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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References: <ZEnb7KuOWmu5P+V9@ovpn-8-24.pek2.redhat.com>
 <ZEny7Izr8iOc/23B@casper.infradead.org>
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Hello Matthew,

On Thu, Apr 27, 2023 at 04:58:36AM +0100, Matthew Wilcox 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.
> 
> That's a feature, not a bug ... the dd should continue indefinitely?

Can you explain what the feature is? And not see such 'issue' or 'feature'
on xfs.

The device has been gone, so IMO it is reasonable to see FS buffered write IO
failed. Actually dmesg has shown that 'EXT4-fs (nvme0n1): Remounting
filesystem read-only'. Seems these things may confuse user.

> 
> balance_dirty_pages() is sleeping in KILLABLE state, so kill -9 of
> the dd process should succeed.

Yeah, dd can be killed, however it may be any application(s), :-)

Fortunately it won't cause trouble during reboot/power off, given
userspace will be killed at that time.


Thanks,
Ming