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Date:   Sat, 6 Apr 2019 19:03:52 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Jiufei Xue <jiufei.xue@...ux.alibaba.com>
Cc:     linux-ext4@...r.kernel.org, jack@...e.cz, renzhen@...ux.alibaba.com
Subject: Re: [PATCH v2] jbd2: check superblock mapped prior to committing

On Thu, Mar 28, 2019 at 06:07:50PM +0800, Jiufei Xue wrote:
> We hit a BUG at fs/buffer.c:3057 if we detached the nbd device
> before unmounting ext4 filesystem.
> 
> The typical chain of events leading to the BUG:
> jbd2_write_superblock
>   submit_bh
>     submit_bh_wbc
>       BUG_ON(!buffer_mapped(bh));
> 
> The block device is removed and all the pages are invalidated. JBD2
> was trying to write journal superblock to the block device which is
> no longer present.
> 
> Fix this by checking the journal superblock's buffer head prior to
> submitting.

Thanks, applied.

					- Ted

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