linux-ext4 - Re: Online resizing of filesystem mounted with backup superblock causes corruption

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Message-ID: <20141224223316.GD29088@thunk.org>
Date:	Wed, 24 Dec 2014 17:33:16 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Maxim Malkov <malkov@...ras.ru>
Cc:	linux-ext4@...r.kernel.org, Andrey Tsyvarev <tsyvarev@...ras.ru>,
	Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Re: Online resizing of filesystem mounted with backup superblock
 causes corruption

On Wed, Dec 24, 2014 at 01:32:42AM +0300, Maxim Malkov wrote:
> # mkfs.ext4 /dev/loop0 100M # adjust if necessary, should be less than
> partition size
> 
> Superblock backups stored on blocks:
>         8193, 24577, 40961, 57345, 73729
> (pick one)
> 
> # mount -t ext4 -o sb=40961 /dev/loop0 /media/
> # resize2fs /dev/loop0
> resize2fs 1.42.12 (29-Aug-2014)
> Filesystem at /dev/loop0 is mounted on /media; on-line resizing required
> old_desc_blocks = 1, new_desc_blocks = 4
> 
> Now, depending on your luck, various things may happen. resize2fs may hang.
> You might see BUG() in your dmesg. Either way, system will become
> severely corrupted (as indicated by fsck). You won't be able to mount
> it, either.

Thanks for the bug report; I've can reproduce this using a 3.18 based
kernel.

It may be that simplest bug fix is to simply disallow resizing if we
are using a backup superblock.

						- Ted
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