linux-ext4 - Re: [bug report] tune2fs: filesystem inconsistency occurs by concurrent write

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Date:   Mon, 26 Jun 2023 19:56:45 +0800
From:   zhanchengbin <zhanchengbin1@...wei.com>
To:     Theodore Ts'o <tytso@....edu>
CC:     <linux-ext4@...r.kernel.org>, linfeilong <linfeilong@...wei.com>,
        <louhongxiang@...wei.com>, <liuzhiqiang26@...wei.com>
Subject: Re: [bug report] tune2fs: filesystem inconsistency occurs by
 concurrent write

Thanks for your explanation, it's a great idea.

  - bin.

On 2023/6/26 10:17, Theodore Ts'o wrote:
> On Mon, Jun 26, 2023 at 12:00:08AM +0800, zhanchengbin wrote:
>> Tune2fs does not recognize writes to the manipulated filesystem in another
>> namespace, there will be two simultaneous write operations on a
>> block, resulting in filesystem inconsistencies.
> 
> What you are reporting has nothing to do with namespaces, since
> "tune2fs -e remount-ro /dev/sdb" is something which is allowed
> regardless of whether the file system is mounted.  What reproduction
> is effectively doing is trying to set up a race between when tune2fs
> writes a byte to update to update the errors behavior, and when the
> actual unmount of the file system happens (e.g., when the last
> namespace unmounts the file system).  At that point, the kernel is
> going to be updating the superblock as part of the unmount, and then
> it calculates the superblock, and then it writes out the superblock.
> 
> If the tune2fs races with the unmount, it's possible for the tune2fs
> update of the error beavhiour bit, and the update of the superblock
> checksum, to race with the kernel's final update of the superblock,
> includinig its attempt to update the checksum.
> 
> There are some workarounds to this, but ultimately, we need to replace
> the ad-hoc modification of the block device by tune2fs with some
> ioctls which specifically update superblock when the file system
> mounted. >
> As far as whether or not tune2fs can detect if the file system is
> mounted, what we can do is check to see if the block device is busy.
> If it is mounted in some other namespace, we won't be able to see it
> mounted in /proc/self/mounts, but we can see that it's not possible to
> open the block device with O_EXCL.
> 
> Compare:
> 
> root@...-xfstests:~# /vtmp/tst_ismounted /dev/vdc
> Device /dev/vdc reports flags 31
>          /dev/vdc is apparently in use.
>          /dev/vdc is mounted.
>          /dev/vdc is mounted on /vdc.
> 
> and then "unshare -m" in another terminal, followed by umount /dev/vdc
> in the first terminal:
> 
> root@...-xfstests:~# /vtmp/tst_ismounted /dev/vdc
> Device /dev/vdc reports flags 10
>          /dev/vdc is apparently in use.
> 
> ... and then after we exit the last mount namespace which was keeping
> /dev/vdc mounted:
> 
> root@...-xfstests:~# [ 2409.811328] EXT4-fs (vdc): unmounting filesystem bdc026fd-85a8-4ccf-94f8-961487000293.
> root@...-xfstests:~# /vtmp/tst_ismounted /dev/vdc
> Device /dev/vdc reports flags 00
> 
> Cheers,
> 
> 						- Ted
> 
> .
>