linux-ext4 - Re: [RFC] ext4: Fix fs can't panic when abort by user

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Date:   Fri, 9 Apr 2021 17:28:54 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Ye Bin <yebin10@...wei.com>
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] ext4: Fix fs can't panic when abort by user

On Thu, Apr 01, 2021 at 04:19:03PM +0800, Ye Bin wrote:
> Test steps:
> 1. mount /dev/sda -o errors=panic test
> 2. mount /dev/sda -o remount,ro test
> 3. mount /dev/sda -o remount,abort test
> 
> Before 014c9caa29d3 not been merged there will trigger panic. But
> 014c9caa29d3 change this behavior.

This makes sense, but I'll note this behavior has changed over time.

root@...-xfstests:~# mount -o errors=panic /dev/vdc /vdc
[   20.252713] EXT4-fs (vdc): mounted filesystem with ordered data mode. Opts: errors=panic
root@...-xfstests:~# mount -o remount,ro /dev/vdc
[   24.832448] EXT4-fs (vdc): re-mounted. Opts: (null)
root@...-xfstests:~# mount -o remount,abort /dev/vdc
[   30.833543] EXT4-fs error (device vdc): ext4_remount:5340: Abort forced by user
mount: /vdc: cannot remount /dev/vdc read-write, is write-protected.
root@...-xfstests:~# mount -o remount,abort,ro /dev/vdc
[   34.545549] EXT4-fs error (device vdc): ext4_remount:5340: Abort forced by user
[   34.555475] EXT4-fs (vdc): re-mounted. Opts: abort
root@...-xfstests:~# uname -a
Linux kvm-xfstests 4.19.163-xfstests #1 SMP Sat Dec 19 23:55:11 EST 2020 x86_64 GNU/Linux
root@...-xfstests:~#

The same is true for the 5.4 kernel.

I do agree that it *should* force a panic, and the fact that
superblock is read-only shouldn't make a difference as to how
errors=panic is handled.

So I think the patch is correct, but I'll note that it also changes
this case:

1)  mount -o /dev/sda -o ro,errors=panic test
2)  echo test > /sys/fs/ext4/sda/trigger_fs_error

With this patch, this will also now panic, whereas before, an
ext4_error() would not trigger a panic.  I think that's better because
it makes things more consistent --- but it is a change in behavior
which could potentially surprise some people.  But since they can
easily get the previous behavior with an explicit "mount -o
ro,errors=continue", I think that's acceptable.

I'll apply the patch with a modified commit description to warn of
this particular change in behavior.

						- Ted