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Subject: Re: BUG: unable to handle kernel NULL pointer dereference at
 00000000000000a8 in nilfs_segctor_do_co
To:     Hideki EIRAKU <hdk1983@gmail.com>, linux-nilfs@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org
References: <20200328.182640.1933740379722138264.hermes@ceres.dti.ne.jp>
 <20200430.213842.00392641.hdk1983@gmail.com>
From:   Tom <tommytoad0@gmail.com>
Message-ID: <ee5677b7-802b-f524-36cc-9d5ae071859b@gmail.com>
Date:   Thu, 30 Apr 2020 08:27:47 -0700
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Thank you!  This is very helpful information, and does seem to be a 
workaround.

Like you, I have my home directory on a separate NILFS2 filesystem. As a 
temporary solution, I removed the line from /etc/fstab for that 
filesystem and added your dd suggestion along with a manual mount of the 
home filesystem to /etc/rc.local.  /home is now mounted properly at boot 
with any of the newer kernels I tried.

Thanks,
Tom

On 4/30/20 5:38 AM, Hideki EIRAKU wrote:
>> In Msg <874kuapb2s.fsf@logand.com>;
>>     Subject "Re: BUG: unable to handle kernel NULL pointer dereference at 00000000000000a8 in nilfs_segctor_do_construct":
>>
>>> Tomas Hlavaty <tom@logand.com> writes:
>>>>>> 2) Can you mount the corrupted(?) partition from a recent version of
>>>>>> kernel ?
>>>
>>> I tried the following Linux kernel versions:
>>>
>>> - v4.19
>>> - v5.4
>>> - v5.5.11
>>>
>>> and still get the crash
> 
> I found conditions to reproduce this issue with Linux 5.7-rc3:
> 
> - CONFIG_MEMCG=y *and* CONFIG_BLK_CGROUP=y
> 
> - When the NILFS2 file system writes to a device, the device file has
>    never written by other programs since boot
> 
> The following is an example with CONFIG_MEMCG=y and
> CONFIG_BLK_CGROUP=y kernel.  If you do mkfs and mount it, it works
> because the mkfs command has written data to the device file before
> mounting:
> 
> # mkfs -t nilfs2 /dev/sda1
> mkfs.nilfs2 (nilfs-utils 2.2.7)
> Start writing file system initial data to the device
>         Blocksize:4096  Device:/dev/sda1  Device Size:267386880
> File system initialization succeeded !!
> # mount /dev/sda1 /mnt
> # touch /mnt
> # sync
> #
> 
> Loopback mount seems to be the same - if you do losetup, mkfs and
> mount on a loopback device, it works:
> 
> # losetup /dev/loop0 foo
> # mkfs -t nilfs2 /dev/loop0
> mkfs.nilfs2 (nilfs-utils 2.2.7)
> Start writing file system initial data to the device
>         Blocksize:4096  Device:/dev/loop0  Device Size:267386880
> File system initialization succeeded !!
> # mount /dev/sda1 /mnt
> # touch /mnt
> # sync
> #
> 
> But if you do mkfs on a file and use mount -o loop, it may fail,
> depending on whether the loopback device assigned by the mount command
> was used or not before mounting:
> 
> # /sbin/mkfs.nilfs2 ./foo
> mkfs.nilfs2 (nilfs-utils 2.2.7)
> Start writing file system initial data to the device
>         Blocksize:4096  Device:./foo  Device Size:268435456
> File system initialization succeeded !!
> # mount -o loop ./foo /mnt
> [   36.371331] NILFS (loop0): segctord starting. Construction interval = 5 seconds, CP frequency < 30 seconds
> # touch /mnt
> # sync
> [   40.252869] BUG: kernel NULL pointer dereference, address: 00000000000000a8
> (snip)
> 
> After reboot, it fails:
> 
> # mount /dev/sda1 /mnt
> [   14.021188] NILFS (sda1): segctord starting. Construction interval = 5 seconds, CP frequency < 30 seconds
> # touch /mnt
> # sync
> [   20.576309] BUG: kernel NULL pointer dereference, address: 00000000000000a8
> (snip)
> 
> But if you do dummy write to the device file before mounting, it
> works:
> 
> # dd if=/dev/sda1 of=/dev/sda1 count=1
> 1+0 records in
> 1+0 records out
> 512 bytes copied, 0.0135982 s, 37.7 kB/s
> # mount /dev/sda1 /mnt
> [   52.604560] NILFS (sda1): mounting unchecked fs
> [   52.613335] NILFS (sda1): recovery complete
> [   52.613877] NILFS (sda1): segctord starting. Construction interval = 5 seconds, CP frequency < 30 seconds
> # touch /mnt
> # sync
> #
> 
> # losetup /dev/loop0 foo
> # dd if=/dev/loop0 of=/dev/loop0 count=1
> 1+0 records in
> 1+0 records out
> 512 bytes copied, 0.0243797 s, 21.0 kB/s
> # mount /dev/loop0 /mnt
> [  271.915595] NILFS (loop0): mounting unchecked fs
> [  272.049603] NILFS (loop0): recovery complete
> [  272.049724] NILFS (loop0): segctord starting. Construction interval = 5 seconds, CP frequency < 30 seconds
> # touch /mnt
> # sync
> #
> 
> I think the dummy write is a simple workaround for now, unless
> mounting NILFS2 at boot time.  But I have been using NILFS2 /home for
> years, I would like to know better workarounds.
>