[RFC PATCH 0/1] nbd: fix crash when unmaping nbd device with fs still mounted

[Ming Lin]

Hi all,

I run into a BUG_ON(!buffer_mapped(bh)) crash with below script.

 $ rbd-nbd map mypool/myimg
 $ mkfs.ext4 /dev/nbd0
 $ mount /dev/nbd0 /mnt/
 $ rbd-nbd unmap /dev/nbd0
 $ umount /mnt

[ 1248.870131] kernel BUG at /home/mlin/linux/fs/buffer.c:3103!
[ 1248.871214] invalid opcode: 0000 [#1] SMP
[ 1248.879468] CPU: 0 PID: 2450 Comm: umount Tainted: G            E   4.11.0-rc2+ #2
[ 1248.896579] Call Trace:
[ 1248.897056]  __sync_dirty_buffer+0x6e/0xe0
[ 1248.897870]  ext4_commit_super+0x1eb/0x290 [ext4]
[ 1248.898795]  ext4_put_super+0x2fa/0x3c0 [ext4]
[ 1248.899662]  generic_shutdown_super+0x6f/0x100
[ 1248.900525]  kill_block_super+0x27/0x70
[ 1248.901257]  deactivate_locked_super+0x43/0x70
[ 1248.902112]  deactivate_super+0x46/0x60
[ 1248.902869]  cleanup_mnt+0x3f/0x80
[ 1248.903526]  __cleanup_mnt+0x12/0x20
[ 1248.904218]  task_work_run+0x83/0xb0
[ 1248.904941]  exit_to_usermode_loop+0x59/0x7b
[ 1248.905769]  do_syscall_64+0x165/0x180
[ 1248.907603]  entry_SYSCALL64_slow_path+0x25/0x25

Last year, Ratna posted a patch to fix it.
https://lkml.org/lkml/2016/4/20/257

Ratna's script to reproduce the bug.

 $ qemu-img create -f qcow2 f.img 1G
 $ mkfs.ext4 f.img
 $ qemu-nbd -c /dev/nbd0 f.img
 $ mount /dev/nbd0 dir
 $ killall -KILL qemu-nbd
 $ sleep 1
 $ ls dir
 $ umount dir

I ported Rantna's patch to 4.11-rc2 and confirmed that it fixes the crash.

Jan Kara had some comments about this bug:
http://www.kernelhub.org/?p=2&msg=361407

I hope to fix this bug in the upstream kernel first and then back port it to 
our production system.

Please see "PATCH 1/1" for detail.

Thanks,
Ming