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

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