When a filesystem is mounted on a nbd device and on a disconnect, because
of kill_bdev(), and resetting bdev size to zero, buffer_head mappings are
getting destroyed under mounted filesystem.
After a bdev size reset(i.e bdev->bd_inode->i_size = 0) on a disconnect,
followed by a sys_umount(),
generic_shutdown_super()->...
->__sync_blockdev()->...
-blkdev_writepages()->...
->do_invalidatepage()->...
-discard_buffer() is discarding superblock buffer_head assumed
to be in mapped state by ext4_commit_super().
[mlin: ported to 4.11-rc2]
Signed-off-by: Ratna Manoj Bolla <manoj.br@gmail.com
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
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