Comment 0 for bug 1772575

Release 16.04, kernel 4.4.0-124-generic

There's an issue in kernel's NBD module which prevents some larger images to be
correctly "connected" , largely described here:
https://github.com/NetworkBlockDevice/nbd/issues/44

There is a small patch here that was accepted into mainline 4.10:
https://www.spinics.net/lists/linux-block/msg07060.html

This is a regression from the previous LTS 3.13 kernel.

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Here is a small example of the faulty behaviour:

# qemu-img create -f qcow2 test.img 1100G
Formatting 'test.img', fmt=qcow2 size=1181116006400 cluster_size=65536 lazy_refcounts=off refcount_bits=16
# qemu-nbd -c /dev/nbd0 test.img
# blockdev --getsize64 /dev/nbd0
18446743055802302464

The correct response would be 1181116006400; this breaks most tools and makes the image unusable, e.g.

# fdisk -l /dev/nbd0
fdisk: cannot open /dev/nbd0: Invalid argument