qemu-nbd -c /dev/nbd0 fails to map partitions

[Chris J Arges]

[impact]
4.4.0-101+ (also 4.4.0-109) Ubuntu kernels no longer can use qemu-nbd to mount virtual disks onto a local filesystem (see test case for reproducer). 4.4.0-98 works without issue.

I was able to bisect the issue down to 907196aba0d891c6d580abe69c36d3b4fd512dcc (a cherry-pick from 37091fdd831f28a6509008542174ed324dd645bc). Reverting this from 4.4 fixes the issue.

Installing the mainline kernel shows mainline is _not_ affected by this bug.

In addition this is also reported in debian here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=829532

[test case]
# given a vmdk disk with multiple partitions
sudo modprobe nbd
sudo qemu-nbd -d /dev/nbd0
sudo qemu-nbd -r -c /dev/nbd0 disk.vmdk
ls /dev/nbd0*
# here you won't see multiple partition, nor be able to mount a partition locally

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1743026

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Chris J Arges (arges)]

This patch seems to fix the issue:
```
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 5f807a6..b761971 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -98,11 +98,6 @@ static inline struct device *nbd_to_dev(struct nbd_device *nbd)
        return disk_to_dev(nbd->disk);
 }

-static bool nbd_is_connected(struct nbd_device *nbd)
-{
- return !!nbd->task_recv;
-}
-
 static const char *nbdcmd_to_ascii(int cmd)
 {
        switch (cmd) {
@@ -126,9 +121,6 @@ static int nbd_size_clear(struct nbd_device *nbd, struct block_device *bdev)

 static void nbd_size_update(struct nbd_device *nbd, struct block_device *bdev)
 {
- if (!nbd_is_connected(nbd))
- return;
-
        bdev->bd_inode->i_size = nbd->bytesize;
        set_capacity(nbd->disk, nbd->bytesize >> 9);
        kobject_uevent(&nbd_to_dev(nbd)->kobj, KOBJ_CHANGE);
```

[Chris J Arges (arges)]

Just a note the patch in #2 is most likely not correct, just a way for me to narrow down the problem. Perhaps we missed a patch (or there is a fix) that makes nbd_is_connected to the correct thing.

[heavendang (heavendang)]

there is a way to workaround it, use partprobe

it is the original solution:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=824553