FreeBSD: Intialize zlog in zvol_first_open, this prevents a NULL reference later.

[oshogbo]

Motivation and Context

In the past the zilog was initialized in the zvol_first_open (https://svnweb.freebsd.org/base/stable/12/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zvol.c?revision=365689&view=markup#l842).
While the openzfs upgrade this was dropped.
This cause an page fault in the zvol_cdev_open function while accessing the ZVOL.

https://drive.google.com/file/d/1-dvj8eoUNqRWL5mcVtH5Px4NbrhLfzML/view?usp=sharing

Description

Initialize zlog in the zvol_first_open like in old version of ZFS.

How Has This Been Tested?

Tested on FreeBSD box.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[ghost]

The zil is opened on first write. It looks like the correct fix here is to check that the zvol has been written to before calling zil_async_to_sync in zvol_cdev_open:

--- a/module/os/freebsd/zfs/zvol_os.c
+++ b/module/os/freebsd/zfs/zvol_os.c
@@ -891,7 +891,8 @@ zvol_cdev_open(struct cdev *dev, int flags, int fmt, struct thread *td)
        if (flags & (FSYNC | FDSYNC)) {
                zsd = &zv->zv_zso->zso_dev;
                zsd->zsd_sync_cnt++;
-               if (zsd->zsd_sync_cnt == 1)
+               if (zsd->zsd_sync_cnt == 1 &&
+                   (zv->zv_flags & ZVOL_WRITTEN_TO) != 0)
                        zil_async_to_sync(zv->zv_zilog, ZVOL_OBJ);
        }

[behlendorf]

It looks like the correct fix here is to check that the zvol has been written to before calling zil_async_to_sync in zvol_cdev_open:

That's right, for additional context see 1eacf2b which added the ZVOL_WRITTEN_TO flag.

[oshogbo]

Thank you for pointing out that to me.
I fixed the patch.

[ghost]

Thanks for the PR!

[behlendorf]

@oshogbo can you rebase this on the latest commits from the master branch and force update the PR. This will resolve the Fedora and FreeBSD build failures and let is get a proper test run with the change.

[oshogbo]

Done.