[Bug 244685] gjournal kernel panic with vfs.write_behind=0

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=244685

            Bug ID: 244685
           Summary: gjournal kernel panic with vfs.write_behind=0
           Product: Base System
           Version: 12.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: spy at spy.zp.ua

Trying to copy huge file to gjournaled filesystem causes reboot just after a
few seconds, if vfs.write_behind=0, and works fine with vfs.write_behind=1

Tested in such conditions under 12.1-RELEASE and 11.3-RELEASE:

#/dev/gpt/ada0-data - 200Gb on SATA disk
#/dev/gpt/nvd0-log - 1Gb on NVME disk

gjournal label -vh /dev/gpt/ada0-data /dev/gpt/nvd0-log

mount -o "async,atime=off" /dev/gpt/ada0-data.journal /dst
mount -o "async,atime=off" /dev/gpt/nvd0-data /src

dd if=/dev/random of=/src/60gb bs=1m count=60000
sysctl vfs.write_behind=0
cp /src/60gb /dst/


I didn't test it neither with slow /src device, nor with fast /dst device.

With bigger log device, we can get panic later, and with huge enough log device
(I tried 30Gb), it can work fine without reboots.

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