persistent integer divide fault panic in zfs_rmnode

[Steven Schlansker]

Hi freebsd-fs,

I have a 8-way raidz2 system running FreeBSD 12.2-RELEASE-p1 GENERIC
Approximately since upgrading to FreeBSD 12.2-RELEASE, I receive a nasty
panic when trying to unlink any of a large number of files.

Fatal trap 18: integer divide fault while in kernel mode


The pool reports as healthy:

  pool: universe
 state: ONLINE
status: One or more devices are configured to use a non-native block size.
        Expect reduced performance.
action: Replace affected devices with devices that support the
        configured block size, or migrate data to a properly configured
        pool.
  scan: resilvered 416M in 0 days 00:08:35 with 0 errors on Thu Jan  7
02:16:03 2021
When some files are unlinked, the system panics with a partial backtrace of:

#6 0xffffffff82a148ce at zfs_rmnode+0x5e
#7 0xffffffff82a35612 at zfs_freebsd_reclaim+0x42
#8 0xffffffff812482db at VOP_RECLAIM_APV+0x7b
#9 0xffffffff80c8e376 at vgonel+0x216
#10 0xffffffff80c8e9c5 at vrecycle+0x45

I captured a dump, and using kgdb extracted a full backtrace, and filed it
as https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=250784

#8  0xffffffff82963725 in get_next_chunk (dn=0xfffff804325045c0,
start=<optimized out>, minimum=0, l1blks=<optimized out>)
    at /usr/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu.c:721
warning: Source file is more recent than executable.
721                 (roundup(*start, iblkrange) - (minimum / iblkrange *
iblkrange)) /
(kgdb) list
716              * L1 blocks in this range have data. If we can, we use this
717              * worst case value as an estimate so we can avoid having
to look
718              * at the object's actual data.
719              */
720             uint64_t total_l1blks =
721                 (roundup(*start, iblkrange) - (minimum / iblkrange *
iblkrange)) /
722                 iblkrange;
723             if (total_l1blks <= maxblks) {
724                     *l1blks = total_l1blks;
725                     *start = minimum;
(kgdb) print iblkrange
$1 = 0
(kgdb) print minimum
$2 = 0

It looks like it is attempting to compute 0 / 0, causing the panic.

How can I restore my zpool to a working state?  Thank you for any
assistance.