[Bug 252821] UFS filesystem panics after deadlock during mksnap_ffs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252821

            Bug ID: 252821
           Summary: UFS filesystem panics after deadlock during mksnap_ffs
           Product: Base System
           Version: 12.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: ml at netfence.it

Backstory of this is in #244048.
Briefly, a test VM with all debugs turned on (INVARIANT, WITNESS, etc...)
deadlocked while taking a snapshot of a non-root filesystem and had to be
reset.

Now I've got a snapshot where:
_ at reboot fsck is run on /, then system starts; after 1 minute a background
fsck is run and this causes a panic;
_ I can boot in single user mode, run fsck -y on the filesystem and boot works
again; however I get the same panic if I try taking a new snapshot.

This makes me think the filesystem is somehow ruined and fsck -y won't fix it.

Panic trace is:
panic: ffs_copyonwrite: bad copy block
cpuid = 0
time = 1581243816
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe001beef0e0
vpanic() at vpanic+0x19d/frame 0xfffffe001beef130
panic() at panic+0x43/frame 0xfffffe001beef190
ffs_copyonwrite() at ffs_copyonwrite+0x74c/frame 0xfffffe001beef230
ffs_geom_strategy() at ffs_geom_strategy+0x8c/frame 0xfffffe001beef260
ufs_strategy() at ufs_strategy+0x83/frame 0xfffffe001beef290
VOP_STRATEGY_APV() at VOP_STRATEGY_APV+0xc9/frame 0xfffffe001beef2c0
bufstrategy() at bufstrategy+0x44/frame 0xfffffe001beef2f0
bufwrite() at bufwrite+0x230/frame 0xfffffe001beef330
ffs_snapshot() at ffs_snapshot+0x8e0/frame 0xfffffe001beef630
ffs_mount() at ffs_mount+0xb3a/frame 0xfffffe001beef7d0
vfs_domount() at vfs_domount+0x8b6/frame 0xfffffe001beef9f0
vfs_donmount() at vfs_donmount+0x7e7/frame 0xfffffe001beefa90
sys_nmount() at sys_nmount+0xf2/frame 0xfffffe001beefac0
amd64_syscall() at amd64_syscall+0x281/frame 0xfffffe001beefbf0
fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe001beefbf0
--- syscall (378, FreeBSD ELF64, sys_nmount), rip = 0x8002d88ba, rsp =
0x7fffffffd288, rbp = 0x7fffffffeae0 ---
KDB: enter: panic

I can connect with remote GDB to this VM, but my knowledge of kernel internal
is not enough to debug this all by myself.

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