panic: vm_page_unhold: hold count < 0!!!
cpuid = 0
time = 1559918772
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x47/frame 0xfffffe0020e456d0
vpanic() at vpanic+0x1e0/frame 0xfffffe0020e45730
panic() at panic+0x43/frame 0xfffffe0020e45790
vm_page_unhold_pages() at vm_page_unhold_pages+0x1e3/frame 0xfffffe0020e457e0
pipe_write() at pipe_write+0x16d8/frame 0xfffffe0020e458b0
dofilewrite() at dofilewrite+0xfd/frame 0xfffffe0020e45910
kern_writev() at kern_writev+0x66/frame 0xfffffe0020e45950
sys_writev() at sys_writev+0x50/frame 0xfffffe0020e45980
amd64_syscall() at amd64_syscall+0x436/frame 0xfffffe0020e45ab0
fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe0020e45ab0
--- syscall (198, FreeBSD ELF64, nosys), rip = 0x4131ba, rsp = 0x7fffdffdcf38, rbp = 0x3 ---
KDB: enter: panic
[ thread pid 8175 tid 101025 ]
Stopped at      kdb_enter+0x6a: movq    $0,kdb_why