[Bug 247552] FreeBSD virtual server fatal trap 12: page fault, supervisor read data, page not present

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=247552

            Bug ID: 247552
           Summary: FreeBSD virtual server fatal trap 12: page fault,
                    supervisor read data, page not present
           Product: Base System
           Version: 12.1-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: matthew.grice at anca.com

I am running FreeBSD 12.1 with the latest update on a vmware vSphere virtual
machine. The server is a web server running nginx, uwsgi and mariadb.

Twice this morning the machine has had a kernel page fault and dumped core. The
message was as follows:

Jun 26 12:58:41 ws-ag-au-17 kernel: Fatal trap 12: page fault while in kernel
mode
Jun 26 12:58:41 ws-ag-au-17 kernel: cpuid = 1; apic id = 02
Jun 26 12:58:41 ws-ag-au-17 kernel: fault virtual address       = 0x8
Jun 26 12:58:41 ws-ag-au-17 kernel: fault code          = supervisor read data,
page not present
Jun 26 12:58:41 ws-ag-au-17 kernel: instruction pointer =
0x20:0xffffffff80c658e0
Jun 26 12:58:41 ws-ag-au-17 kernel: stack pointer               =
0x28:0xfffffe00005cf7a0
Jun 26 12:58:41 ws-ag-au-17 kernel: frame pointer               =
0x28:0xfffffe00005cf7d0
Jun 26 12:58:41 ws-ag-au-17 kernel: code segment                = base 0x0,
limit 0xfffff, type 0x1b
Jun 26 12:58:41 ws-ag-au-17 kernel:                     = DPL 0, pres 1, long
1, def32 0, gran 1
Jun 26 12:58:41 ws-ag-au-17 kernel: processor eflags    = interrupt enabled,
resume, IOPL = 0
Jun 26 12:58:41 ws-ag-au-17 kernel: current process             = 1330 (nginx)
Jun 26 12:58:41 ws-ag-au-17 kernel: trap number         = 12
Jun 26 12:58:41 ws-ag-au-17 kernel: panic: page fault
Jun 26 12:58:41 ws-ag-au-17 kernel: cpuid = 1
Jun 26 12:58:41 ws-ag-au-17 kernel: time = 1593140291
Jun 26 12:58:41 ws-ag-au-17 kernel: KDB: stack backtrace:
Jun 26 12:58:41 ws-ag-au-17 kernel: #0 0xffffffff80c1d297 at kdb_backtrace+0x67
Jun 26 12:58:41 ws-ag-au-17 kernel: #1 0xffffffff80bd05cd at vpanic+0x19d
Jun 26 12:58:41 ws-ag-au-17 kernel: #2 0xffffffff80bd0423 at panic+0x43
Jun 26 12:58:41 ws-ag-au-17 kernel: #3 0xffffffff810a7dcc at trap_fatal+0x39c
Jun 26 12:58:41 ws-ag-au-17 kernel: #4 0xffffffff810a7e19 at trap_pfault+0x49
Jun 26 12:58:41 ws-ag-au-17 kernel: #5 0xffffffff810a740f at trap+0x29f
Jun 26 12:58:41 ws-ag-au-17 kernel: #6 0xffffffff81081a0c at calltrap+0x8
Jun 26 12:58:41 ws-ag-au-17 kernel: #7 0xffffffff80c64a01 at sbdestroy+0x41
Jun 26 12:58:41 ws-ag-au-17 kernel: #8 0xffffffff80c67225 at sofree+0x275
Jun 26 12:58:41 ws-ag-au-17 kernel: #9 0xffffffff80c67d77 at soclose+0x2f7
Jun 26 12:58:41 ws-ag-au-17 kernel: #10 0xffffffff80b7802a at _fdrop+0x1a
Jun 26 12:58:41 ws-ag-au-17 kernel: #11 0xffffffff80b7b151 at closef+0x241
Jun 26 12:58:41 ws-ag-au-17 kernel: #12 0xffffffff80b78547 at closefp+0x97
Jun 26 12:58:41 ws-ag-au-17 kernel: #13 0xffffffff810a8984 at
amd64_syscall+0x364
Jun 26 12:58:41 ws-ag-au-17 kernel: #14 0xffffffff81082330 at
fast_syscall_common+0x101
Jun 26 12:58:41 ws-ag-au-17 kernel: Uptime: 1h33m41s
Jun 26 12:58:41 ws-ag-au-17 kernel: Dumping 274 out of 4057
MB:..6%..12%..24%..36%..41%..53%..65%..71%..82%..94%
Jun 26 12:58:41 ws-ag-au-17 kernel: Dump complete

I have just enabled the writing of core dumps to disk and will post the dump
when it happens again.

The web server is usually being accessed when the panic occurs, but only
lightly (maximum of two users at once). Memory usage is 500Mb out of 4GB and
the dual core CPU does not seem to get above 10% while serving pages.

At this point I cannot deliberately reproduce the bug as it seems to be
spurious.

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