panic after LOR, 2nd netmap_mem2.c, 1st vm_fault.c

[Harry Schmalzbauer]

 lock order reversal: (sleepable after non-sleepable)
 1st 0xfffff8007519a960 vm object (vm object) @
/usr/local/share/deploy-tools/RELENG_11/src/sys/vm/vm_fault.c:572
 2nd 0xfffff8003299b000 (d)->nm_mtx ((d)->nm_mtx) @
/usr/local/share/deploy-tools/RELENG_11/src/sys/dev/netmap/netmap_mem2.c:577
stack backtrace:
#0 0xffffffff805e7900 at witness_debugger+0x70
#1 0xffffffff805e77f3 at witness_checkorder+0xe23
#2 0xffffffff80591f4e at _sx_xlock+0x5e
#3 0xffffffff8042275a at netmap_mem2_ofstophys+0x2a
#4 0xffffffff8041f3ab at netmap_dev_pager_fault+0x3b
#5 0xffffffff8082d834 at dev_pager_getpages+0x74
#6 0xffffffff80856d2a at vm_pager_get_pages+0x4a
#7 0xffffffff8083aa92 at vm_fault_hold+0xa52
#8 0xffffffff80839ff5 at vm_fault+0x75
#9 0xffffffff8088048f at trap_pfault+0xff
#10 0xffffffff8087fc38 at trap+0x348
#11 0xffffffff808645d1 at calltrap+0x8

KDB: stack backtrace:
#0 0xffffffff805ca4b7 at kdb_backtrace+0x67
#1 0xffffffff8058a3f6 at vpanic+0x186
#2 0xffffffff8058a473 at panic+0x43
#3 0xffffffff8056b564 at __mtx_assert+0xb4
#4 0xffffffff80544780 at knlist_add+0x20
#5 0xffffffff8041ead0 at netmap_kqfilter+0x110
#6 0xffffffff804657f7 at devfs_kqfilter_f+0x77
#7 0xffffffff80542a0b at kqueue_register+0x78b
#8 0xffffffff80543432 at kqueue_kevent+0x92
#9 0xffffffff80543336 at kern_kevent_fp+0x96
#10 0xffffffff8054324f at kern_kevent+0x9f
#11 0xffffffff80543058 at sys_kevent+0x138
#12 0xffffffff80880dda at amd64_syscall+0x57a
#13 0xffffffff808648bb at Xfast_syscall+0xfb

#0  doadump (textdump=<value optimized out>) at pcpu.h:222
#1  0xffffffff80589e70 in kern_reboot (howto=260) at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_shutdown.c:366
#2  0xffffffff8058a430 in vpanic (fmt=<value optimized out>, ap=<value
optimized out>)
    at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_shutdown.c:759
#3  0xffffffff8058a473 in panic (fmt=<value optimized out>) at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_shutdown.c:690
#4  0xffffffff8056b564 in __mtx_assert (c=0x0, what=0, file=0x0, line=0)
at /usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_mutex.c:1000
#5  0xffffffff80544780 in knlist_add (knl=0xfffffe000a055450,
kn=0xfffff8026d097e80, islocked=1)
    at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_event.c:2089
#6  0xffffffff8041ead0 in netmap_kqfilter (dev=<value optimized out>,
kn=0xfffff8026d097e80)
    at
/usr/local/share/deploy-tools/RELENG_11/src/sys/dev/netmap/netmap_freebsd.c:1354
#7  0xffffffff804657f7 in devfs_kqfilter_f (fp=0xfffff8001faf8780,
kn=0xfffff8026d097e80)
    at
/usr/local/share/deploy-tools/RELENG_11/src/sys/fs/devfs/devfs_vnops.c:837
#8  0xffffffff80542a0b in kqueue_register (kq=0xfffff8001a65c000,
kev=0xfffffe045b4dc650, td=0xfffff8014ca71000, waitok=<value optimized out>)
    at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_event.c:1334
#9  0xffffffff80543432 in kqueue_kevent (kq=0xfffff8001a65c000,
td=0xfffff8014ca71000, nchanges=4, nevents=<value optimized out>,
k_ops=0xfffffe045b4dc8a0,
    timeout=<value optimized out>) at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_event.c:1019
#10 0xffffffff80543336 in kern_kevent_fp (td=0xfffff8014ca71000,
fp=<value optimized out>, nchanges=4, nevents=<value optimized out>,
    k_ops=<value optimized out>, timeout=<value optimized out>) at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_event.c:1050
#11 0xffffffff8054324f in kern_kevent (td=0xfffff8014ca71000, fd=7,
nchanges=4, nevents=0, k_ops=0xfffffe045b4dc8a0, timeout=0x0)
    at /usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_event.c:993
#12 0xffffffff80543058 in sys_kevent (td=0xfffff8014ca71000,
uap=0xfffffe045b4dca30) at
/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_event.c:925
#13 0xffffffff80880dda in amd64_syscall (td=0xfffff8014ca71000,
traced=0) at subr_syscall.c:135
#14 0xffffffff808648bb in Xfast_syscall () at
/usr/local/share/deploy-tools/RELENG_11/src/sys/amd64/amd64/exception.S:396
#15 0x000000080122813a in ?? ()

Sorry, again me, again helpless without you.

This is on r319032, stable/11 with netmap merged from HEAD. Since I was
advised to replace netmap with github sources, I thought it may be
beneficial to get the various HEAD-fixes as well.
Seems it's beyond my scope doing such things.

Does anybody have a quick idea if it's easily fixable, or a complicated
issue, possibly caused due to MFC boch?

Thanks,

-harry