6.1 panic in mutex_vector_enter() called from genfs_do_putpages()

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I had a panic on a 6.1/amd64 system during the release of a number of fss(4) 
snapshots. Of course I cannot tell for sure the panic  was caused by 
fssconfig -u, only it happened at that time:

uvm_fault(0xffffffff80724860, 0x0, 1) -> e
fatal page fault in supervisor mode
trap type 6 code 0 rip ffffffff8025cfbf cs 8 rflags 10283 cr2  8 cpl 0 rsp fffffe811d8be800
kernel: page fault trap, code=0
Stopped in pid 0,80 (system) at netbsd:mutex_vevtor_enter+0x80: movq	18(%r15),%rax

show registers reveals r15 contains fffffffffffffff0.

bt gives:
mutex_vector_enter() at netbsd:mutex_vector_enter+0x80
genfs_do_putpages() at netbsd:genfs_do_putpages+0xb19
VOP_PUTPAGES at netbsd:VOP_PUTPAGES+0x3a
uvm_pageout() at netbsd:uvm_pageout+0x20a

(thats all manual OCR, of course.)

I'm creating and destroying these snapshots daily (I hold them during backup), 
so it can't be a general problem related to snapshotting these file systems.

Any clues?