9.2 + ZFS + i386 = panic: pmap_enter: attempted pmap_enter on 4MB page

[J David]

What are the necessary loader.conf / kernel config invocations to make
ZFS stable on 9.2-RELEASE i386 node with 3GB RAM?

This machine was rock solid under 8.4, but since upgrading to 9.2 it
has been a disaster.  It crashes every few hours with "panic:
pmap_enter: attempted pmap_enter on 4MB page."

Here are a couple of stack traces:

panic: pmap_enter: attempted pmap_enter on 4MB page
cpuid = 0
KDB: stack backtrace:
#0 0xc0afc092 at kdb_backtrace+0x52
#1 0xc0ac249c at panic+0x1bc
#2 0xc0f1825d at pmap_enter+0x63d
#3 0xc0d34ee5 at vm_fault_hold+0x1c45
#4 0xc0d33262 at vm_fault+0x82
#5 0xc0f1f5d6 at trap_pfault+0x186
#6 0xc0f1ed4b at trap+0x51b
#7 0xc0f0887c at calltrap+0x6
#8 0xc16770b6 at zio_execute+0x116
#9 0xc15d9460 at taskq_run_safe+0x10
#10 0xc0b08f26 at taskqueue_run_locked+0xe6
#11 0xc0b097f7 at taskqueue_thread_loop+0xb7
#12 0xc0a90a73 at fork_exit+0xa3
#13 0xc0f08924 at fork_trampoline+0x8

panic: pmap_enter: attempted pmap_enter on 4MB page
cpuid = 1
KDB: stack backtrace:
#0 0xc0afc092 at kdb_backtrace+0x52
#1 0xc0ac249c at panic+0x1bc
#2 0xc0f1825d at pmap_enter+0x63d
#3 0xc0d34ee5 at vm_fault_hold+0x1c45
#4 0xc0d33262 at vm_fault+0x82
#5 0xc0f1f5d6 at trap_pfault+0x186
#6 0xc0f1ed4b at trap+0x51b
#7 0xc0f0887c at calltrap+0x6
#8 0xc1679bdf at zio_dva_allocate+0x9f
#9 0xc16770b6 at zio_execute+0x116
#10 0xc15d9460 at taskq_run_safe+0x10
#11 0xc0b08f26 at taskqueue_run_locked+0xe6
#12 0xc0b097f7 at taskqueue_thread_loop+0xb7
#13 0xc0a90a73 at fork_exit+0xa3
#14 0xc0f08924 at fork_trampoline+0x8

For the last crash, top was running and this is what was on the screen
when it died:

CPU:  0.6% user,  0.0% nice,  2.2% system,  0.2% interrupt, 97.1% idle
Mem: 442M Active, 91M Inact, 79M Wired, 3764K Cache, 4960K Buf, 2380M Free
ARC: 40M Total, 8520K MFU, 30M MRU, 304K Anon, 494K Header, 1062K Other
Swap: 4096M Total, 4096M Free
Write failed: Broken pipe

It doesn't seem to matter what KVA_PAGES, vm.kmem_size,
vfs.zfs.arc_max or vfs.zfs.vdev.cache.size is set to, and the ZFS
tuning guides in the wiki (albeit appearing to be dating to the 7.x
era) provides guidelines for tuning down to 768M.  So 3GB should be
enough for a machine that is 99% idle.  (Particularly given that it
dies with >2GB free.)

This seems to be specific to i386, this problem hasn't cropped up on
any amd64 nodes.  No compression, deduplication, snapshots or anything
like that is in use.

Thanks for any suggestions!