I built a 6.3 kernel with tmpfs enabled. Here's a diff of this kernel with GENERIC:
# diff -u GENERIC GENERIC_TMPFS --- GENERIC Thu Mar 15 03:52:16 2018 +++ GENERIC_TMPFS Thu Apr 5 00:56:50 2018 @@ -11,6 +11,7 @@ machine amd64 include "../../../conf/GENERIC" +option TMPFS # efficient memory file system maxusers 80 # estimated number of users option USER_PCICONF # user-space PCI configuration And I tried to mount tmpfs on this kernel, but it didn't: # mount_tmpfs tmpfs /root/mnt_test mount_tmpfs: tmpfs on /root/mnt_test: Bad address It worked up to 6.2 release. full dmesg: OpenBSD 6.3-stable (GENERIC_TMPFS) #0: Thu Apr 5 01:39:39 JST 2018 root@nimbus12.localnet:/usr/src/sys/arch/amd64/compile/GENERIC_TMPFS real mem = 3463262208 (3302MB) avail mem = 3351334912 (3196MB) mpath0 at root scsibus0 at mpath0: 256 targets mainbus0 at root bios0 at mainbus0: SMBIOS rev. 2.4 @ 0xf70b0 (61 entries) bios0: vendor Dell Inc. version "A05" date 04/02/2007 bios0: Dell Inc. Latitude D520 acpi0 at bios0: rev 0 acpi0: sleep states S0 S3 S4 S5 acpi0: tables DSDT FACP HPET APIC MCFG SLIC SSDT acpi0: wakeup devices LID_(S3) PBTN(S4) PCI0(S4) USB0(S0) USB1(S0) USB2(S0) USB3(S0) EHCI(S0) AZAL(S3) PCIE(S4) RP01(S3) RP02(S4) RP03(S3) RP04(S3) RP05(S3) RP06(S3) acpitimer0 at acpi0: 3579545 Hz, 24 bits acpihpet0 at acpi0: 14318179 Hz acpimadt0 at acpi0 addr 0xfee00000: PC-AT compat cpu0 at mainbus0: apid 0 (boot processor) cpu0: Intel(R) Core(TM)2 CPU T5500 @ 1.66GHz, 1662.73 MHz cpu0: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE,SSE3,DTES64,MWAIT,DS-CPL,VMX,EST,TM2,SSSE3,CX16,xTPR,PDCM,NXE,LONG,LAHF,PERF,SENSOR,MELTDOWN cpu0: 2MB 64b/line 8-way L2 cache cpu0: smt 0, core 0, package 0 mtrr: Pentium Pro MTRR support, 8 var ranges, 88 fixed ranges cpu0: apic clock running at 166MHz cpu0: mwait min=64, max=64, C-substates=0.2.2.2.2, IBE cpu at mainbus0: not configured ioapic0 at mainbus0: apid 2 pa 0xfec00000, version 20, 24 pins , remapped to apid 2 acpimcfg0 at acpi0 addr 0xf0000000, bus 0-63 acpiprt0 at acpi0: bus 0 (PCI0) acpiprt1 at acpi0: bus 2 (PCIE) acpiprt2 at acpi0: bus 11 (RP01) acpiprt3 at acpi0: bus 12 (RP02) acpiprt4 at acpi0: bus -1 (RP03) acpiprt5 at acpi0: bus -1 (RP04) acpiprt6 at acpi0: bus -1 (RP05) acpiprt7 at acpi0: bus -1 (RP06) acpicpu0 at acpi0: !C3(100@57 io@0x1016), !C2(500@1 io@0x1014), C1(1000@1 halt), PSS acpitz0 at acpi0: critical temperature is 126 degC "*pnp0c14" at acpi0 not configured acpiac0 at acpi0: AC unit online acpibat0 at acpi0: BAT0 model "DELL 0000Y8" serial 576 type LION oem "Sony" acpibat1 at acpi0: BAT1 not present acpibtn0 at acpi0: LID_ acpibtn1 at acpi0: PBTN acpibtn2 at acpi0: SBTN acpidock0 at acpi0: GDCK not docked (0) acpivideo0 at acpi0: VID_ acpivout0 at acpivideo0: LCD_ acpivideo1 at acpi0: VID2 cpu0: Enhanced SpeedStep 1662 MHz: speeds: 1667, 1333, 1000 MHz pci0 at mainbus0 bus 0 pchb0 at pci0 dev 0 function 0 "Intel 82945GM Host" rev 0x03 inteldrm0 at pci0 dev 2 function 0 "Intel 82945GM Video" rev 0x03 drm0 at inteldrm0 intagp0 at inteldrm0 agp0 at intagp0: aperture at 0xd0000000, size 0x10000000 inteldrm0: apic 2 int 16 inteldrm0: 848x480, 32bpp wsdisplay0 at inteldrm0 mux 1: console (std, vt100 emulation) wsdisplay0: screen 1-5 added (std, vt100 emulation) "Intel 82945GM Video" rev 0x03 at pci0 dev 2 function 1 not configured azalia0 at pci0 dev 27 function 0 "Intel 82801GB HD Audio" rev 0x01: msi azalia0: codecs: Sigmatel STAC9200, Conexant/0x2bfa, using Sigmatel STAC9200 audio0 at azalia0 ppb0 at pci0 dev 28 function 0 "Intel 82801GB PCIE" rev 0x01: msi pci1 at ppb0 bus 11 ppb1 at pci0 dev 28 function 1 "Intel 82801GB PCIE" rev 0x01: msi pci2 at ppb1 bus 12 wpi0 at pci2 dev 0 function 0 "Intel PRO/Wireless 3945ABG" rev 0x02: msi, JPN, address 00:1b:77:01:4e:fe uhci0 at pci0 dev 29 function 0 "Intel 82801GB USB" rev 0x01: apic 2 int 20 uhci1 at pci0 dev 29 function 1 "Intel 82801GB USB" rev 0x01: apic 2 int 21 uhci2 at pci0 dev 29 function 2 "Intel 82801GB USB" rev 0x01: apic 2 int 22 uhci3 at pci0 dev 29 function 3 "Intel 82801GB USB" rev 0x01: apic 2 int 23 ehci0 at pci0 dev 29 function 7 "Intel 82801GB USB" rev 0x01: apic 2 int 20 usb0 at ehci0: USB revision 2.0 uhub0 at usb0 configuration 1 interface 0 "Intel EHCI root hub" rev 2.00/1.00 addr 1 ppb2 at pci0 dev 30 function 0 "Intel 82801BAM Hub-to-PCI" rev 0xe1 pci3 at ppb2 bus 2 bce0 at pci3 dev 0 function 0 "Broadcom BCM4401B1" rev 0x02: apic 2 int 17, address 00:19:b9:6c:60:dd bmtphy0 at bce0 phy 1: BCM4401 10/100baseTX PHY, rev. 0 cbb0 at pci3 dev 1 function 0 "O2 Micro OZ711EZ1 CardBus" rev 0x21: apic 2 int 19 "O2 Micro Firewire" rev 0x02 at pci3 dev 1 function 4 not configured cardslot0 at cbb0 slot 0 flags 0 cardbus0 at cardslot0: bus 3 device 0 cacheline 0x0, lattimer 0x20 pcmcia0 at cardslot0 pcib0 at pci0 dev 31 function 0 "Intel 82801GBM LPC" rev 0x01 pciide0 at pci0 dev 31 function 2 "Intel 82801GBM SATA" rev 0x01: DMA, channel 0 wired to compatibility, channel 1 wired to compatibility wd0 at pciide0 channel 0 drive 0: <MARSHAL MAL2320SA-T54> wd0: 16-sector PIO, LBA48, 305245MB, 625142448 sectors wd0(pciide0:0:0): using PIO mode 4, Ultra-DMA mode 5 atapiscsi0 at pciide0 channel 1 drive 0 scsibus1 at atapiscsi0: 2 targets cd0 at scsibus1 targ 0 lun 0: <TSSTcorp, CDRW/DVD TSL462D, DE01> ATAPI 5/cdrom removable cd0(pciide0:1:0): using PIO mode 4, Ultra-DMA mode 2 ichiic0 at pci0 dev 31 function 3 "Intel 82801GB SMBus" rev 0x01: apic 2 int 17 iic0 at ichiic0 spdmem0 at iic0 addr 0x50: 2GB DDR2 SDRAM non-parity PC2-5300CL5 SO-DIMM spdmem1 at iic0 addr 0x52: 2GB DDR2 SDRAM non-parity PC2-5300CL5 SO-DIMM usb1 at uhci0: USB revision 1.0 uhub1 at usb1 configuration 1 interface 0 "Intel UHCI root hub" rev 1.00/1.00 addr 1 usb2 at uhci1: USB revision 1.0 uhub2 at usb2 configuration 1 interface 0 "Intel UHCI root hub" rev 1.00/1.00 addr 1 usb3 at uhci2: USB revision 1.0 uhub3 at usb3 configuration 1 interface 0 "Intel UHCI root hub" rev 1.00/1.00 addr 1 usb4 at uhci3: USB revision 1.0 uhub4 at usb4 configuration 1 interface 0 "Intel UHCI root hub" rev 1.00/1.00 addr 1 isa0 at pcib0 isadma0 at isa0 com0 at isa0 port 0x3f8/8 irq 4: ns16550a, 16 byte fifo pckbc0 at isa0 port 0x60/5 irq 1 irq 12 pckbd0 at pckbc0 (kbd slot) wskbd0 at pckbd0: console keyboard, using wsdisplay0 pms0 at pckbc0 (aux slot) wsmouse0 at pms0 mux 0 pms0: ALPS Glidepoint, version 0x7321 pcppi0 at isa0 port 0x61 spkr0 at pcppi0 uhub5 at uhub0 port 2 configuration 1 interface 0 "Dell product 0xa005" rev 2.00/50.18 addr 2 umass0 at uhub0 port 4 configuration 1 interface 0 "Western Digital Ext HDD 1021" rev 2.00/20.02 addr 3 umass0: using SCSI over Bulk-Only scsibus2 at umass0: 2 targets, initiator 0 sd0 at scsibus2 targ 1 lun 0: <WD, Ext HDD 1021, 2002> SCSI2 0/direct fixed serial.10581021343533383436 sd0: 1430797MB, 512 bytes/sector, 2930272256 sectors vscsi0 at root scsibus3 at vscsi0: 256 targets softraid0 at root scsibus4 at softraid0: 256 targets root on sd0f (89fb3b5bd471024d.f) swap on sd0b dump on sd0b