6.2 -> 7.0 now mlock(2) fails

[Dieter]

I never saw mlock(2) fail in 6.2 but with 7.0 I sometimes
get mlock(2) failed: Resource temporarily unavailable.

I commented out a ton of drivers in the kernel config file,
which I didn't do in 6.2, so the 7.0 kernel should be using less
memory, unless something still in there gained a lot of bloat.

dmesg says:
  usable memory = 2139107328 (2040 MB)
  avail memory  = 2064769024 (1969 MB)

pstat -s says:
  Device          1M-blocks     Used    Avail Capacity
  /dev/ad6s10          4482        0     4482     0%

so there should be far more than enough memory.

systat -vmstat says:

  Mem:KB    REAL            VIRTUAL                       VN PAGER   SWAP PAGER
          Tot   Share      Tot    Share    Free           in   out     in   out
  Act  213464    8460   442088    17796   72928  count    
  All  255092    8900  4688532    19072          pages  

and mlock is failing.

What changed and how do I fix it?