The KMEMSTATS option, formerly silently turned on by a #define in
<sys/malloc.h>, is now turned off by default.  If you build your kernel with
"options KMEMSTATS" or "options DEBUG" you will turn it back on.

It's turned off because it causes a performance lose by disabling the MALLOC()
and FREE() forms of the kernel allocator, which are faster than the function
form and are used in certain critical paths.

Note that vmstat -m does not report useful data if KMEMSTATS is off; however,
the proposition that most users would prefer to have that functionality at
the expense of possibly a significant amount of performance is dubious at
best.

See the options(4) manual page for more information.

Thor