"free" doesn't account for slab cache

Binary package hint: procps

Using procps 1:3.2.7-3ubuntu2 from Feisty, do "free -m":

             total used free shared buffers cached
Mem: 2026 1967 59 0 629 665
-/+ buffers/cache: 672 1354
Swap: 0 0 0

Historically I've used the "-/+ buffers/cache" line as a way to determine how much memory my system is "really" using. "Really" in this case means "memory in use that won't just be freed up when I want it for something".

Since the "-/+ buffers/cache" line doesn't include the Slab cache (the "Slab:" line in /proc/meminfo), and the Slab cache can grow quite big, "free" cannot be used to determine the amount of free memory in the machine :-(.

I would like "free" to count the Slab: memory as free, at least on the "-/+ buffers/cache" line.

Or that somebody could tell me an alternative way of finding out the amount of free memory in my machine without having to manually add things up from /proc/meminfo.