Cached memory never gets released

Linux 2.4.23 on x86, 128M memory available
Free outputs this:
total used free shared buffers cached
Mem: 119204 109592 9612 0 60 92136
-/+ buffers/cache: 17396 101808
Swap: 0 0 0

The root filesystem is NFS, no hard drives involved.
When I run a simple program that just mallocs memory and fills it with random data, the kernel kills the process after it has only allocated 8 or 9 megs. The 92136K of cached memory can't get released for some reason. What is happening?

Thanks very much!
-Dave