Would it help if I say I have a feeling it happens when running out of physical memory ? Or when allocating/using a lot of memory in a short timespan and releasing it under this condition.
I am having this exact same problem, with a machine that has 2G of swap and 768M of physical memory. I filed a bug in Red Hat Bugzilla, and was told to produce the output of /proc/slabinfo, but it takes the machine a while to consume all of its resources to the point where everything essentially is being run out of the swap file.
I will produce the Bug number once Bugzilla is alive again.