This turned out to be a huge win on 32-bit i386 in PAE mode, but it is 
likely not as significant on x86_64; I don't know because I haven't 
actually measured the cost.  I don't have 64-bit hardware that I have 
the luxury of rebooting right now, so this patch is untested, but if 
someone wants to try this out, it might actually show a measurable win 
on fork/exit.  I lost my cycle count measurement diffs, but I don't 
think they would apply cleanly to x86_64 anyways.  This patch at least 
looks good, and compiles cleanly on 2.6.13-rc5-mm1, thus passing some 
level of testing.

Also, it might show reduced latency on pre-emptible kernels during heavy 
fork/exit activity, possibly allowing ZAP_BLOCK_SIZE to be raised for 
some architectures (I measured a ~30-50% reduction in cycle timings for 
zap_pte_range on i386 with CONFIG_PREEMPT with the analogous patch).

Zach