possible performance issue in 4-level page tables

[Zou Nan hai]

There is a performance regression of lmbench
lat_proc fork result on ia64.

in 
2.6.10 

I got 
Process fork+exit:164.8438 microseconds.

in 2.6.11-rc2
Process fork+exit:183.8621 microseconds.

I believe this regression was caused by 
the 4-level page tables change.

Since most of the kernel time spend in lat_proc fork is copy_page_range
in fork path and clear_page_range in the exit path. Now they are 1 level
deeper.

Though pud and pgd is same on IA64, there is still some overhead
introduced I think.
 
Are any other architectures seeing the same sort of results?

Zou Nan hai

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